1=head1 NAME 2X<function> 3 4perlfunc - Perl builtin functions 5 6=head1 DESCRIPTION 7 8The functions in this section can serve as terms in an expression. 9They fall into two major categories: list operators and named unary 10operators. These differ in their precedence relationship with a 11following comma. (See the precedence table in L<perlop>.) List 12operators take more than one argument, while unary operators can never 13take more than one argument. Thus, a comma terminates the argument of 14a unary operator, but merely separates the arguments of a list 15operator. A unary operator generally provides scalar context to its 16argument, while a list operator may provide either scalar or list 17contexts for its arguments. If it does both, scalar arguments 18come first and list argument follow, and there can only ever 19be one such list argument. For instance, 20L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> has three scalar arguments 21followed by a list, whereas L<C<gethostbyname>|/gethostbyname NAME> has 22four scalar arguments. 23 24In the syntax descriptions that follow, list operators that expect a 25list (and provide list context for elements of the list) are shown 26with LIST as an argument. Such a list may consist of any combination 27of scalar arguments or list values; the list values will be included 28in the list as if each individual element were interpolated at that 29point in the list, forming a longer single-dimensional list value. 30Commas should separate literal elements of the LIST. 31 32Any function in the list below may be used either with or without 33parentheses around its arguments. (The syntax descriptions omit the 34parentheses.) If you use parentheses, the simple but occasionally 35surprising rule is this: It I<looks> like a function, therefore it I<is> a 36function, and precedence doesn't matter. Otherwise it's a list 37operator or unary operator, and precedence does matter. Whitespace 38between the function and left parenthesis doesn't count, so sometimes 39you need to be careful: 40 41 print 1+2+4; # Prints 7. 42 print(1+2) + 4; # Prints 3. 43 print (1+2)+4; # Also prints 3! 44 print +(1+2)+4; # Prints 7. 45 print ((1+2)+4); # Prints 7. 46 47If you run Perl with the L<C<use warnings>|warnings> pragma, it can warn 48you about this. For example, the third line above produces: 49 50 print (...) interpreted as function at - line 1. 51 Useless use of integer addition in void context at - line 1. 52 53A few functions take no arguments at all, and therefore work as neither 54unary nor list operators. These include such functions as 55L<C<time>|/time> and L<C<endpwent>|/endpwent>. For example, 56C<time+86_400> always means C<time() + 86_400>. 57 58For functions that can be used in either a scalar or list context, 59nonabortive failure is generally indicated in scalar context by 60returning the undefined value, and in list context by returning the 61empty list. 62 63Remember the following important rule: There is B<no rule> that relates 64the behavior of an expression in list context to its behavior in scalar 65context, or vice versa. It might do two totally different things. 66Each operator and function decides which sort of value would be most 67appropriate to return in scalar context. Some operators return the 68length of the list that would have been returned in list context. Some 69operators return the first value in the list. Some operators return the 70last value in the list. Some operators return a count of successful 71operations. In general, they do what you want, unless you want 72consistency. 73X<context> 74 75A named array in scalar context is quite different from what would at 76first glance appear to be a list in scalar context. You can't get a list 77like C<(1,2,3)> into being in scalar context, because the compiler knows 78the context at compile time. It would generate the scalar comma operator 79there, not the list concatenation version of the comma. That means it 80was never a list to start with. 81 82In general, functions in Perl that serve as wrappers for system calls 83("syscalls") of the same name (like L<chown(2)>, L<fork(2)>, 84L<closedir(2)>, etc.) return true when they succeed and 85L<C<undef>|/undef EXPR> otherwise, as is usually mentioned in the 86descriptions below. This is different from the C interfaces, which 87return C<-1> on failure. Exceptions to this rule include 88L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS>, and 89L<C<syscall>|/syscall NUMBER, LIST>. System calls also set the special 90L<C<$!>|perlvar/$!> variable on failure. Other functions do not, except 91accidentally. 92 93Extension modules can also hook into the Perl parser to define new 94kinds of keyword-headed expression. These may look like functions, but 95may also look completely different. The syntax following the keyword 96is defined entirely by the extension. If you are an implementor, see 97L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such 98a module, see the module's documentation for details of the syntax that 99it defines. 100 101=head2 Perl Functions by Category 102X<function> 103 104Here are Perl's functions (including things that look like 105functions, like some keywords and named operators) 106arranged by category. Some functions appear in more 107than one place. Any warnings, including those produced by 108keywords, are described in L<perldiag> and L<warnings>. 109 110=over 4 111 112=item Functions for SCALARs or strings 113X<scalar> X<string> X<character> 114 115=for Pod::Functions =String 116 117L<C<chomp>|/chomp VARIABLE>, L<C<chop>|/chop VARIABLE>, 118L<C<chr>|/chr NUMBER>, L<C<crypt>|/crypt PLAINTEXT,SALT>, 119L<C<fc>|/fc EXPR>, L<C<hex>|/hex EXPR>, 120L<C<index>|/index STR,SUBSTR,POSITION>, L<C<lc>|/lc EXPR>, 121L<C<lcfirst>|/lcfirst EXPR>, L<C<length>|/length EXPR>, 122L<C<oct>|/oct EXPR>, L<C<ord>|/ord EXPR>, 123L<C<pack>|/pack TEMPLATE,LIST>, 124L<C<qE<sol>E<sol>>|/qE<sol>STRINGE<sol>>, 125L<C<qqE<sol>E<sol>>|/qqE<sol>STRINGE<sol>>, L<C<reverse>|/reverse LIST>, 126L<C<rindex>|/rindex STR,SUBSTR,POSITION>, 127L<C<sprintf>|/sprintf FORMAT, LIST>, 128L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT>, 129L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>, L<C<uc>|/uc EXPR>, 130L<C<ucfirst>|/ucfirst EXPR>, 131L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>> 132 133L<C<fc>|/fc EXPR> is available only if the 134L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is 135prefixed with C<CORE::>. The 136L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically 137with a C<use v5.16> (or higher) declaration in the current scope. 138 139=item Regular expressions and pattern matching 140X<regular expression> X<regex> X<regexp> 141 142=for Pod::Functions =Regexp 143 144L<C<mE<sol>E<sol>>|/mE<sol>E<sol>>, L<C<pos>|/pos SCALAR>, 145L<C<qrE<sol>E<sol>>|/qrE<sol>STRINGE<sol>>, 146L<C<quotemeta>|/quotemeta EXPR>, 147L<C<sE<sol>E<sol>E<sol>>|/sE<sol>E<sol>E<sol>>, 148L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>, 149L<C<study>|/study SCALAR> 150 151=item Numeric functions 152X<numeric> X<number> X<trigonometric> X<trigonometry> 153 154=for Pod::Functions =Math 155 156L<C<abs>|/abs VALUE>, L<C<atan2>|/atan2 Y,X>, L<C<cos>|/cos EXPR>, 157L<C<exp>|/exp EXPR>, L<C<hex>|/hex EXPR>, L<C<int>|/int EXPR>, 158L<C<log>|/log EXPR>, L<C<oct>|/oct EXPR>, L<C<rand>|/rand EXPR>, 159L<C<sin>|/sin EXPR>, L<C<sqrt>|/sqrt EXPR>, L<C<srand>|/srand EXPR> 160 161=item Functions for real @ARRAYs 162X<array> 163 164=for Pod::Functions =ARRAY 165 166L<C<each>|/each HASH>, L<C<keys>|/keys HASH>, L<C<pop>|/pop ARRAY>, 167L<C<push>|/push ARRAY,LIST>, L<C<shift>|/shift ARRAY>, 168L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>, 169L<C<unshift>|/unshift ARRAY,LIST>, L<C<values>|/values HASH> 170 171=item Functions for list data 172X<list> 173 174=for Pod::Functions =LIST 175 176L<C<grep>|/grep BLOCK LIST>, L<C<join>|/join EXPR,LIST>, 177L<C<map>|/map BLOCK LIST>, L<C<qwE<sol>E<sol>>|/qwE<sol>STRINGE<sol>>, 178L<C<reverse>|/reverse LIST>, L<C<sort>|/sort SUBNAME LIST>, 179L<C<unpack>|/unpack TEMPLATE,EXPR> 180 181=item Functions for real %HASHes 182X<hash> 183 184=for Pod::Functions =HASH 185 186L<C<delete>|/delete EXPR>, L<C<each>|/each HASH>, 187L<C<exists>|/exists EXPR>, L<C<keys>|/keys HASH>, 188L<C<values>|/values HASH> 189 190=item Input and output functions 191X<I/O> X<input> X<output> X<dbm> 192 193=for Pod::Functions =I/O 194 195L<C<binmode>|/binmode FILEHANDLE, LAYER>, L<C<close>|/close FILEHANDLE>, 196L<C<closedir>|/closedir DIRHANDLE>, L<C<dbmclose>|/dbmclose HASH>, 197L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, L<C<die>|/die LIST>, 198L<C<eof>|/eof FILEHANDLE>, L<C<fileno>|/fileno FILEHANDLE>, 199L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<format>|/format>, 200L<C<getc>|/getc FILEHANDLE>, L<C<print>|/print FILEHANDLE LIST>, 201L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, 202L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>, 203L<C<readdir>|/readdir DIRHANDLE>, L<C<readline>|/readline EXPR>, 204L<C<rewinddir>|/rewinddir DIRHANDLE>, L<C<say>|/say FILEHANDLE LIST>, 205L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 206L<C<seekdir>|/seekdir DIRHANDLE,POS>, 207L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, 208L<C<syscall>|/syscall NUMBER, LIST>, 209L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>, 210L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>, 211L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, 212L<C<tell>|/tell FILEHANDLE>, L<C<telldir>|/telldir DIRHANDLE>, 213L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<warn>|/warn LIST>, 214L<C<write>|/write FILEHANDLE> 215 216L<C<say>|/say FILEHANDLE LIST> is available only if the 217L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is 218prefixed with C<CORE::>. The 219L<C<"say"> feature|feature/The 'say' feature> is enabled automatically 220with a C<use v5.10> (or higher) declaration in the current scope. 221 222=item Functions for fixed-length data or records 223 224=for Pod::Functions =Binary 225 226L<C<pack>|/pack TEMPLATE,LIST>, 227L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>, 228L<C<syscall>|/syscall NUMBER, LIST>, 229L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>, 230L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>, 231L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, 232L<C<unpack>|/unpack TEMPLATE,EXPR>, L<C<vec>|/vec EXPR,OFFSET,BITS> 233 234=item Functions for filehandles, files, or directories 235X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink> 236 237=for Pod::Functions =File 238 239L<C<-I<X>>|/-X FILEHANDLE>, L<C<chdir>|/chdir EXPR>, 240L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>, 241L<C<chroot>|/chroot FILENAME>, 242L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>, L<C<glob>|/glob EXPR>, 243L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, 244L<C<link>|/link OLDFILE,NEWFILE>, L<C<lstat>|/lstat FILEHANDLE>, 245L<C<mkdir>|/mkdir FILENAME,MODE>, L<C<open>|/open FILEHANDLE,MODE,EXPR>, 246L<C<opendir>|/opendir DIRHANDLE,EXPR>, L<C<readlink>|/readlink EXPR>, 247L<C<rename>|/rename OLDNAME,NEWNAME>, L<C<rmdir>|/rmdir FILENAME>, 248L<C<select>|/select FILEHANDLE>, L<C<stat>|/stat FILEHANDLE>, 249L<C<symlink>|/symlink OLDFILE,NEWFILE>, 250L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, 251L<C<umask>|/umask EXPR>, L<C<unlink>|/unlink LIST>, 252L<C<utime>|/utime LIST> 253 254=item Keywords related to the control flow of your Perl program 255X<control flow> 256 257=for Pod::Functions =Flow 258 259L<C<break>|/break>, L<C<caller>|/caller EXPR>, 260L<C<continue>|/continue BLOCK>, L<C<die>|/die LIST>, L<C<do>|/do BLOCK>, 261L<C<dump>|/dump LABEL>, L<C<eval>|/eval EXPR>, 262L<C<evalbytes>|/evalbytes EXPR>, L<C<exit>|/exit EXPR>, 263L<C<__FILE__>|/__FILE__>, L<C<goto>|/goto LABEL>, 264L<C<last>|/last LABEL>, L<C<__LINE__>|/__LINE__>, 265L<C<method>|/method NAME BLOCK>, 266L<C<next>|/next LABEL>, L<C<__PACKAGE__>|/__PACKAGE__>, 267L<C<redo>|/redo LABEL>, L<C<return>|/return EXPR>, 268L<C<sub>|/sub NAME BLOCK>, L<C<__SUB__>|/__SUB__>, 269L<C<wantarray>|/wantarray> 270 271L<C<break>|/break> is available only if you enable the experimental 272L<C<"switch"> feature|feature/The 'switch' feature> or use the C<CORE::> 273prefix. The L<C<"switch"> feature|feature/The 'switch' feature> also 274enables the C<default>, C<given> and C<when> statements, which are 275documented in L<perlsyn/"Switch Statements">. 276The L<C<"switch"> feature|feature/The 'switch' feature> is enabled 277automatically with a C<use v5.10> (or higher) declaration in the current 278scope. In Perl v5.14 and earlier, L<C<continue>|/continue BLOCK> 279required the L<C<"switch"> feature|feature/The 'switch' feature>, like 280the other keywords. 281 282L<C<evalbytes>|/evalbytes EXPR> is only available with the 283L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features> 284(see L<feature>) or if prefixed with C<CORE::>. L<C<__SUB__>|/__SUB__> 285is only available with the 286L<C<"current_sub"> feature|feature/The 'current_sub' feature> or if 287prefixed with C<CORE::>. Both the 288L<C<"evalbytes">|feature/The 'unicode_eval' and 'evalbytes' features> 289and L<C<"current_sub">|feature/The 'current_sub' feature> features are 290enabled automatically with a C<use v5.16> (or higher) declaration in the 291current scope. 292 293=item Keywords related to scoping 294 295=for Pod::Functions =Namespace 296 297L<C<caller>|/caller EXPR>, 298L<C<class>|/class NAMESPACE>, 299L<C<field>|/field VARNAME>, 300L<C<import>|/import LIST>, 301L<C<local>|/local EXPR>, 302L<C<my>|/my VARLIST>, 303L<C<our>|/our VARLIST>, 304L<C<package>|/package NAMESPACE>, 305L<C<state>|/state VARLIST>, 306L<C<use>|/use Module VERSION LIST> 307 308L<C<state>|/state VARLIST> is available only if the 309L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is 310prefixed with C<CORE::>. The 311L<C<"state"> feature|feature/The 'state' feature> is enabled 312automatically with a C<use v5.10> (or higher) declaration in the current 313scope. 314 315=item Miscellaneous functions 316 317=for Pod::Functions =Misc 318 319L<C<defined>|/defined EXPR>, L<C<formline>|/formline PICTURE,LIST>, 320L<C<lock>|/lock THING>, L<C<prototype>|/prototype FUNCTION>, 321L<C<reset>|/reset EXPR>, L<C<scalar>|/scalar EXPR>, 322L<C<undef>|/undef EXPR> 323 324=item Functions for processes and process groups 325X<process> X<pid> X<process id> 326 327=for Pod::Functions =Process 328 329L<C<alarm>|/alarm SECONDS>, L<C<exec>|/exec LIST>, L<C<fork>|/fork>, 330L<C<getpgrp>|/getpgrp PID>, L<C<getppid>|/getppid>, 331L<C<getpriority>|/getpriority WHICH,WHO>, L<C<kill>|/kill SIGNAL, LIST>, 332L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>, 333L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, 334L<C<readpipe>|/readpipe EXPR>, L<C<setpgrp>|/setpgrp PID,PGRP>, 335L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>, 336L<C<sleep>|/sleep EXPR>, L<C<system>|/system LIST>, L<C<times>|/times>, 337L<C<wait>|/wait>, L<C<waitpid>|/waitpid PID,FLAGS> 338 339=item Keywords related to Perl modules 340X<module> 341 342=for Pod::Functions =Modules 343 344L<C<do>|/do EXPR>, L<C<import>|/import LIST>, 345L<C<no>|/no MODULE VERSION LIST>, L<C<package>|/package NAMESPACE>, 346L<C<require>|/require VERSION>, L<C<use>|/use Module VERSION LIST> 347 348=item Keywords related to classes and object-orientation 349X<object> X<class> X<package> 350 351=for Pod::Functions =Objects 352 353L<C<bless>|/bless REF,CLASSNAME>, 354L<C<class>|/class NAMESPACE>, 355L<C<__CLASS__>|/__CLASS__>, 356L<C<dbmclose>|/dbmclose HASH>, 357L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, 358L<C<field>|/field VARNAME>, 359L<C<method>|/method NAME BLOCK>, 360L<C<package>|/package NAMESPACE>, 361L<C<ref>|/ref EXPR>, 362L<C<tie>|/tie VARIABLE,CLASSNAME,LIST>, 363L<C<tied>|/tied VARIABLE>, 364L<C<untie>|/untie VARIABLE>, 365L<C<use>|/use Module VERSION LIST> 366 367=item Low-level socket functions 368X<socket> X<sock> 369 370=for Pod::Functions =Socket 371 372L<C<accept>|/accept NEWSOCKET,GENERICSOCKET>, 373L<C<bind>|/bind SOCKET,NAME>, L<C<connect>|/connect SOCKET,NAME>, 374L<C<getpeername>|/getpeername SOCKET>, 375L<C<getsockname>|/getsockname SOCKET>, 376L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>, 377L<C<listen>|/listen SOCKET,QUEUESIZE>, 378L<C<recv>|/recv SOCKET,SCALAR,LENGTH,FLAGS>, 379L<C<send>|/send SOCKET,MSG,FLAGS,TO>, 380L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>, 381L<C<shutdown>|/shutdown SOCKET,HOW>, 382L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>, 383L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL> 384 385=item System V interprocess communication functions 386X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> 387 388=for Pod::Functions =SysV 389 390L<C<msgctl>|/msgctl ID,CMD,ARG>, L<C<msgget>|/msgget KEY,FLAGS>, 391L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>, 392L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>, 393L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>, 394L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>, 395L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>, 396L<C<shmread>|/shmread ID,VAR,POS,SIZE>, 397L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE> 398 399=item Fetching user and group info 400X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> 401 402=for Pod::Functions =User 403 404L<C<endgrent>|/endgrent>, L<C<endhostent>|/endhostent>, 405L<C<endnetent>|/endnetent>, L<C<endpwent>|/endpwent>, 406L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>, 407L<C<getgrnam>|/getgrnam NAME>, L<C<getlogin>|/getlogin>, 408L<C<getpwent>|/getpwent>, L<C<getpwnam>|/getpwnam NAME>, 409L<C<getpwuid>|/getpwuid UID>, L<C<setgrent>|/setgrent>, 410L<C<setpwent>|/setpwent> 411 412=item Fetching network info 413X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> 414 415=for Pod::Functions =Network 416 417L<C<endprotoent>|/endprotoent>, L<C<endservent>|/endservent>, 418L<C<gethostbyaddr>|/gethostbyaddr ADDR,ADDRTYPE>, 419L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>, 420L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>, 421L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>, 422L<C<getprotobyname>|/getprotobyname NAME>, 423L<C<getprotobynumber>|/getprotobynumber NUMBER>, 424L<C<getprotoent>|/getprotoent>, 425L<C<getservbyname>|/getservbyname NAME,PROTO>, 426L<C<getservbyport>|/getservbyport PORT,PROTO>, 427L<C<getservent>|/getservent>, L<C<sethostent>|/sethostent STAYOPEN>, 428L<C<setnetent>|/setnetent STAYOPEN>, 429L<C<setprotoent>|/setprotoent STAYOPEN>, 430L<C<setservent>|/setservent STAYOPEN> 431 432=item Time-related functions 433X<time> X<date> 434 435=for Pod::Functions =Time 436 437L<C<gmtime>|/gmtime EXPR>, L<C<localtime>|/localtime EXPR>, 438L<C<time>|/time>, L<C<times>|/times> 439 440=item Non-function keywords 441 442=for Pod::Functions =!Non-functions 443 444C<ADJUST>, 445C<and>, 446C<AUTOLOAD>, 447C<BEGIN>, 448C<catch>, 449C<CHECK>, 450C<cmp>, 451C<CORE>, 452C<__DATA__>, 453C<default>, 454C<defer>, 455C<DESTROY>, 456C<else>, 457C<elseif>, 458C<elsif>, 459C<END>, 460C<__END__>, 461C<eq>, 462C<finally>, 463C<for>, 464C<foreach>, 465C<ge>, 466C<given>, 467C<gt>, 468C<if>, 469C<INIT>, 470C<isa>, 471C<le>, 472C<lt>, 473C<ne>, 474C<not>, 475C<or>, 476C<try>, 477C<UNITCHECK>, 478C<unless>, 479C<until>, 480C<when>, 481C<while>, 482C<x>, 483C<xor> 484 485=back 486 487=head2 Portability 488X<portability> X<Unix> X<portable> 489 490Perl was born in Unix and can therefore access all common Unix 491system calls. In non-Unix environments, the functionality of some 492Unix system calls may not be available or details of the available 493functionality may differ slightly. The Perl functions affected 494by this are: 495 496L<C<-I<X>>|/-X FILEHANDLE>, L<C<binmode>|/binmode FILEHANDLE, LAYER>, 497L<C<chmod>|/chmod LIST>, L<C<chown>|/chown LIST>, 498L<C<chroot>|/chroot FILENAME>, L<C<crypt>|/crypt PLAINTEXT,SALT>, 499L<C<dbmclose>|/dbmclose HASH>, L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, 500L<C<dump>|/dump LABEL>, L<C<endgrent>|/endgrent>, 501L<C<endhostent>|/endhostent>, L<C<endnetent>|/endnetent>, 502L<C<endprotoent>|/endprotoent>, L<C<endpwent>|/endpwent>, 503L<C<endservent>|/endservent>, L<C<exec>|/exec LIST>, 504L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>, 505L<C<flock>|/flock FILEHANDLE,OPERATION>, L<C<fork>|/fork>, 506L<C<getgrent>|/getgrent>, L<C<getgrgid>|/getgrgid GID>, 507L<C<gethostbyname>|/gethostbyname NAME>, L<C<gethostent>|/gethostent>, 508L<C<getlogin>|/getlogin>, 509L<C<getnetbyaddr>|/getnetbyaddr ADDR,ADDRTYPE>, 510L<C<getnetbyname>|/getnetbyname NAME>, L<C<getnetent>|/getnetent>, 511L<C<getppid>|/getppid>, L<C<getpgrp>|/getpgrp PID>, 512L<C<getpriority>|/getpriority WHICH,WHO>, 513L<C<getprotobynumber>|/getprotobynumber NUMBER>, 514L<C<getprotoent>|/getprotoent>, L<C<getpwent>|/getpwent>, 515L<C<getpwnam>|/getpwnam NAME>, L<C<getpwuid>|/getpwuid UID>, 516L<C<getservbyport>|/getservbyport PORT,PROTO>, 517L<C<getservent>|/getservent>, 518L<C<getsockopt>|/getsockopt SOCKET,LEVEL,OPTNAME>, 519L<C<glob>|/glob EXPR>, L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, 520L<C<kill>|/kill SIGNAL, LIST>, L<C<link>|/link OLDFILE,NEWFILE>, 521L<C<lstat>|/lstat FILEHANDLE>, L<C<msgctl>|/msgctl ID,CMD,ARG>, 522L<C<msgget>|/msgget KEY,FLAGS>, 523L<C<msgrcv>|/msgrcv ID,VAR,SIZE,TYPE,FLAGS>, 524L<C<msgsnd>|/msgsnd ID,MSG,FLAGS>, L<C<open>|/open FILEHANDLE,MODE,EXPR>, 525L<C<pipe>|/pipe READHANDLE,WRITEHANDLE>, L<C<readlink>|/readlink EXPR>, 526L<C<rename>|/rename OLDNAME,NEWNAME>, 527L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, 528L<C<semctl>|/semctl ID,SEMNUM,CMD,ARG>, 529L<C<semget>|/semget KEY,NSEMS,FLAGS>, L<C<semop>|/semop KEY,OPSTRING>, 530L<C<setgrent>|/setgrent>, L<C<sethostent>|/sethostent STAYOPEN>, 531L<C<setnetent>|/setnetent STAYOPEN>, L<C<setpgrp>|/setpgrp PID,PGRP>, 532L<C<setpriority>|/setpriority WHICH,WHO,PRIORITY>, 533L<C<setprotoent>|/setprotoent STAYOPEN>, L<C<setpwent>|/setpwent>, 534L<C<setservent>|/setservent STAYOPEN>, 535L<C<setsockopt>|/setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL>, 536L<C<shmctl>|/shmctl ID,CMD,ARG>, L<C<shmget>|/shmget KEY,SIZE,FLAGS>, 537L<C<shmread>|/shmread ID,VAR,POS,SIZE>, 538L<C<shmwrite>|/shmwrite ID,STRING,POS,SIZE>, 539L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL>, 540L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, 541L<C<stat>|/stat FILEHANDLE>, L<C<symlink>|/symlink OLDFILE,NEWFILE>, 542L<C<syscall>|/syscall NUMBER, LIST>, 543L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, 544L<C<system>|/system LIST>, L<C<times>|/times>, 545L<C<truncate>|/truncate FILEHANDLE,LENGTH>, L<C<umask>|/umask EXPR>, 546L<C<unlink>|/unlink LIST>, L<C<utime>|/utime LIST>, L<C<wait>|/wait>, 547L<C<waitpid>|/waitpid PID,FLAGS> 548 549For more information about the portability of these functions, see 550L<perlport> and other available platform-specific documentation. 551 552=head2 Alphabetical Listing of Perl Functions 553 554=over 555 556=item -X FILEHANDLE 557X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> 558X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> 559 560=item -X EXPR 561 562=item -X DIRHANDLE 563 564=item -X 565 566=for Pod::Functions a file test (-r, -x, etc) 567 568A file test, where X is one of the letters listed below. This unary 569operator takes one argument, either a filename, a filehandle, or a dirhandle, 570and tests the associated file to see if something is true about it. If the 571argument is omitted, tests L<C<$_>|perlvar/$_>, except for C<-t>, which 572tests STDIN. Unless otherwise documented, it returns C<1> for true and 573C<''> for false. If the file doesn't exist or can't be examined, it 574returns L<C<undef>|/undef EXPR> and sets L<C<$!>|perlvar/$!> (errno). 575With the exception of the C<-l> test they all follow symbolic links 576because they use C<stat()> and not C<lstat()> (so dangling symlinks can't 577be examined and will therefore report failure). 578 579Despite the funny names, precedence is the same as any other named unary 580operator. The operator may be any of: 581 582 -r File is readable by effective uid/gid. 583 -w File is writable by effective uid/gid. 584 -x File is executable by effective uid/gid. 585 -o File is owned by effective uid. 586 587 -R File is readable by real uid/gid. 588 -W File is writable by real uid/gid. 589 -X File is executable by real uid/gid. 590 -O File is owned by real uid. 591 592 -e File exists. 593 -z File has zero size (is empty). 594 -s File has nonzero size (returns size in bytes). 595 596 -f File is a plain file. 597 -d File is a directory. 598 -l File is a symbolic link (false if symlinks aren't 599 supported by the file system). 600 -p File is a named pipe (FIFO), or Filehandle is a pipe. 601 -S File is a socket. 602 -b File is a block special file. 603 -c File is a character special file. 604 -t Filehandle is opened to a tty. 605 606 -u File has setuid bit set. 607 -g File has setgid bit set. 608 -k File has sticky bit set. 609 610 -T File is an ASCII or UTF-8 text file (heuristic guess). 611 -B File is a "binary" file (opposite of -T). 612 613 -M Script start time minus file modification time, in days. 614 -A Same for access time. 615 -C Same for inode change time (Unix, may differ for other 616 platforms) 617 618Example: 619 620 while (<>) { 621 chomp; 622 next unless -f $_; # ignore specials 623 #... 624 } 625 626Note that C<-s/a/b/> does not do a negated substitution. Saying 627C<-exp($foo)> still works as expected, however: only single letters 628following a minus are interpreted as file tests. 629 630These operators are exempt from the "looks like a function rule" described 631above. That is, an opening parenthesis after the operator does not affect 632how much of the following code constitutes the argument. Put the opening 633parentheses before the operator to separate it from code that follows (this 634applies only to operators with higher precedence than unary operators, of 635course): 636 637 -s($file) + 1024 # probably wrong; same as -s($file + 1024) 638 (-s $file) + 1024 # correct 639 640The interpretation of the file permission operators C<-r>, C<-R>, 641C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode 642of the file and the uids and gids of the user. There may be other 643reasons you can't actually read, write, or execute the file: for 644example network filesystem access controls, ACLs (access control lists), 645read-only filesystems, and unrecognized executable formats. Note 646that the use of these six specific operators to verify if some operation 647is possible is usually a mistake, because it may be open to race 648conditions. 649 650Also note that, for the superuser on the local filesystems, the C<-r>, 651C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 652if any execute bit is set in the mode. Scripts run by the superuser 653may thus need to do a L<C<stat>|/stat FILEHANDLE> to determine the 654actual mode of the file, or temporarily set their effective uid to 655something else. 656 657If you are using ACLs, there is a pragma called L<C<filetest>|filetest> 658that may produce more accurate results than the bare 659L<C<stat>|/stat FILEHANDLE> mode bits. 660When under C<use filetest 'access'>, the above-mentioned filetests 661test whether the permission can(not) be granted using the L<access(2)> 662family of system calls. Also note that the C<-x> and C<-X> tests may 663under this pragma return true even if there are no execute permission 664bits set (nor any extra execute permission ACLs). This strangeness is 665due to the underlying system calls' definitions. Note also that, due to 666the implementation of C<use filetest 'access'>, the C<_> special 667filehandle won't cache the results of the file tests when this pragma is 668in effect. Read the documentation for the L<C<filetest>|filetest> 669pragma for more information. 670 671The C<-T> and C<-B> tests work as follows. The first block or so of 672the file is examined to see if it is valid UTF-8 that includes non-ASCII 673characters. If so, it's a C<-T> file. Otherwise, that same portion of 674the file is examined for odd characters such as strange control codes or 675characters with the high bit set. If more than a third of the 676characters are strange, it's a C<-B> file; otherwise it's a C<-T> file. 677Also, any file containing a zero byte in the examined portion is 678considered a binary file. (If executed within the scope of a L<S<use 679locale>|perllocale> which includes C<LC_CTYPE>, odd characters are 680anything that isn't a printable nor space in the current locale.) If 681C<-T> or C<-B> is used on a filehandle, the current IO buffer is 682examined 683rather than the first block. Both C<-T> and C<-B> return true on an empty 684file, or a file at EOF when testing a filehandle. Because you have to 685read a file to do the C<-T> test, on most occasions you want to use a C<-f> 686against the file first, as in C<next unless -f $file && -T $file>. 687 688If any of the file tests (or either the L<C<stat>|/stat FILEHANDLE> or 689L<C<lstat>|/lstat FILEHANDLE> operator) is given the special filehandle 690consisting of a solitary underline, then the stat structure of the 691previous file test (or L<C<stat>|/stat FILEHANDLE> operator) is used, 692saving a system call. (This doesn't work with C<-t>, and you need to 693remember that L<C<lstat>|/lstat FILEHANDLE> and C<-l> leave values in 694the stat structure for the symbolic link, not the real file.) (Also, if 695the stat buffer was filled by an L<C<lstat>|/lstat FILEHANDLE> call, 696C<-T> and C<-B> will reset it with the results of C<stat _>). 697Example: 698 699 print "Can do.\n" if -r $x || -w _ || -x _; 700 701 stat($filename); 702 print "Readable\n" if -r _; 703 print "Writable\n" if -w _; 704 print "Executable\n" if -x _; 705 print "Setuid\n" if -u _; 706 print "Setgid\n" if -g _; 707 print "Sticky\n" if -k _; 708 print "Text\n" if -T _; 709 print "Binary\n" if -B _; 710 711As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file 712test operators, in a way that C<-f -w -x $file> is equivalent to 713C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use 714the return value of C<-f $file> as an argument to another filetest 715operator, no special magic will happen.) 716 717Portability issues: L<perlport/-X>. 718 719To avoid confusing would-be users of your code with mysterious 720syntax errors, put something like this at the top of your script: 721 722 use v5.10; # so filetest ops can stack 723 724=item abs VALUE 725X<abs> X<absolute> 726 727=item abs 728 729=for Pod::Functions absolute value function 730 731Returns the absolute value of its argument. 732If VALUE is omitted, uses L<C<$_>|perlvar/$_>. 733 734=item accept NEWSOCKET,GENERICSOCKET 735X<accept> 736 737=for Pod::Functions accept an incoming socket connect 738 739Accepts an incoming socket connect, just as L<accept(2)> 740does. Returns the packed address if it succeeded, false otherwise. 741See the example in L<perlipc/"Sockets: Client/Server Communication">. 742 743On systems that support a close-on-exec flag on files, the flag will 744be set for the newly opened file descriptor, as determined by the 745value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>. 746 747=item alarm SECONDS 748X<alarm> 749X<SIGALRM> 750X<timer> 751 752=item alarm 753 754=for Pod::Functions schedule a SIGALRM 755 756Arranges to have a SIGALRM delivered to this process after the 757specified number of wallclock seconds has elapsed. If SECONDS is not 758specified, the value stored in L<C<$_>|perlvar/$_> is used. (On some 759machines, unfortunately, the elapsed time may be up to one second less 760or more than you specified because of how seconds are counted, and 761process scheduling may delay the delivery of the signal even further.) 762 763Only one timer may be counting at once. Each call disables the 764previous timer, and an argument of C<0> may be supplied to cancel the 765previous timer without starting a new one. The returned value is the 766amount of time remaining on the previous timer. 767 768For delays of finer granularity than one second, the L<Time::HiRes> module 769(from CPAN, and starting from Perl 5.8 part of the standard 770distribution) provides 771L<C<ualarm>|Time::HiRes/ualarm ( $useconds [, $interval_useconds ] )>. 772You may also use Perl's four-argument version of 773L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the first three 774arguments undefined, or you might be able to use the 775L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)> 776if your system supports it. See L<perlfaq8> for details. 777 778It is usually a mistake to intermix L<C<alarm>|/alarm SECONDS> and 779L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> may be 780internally implemented on your system with L<C<alarm>|/alarm SECONDS>. 781 782If you want to use L<C<alarm>|/alarm SECONDS> to time out a system call 783you need to use an L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> pair. You 784can't rely on the alarm causing the system call to fail with 785L<C<$!>|perlvar/$!> set to C<EINTR> because Perl sets up signal handlers 786to restart system calls on some systems. Using 787L<C<eval>|/eval EXPR>/L<C<die>|/die LIST> always works, modulo the 788caveats given in L<perlipc/"Signals">. 789 790 eval { 791 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required 792 alarm $timeout; 793 my $nread = sysread $socket, $buffer, $size; 794 alarm 0; 795 }; 796 if ($@) { 797 die unless $@ eq "alarm\n"; # propagate unexpected errors 798 # timed out 799 } 800 else { 801 # didn't 802 } 803 804For more information see L<perlipc>. 805 806Portability issues: L<perlport/alarm>. 807 808=item atan2 Y,X 809X<atan2> X<arctangent> X<tan> X<tangent> 810 811=for Pod::Functions arctangent of Y/X in the range -PI to PI 812 813Returns the arctangent of Y/X in the range -PI to PI. 814 815For the tangent operation, you may use the 816L<C<Math::Trig::tan>|Math::Trig/B<tan>> function, or use the familiar 817relation: 818 819 sub tan { sin($_[0]) / cos($_[0]) } 820 821The return value for C<atan2(0,0)> is implementation-defined; consult 822your L<atan2(3)> manpage for more information. 823 824Portability issues: L<perlport/atan2>. 825 826=item bind SOCKET,NAME 827X<bind> 828 829=for Pod::Functions binds an address to a socket 830 831Binds a network address to a socket, just as L<bind(2)> 832does. Returns true if it succeeded, false otherwise. NAME should be a 833packed address of the appropriate type for the socket. See the examples in 834L<perlipc/"Sockets: Client/Server Communication">. 835 836=item binmode FILEHANDLE, LAYER 837X<binmode> X<binary> X<text> X<DOS> X<Windows> 838 839=item binmode FILEHANDLE 840 841=for Pod::Functions prepare binary files for I/O 842 843Arranges for FILEHANDLE to be read or written in "binary" or "text" 844mode on systems where the run-time libraries distinguish between 845binary and text files. If FILEHANDLE is an expression, the value is 846taken as the name of the filehandle. Returns true on success, 847otherwise it returns L<C<undef>|/undef EXPR> and sets 848L<C<$!>|perlvar/$!> (errno). 849 850On some systems (in general, DOS- and Windows-based systems) 851L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not 852working with a text file. For the sake of portability it is a good idea 853always to use it when appropriate, and never to use it when it isn't 854appropriate. Also, people can set their I/O to be by default 855UTF8-encoded Unicode, not bytes. 856 857In other words: regardless of platform, use 858L<C<binmode>|/binmode FILEHANDLE, LAYER> on binary data, like images, 859for example. 860 861If LAYER is present it is a single string, but may contain multiple 862directives. The directives alter the behaviour of the filehandle. 863When LAYER is present, using binmode on a text file makes sense. 864 865If LAYER is omitted or specified as C<:raw> the filehandle is made 866suitable for passing binary data. This includes turning off possible CRLF 867translation and marking it as bytes (as opposed to Unicode characters). 868Note that, despite what may be implied in I<"Programming Perl"> (the 869Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>. 870Other layers that would affect the binary nature of the stream are 871I<also> disabled. See L<PerlIO>, and the discussion about the PERLIO 872environment variable in L<perlrun|perlrun/PERLIO>. 873 874The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the 875form C<:...>, are called I/O I<layers>. The L<open> pragma can be used to 876establish default I/O layers. 877 878I<The LAYER parameter of the L<C<binmode>|/binmode FILEHANDLE, LAYER> 879function is described as "DISCIPLINE" in "Programming Perl, 3rd 880Edition". However, since the publishing of this book, by many known as 881"Camel III", the consensus of the naming of this functionality has moved 882from "discipline" to "layer". All documentation of this version of Perl 883therefore refers to "layers" rather than to "disciplines". Now back to 884the regularly scheduled documentation...> 885 886To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>. 887C<:utf8> just marks the data as UTF-8 without further checking, 888while C<:encoding(UTF-8)> checks the data for actually being valid 889UTF-8. More details can be found in L<PerlIO::encoding>. 890 891In general, L<C<binmode>|/binmode FILEHANDLE, LAYER> should be called 892after L<C<open>|/open FILEHANDLE,MODE,EXPR> but before any I/O is done on the 893filehandle. Calling L<C<binmode>|/binmode FILEHANDLE, LAYER> normally 894flushes any pending buffered output data (and perhaps pending input 895data) on the handle. An exception to this is the C<:encoding> layer 896that changes the default character encoding of the handle. 897The C<:encoding> layer sometimes needs to be called in 898mid-stream, and it doesn't flush the stream. C<:encoding> 899also implicitly pushes on top of itself the C<:utf8> layer because 900internally Perl operates on UTF8-encoded Unicode characters. 901 902The operating system, device drivers, C libraries, and Perl run-time 903system all conspire to let the programmer treat a single 904character (C<\n>) as the line terminator, irrespective of external 905representation. On many operating systems, the native text file 906representation matches the internal representation, but on some 907platforms the external representation of C<\n> is made up of more than 908one character. 909 910All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use 911a single character to end each line in the external representation of text 912(even though that single character is CARRIAGE RETURN on old, pre-Darwin 913flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other 914systems like OS/2, DOS, and the various flavors of MS-Windows, your program 915sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the 916two characters C<\cM\cJ>. That means that if you don't use 917L<C<binmode>|/binmode FILEHANDLE, LAYER> on these systems, C<\cM\cJ> 918sequences on disk will be converted to C<\n> on input, and any C<\n> in 919your program will be converted back to C<\cM\cJ> on output. This is 920what you want for text files, but it can be disastrous for binary files. 921 922Another consequence of using L<C<binmode>|/binmode FILEHANDLE, LAYER> 923(on some systems) is that special end-of-file markers will be seen as 924part of the data stream. For systems from the Microsoft family this 925means that, if your binary data contain C<\cZ>, the I/O subsystem will 926regard it as the end of the file, unless you use 927L<C<binmode>|/binmode FILEHANDLE, LAYER>. 928 929L<C<binmode>|/binmode FILEHANDLE, LAYER> is important not only for 930L<C<readline>|/readline EXPR> and L<C<print>|/print FILEHANDLE LIST> 931operations, but also when using 932L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>, 933L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 934L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>, 935L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> and 936L<C<tell>|/tell FILEHANDLE> (see L<perlport> for more details). See the 937L<C<$E<sol>>|perlvar/$E<sol>> and L<C<$\>|perlvar/$\> variables in 938L<perlvar> for how to manually set your input and output 939line-termination sequences. 940 941Portability issues: L<perlport/binmode>. 942 943=item bless REF,CLASSNAME 944X<bless> 945 946=item bless REF 947 948=for Pod::Functions create an object 949 950C<bless> tells Perl to mark the item referred to by C<REF> as an 951object in a package. The two-argument version of C<bless> is 952always preferable unless there is a specific reason to I<not> 953use it. 954 955=over 956 957=item * Bless the referred-to item into a specific package 958(recommended form): 959 960 bless $ref, $package; 961 962The two-argument form adds the object to the package specified 963as the second argument. 964 965=item * Bless the referred-to item into package C<main>: 966 967 bless $ref, ""; 968 969If the second argument is an empty string, C<bless> adds the 970object to package C<main>. 971 972=item * Bless the referred-to item into the current package (not 973inheritable): 974 975 bless $ref; 976 977If C<bless> is used without its second argument, the object is 978created in the current package. The second argument should 979always be supplied if a derived class might inherit a method 980executing C<bless>. Because it is a potential source of bugs, 981one-argument C<bless> is discouraged. 982 983=back 984 985See L<perlobj> for more about the blessing (and blessings) of 986objects. 987 988L<C<bless>|/bless REF,CLASSNAME> returns its first argument, the 989supplied reference, as the value of the function; since C<bless> 990is commonly the last thing executed in constructors, this means 991that the reference to the object is returned as the 992constructor's value and allows the caller to immediately use 993this returned object in method calls. 994 995C<CLASSNAME> should always be a mixed-case name, as 996all-uppercase and all-lowercase names are meant to be used only 997for Perl builtin types and pragmas, respectively. Avoid creating 998all-uppercase or all-lowercase package names to prevent 999confusion. 1000 1001Also avoid C<bless>ing things into the class name C<0>; this 1002will cause code which (erroneously) checks the result of 1003L<C<ref>|/ref EXPR> to see if a reference is C<bless>ed to fail, 1004as "0", a falsy value, is returned. 1005 1006See L<perlmod/"Perl Modules"> for more details. 1007 1008=item break 1009 1010=for Pod::Functions +switch break out of a C<given> block 1011 1012Break out of a C<given> block. 1013 1014L<C<break>|/break> is available only if the 1015L<C<"switch"> feature|feature/The 'switch' feature> is enabled or if it 1016is prefixed with C<CORE::>. The 1017L<C<"switch"> feature|feature/The 'switch' feature> is enabled 1018automatically with a C<use v5.10> (or higher) declaration in the current 1019scope. 1020 1021=item caller EXPR 1022X<caller> X<call stack> X<stack> X<stack trace> 1023 1024=item caller 1025 1026=for Pod::Functions get context of the current subroutine call 1027 1028Returns the context of the current pure perl subroutine call. In scalar 1029context, returns the caller's package name if there I<is> a caller (that is, if 1030we're in a subroutine or L<C<eval>|/eval EXPR> or 1031L<C<require>|/require VERSION>) and the undefined value otherwise. 1032C<caller> never returns XS subs and they are skipped. The next pure perl 1033sub will appear instead of the XS sub in caller's return values. In 1034list context, caller returns 1035 1036 # 0 1 2 1037 my ($package, $filename, $line) = caller; 1038 1039Like L<C<__FILE__>|/__FILE__> and L<C<__LINE__>|/__LINE__>, the filename and 1040line number returned here may be altered by the mechanism described at 1041L<perlsyn/"Plain Old Comments (Not!)">. 1042 1043With EXPR, it returns some extra information that the debugger uses to 1044print a stack trace. The value of EXPR indicates how many call frames 1045to go back before the current one. 1046 1047 # 0 1 2 3 4 1048 my ($package, $filename, $line, $subroutine, $hasargs, 1049 1050 # 5 6 7 8 9 10 1051 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) 1052 = caller($i); 1053 1054Here, $subroutine is the function that the caller called (rather than the 1055function containing the caller). Note that $subroutine may be C<(eval)> if 1056the frame is not a subroutine call, but an L<C<eval>|/eval EXPR>. In 1057such a case additional elements $evaltext and C<$is_require> are set: 1058C<$is_require> is true if the frame is created by a 1059L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST> 1060statement, $evaltext contains the text of the C<eval EXPR> statement. 1061In particular, for an C<eval BLOCK> statement, $subroutine is C<(eval)>, 1062but $evaltext is undefined. (Note also that each 1063L<C<use>|/use Module VERSION LIST> statement creates a 1064L<C<require>|/require VERSION> frame inside an C<eval EXPR> frame.) 1065$subroutine may also be C<(unknown)> if this particular subroutine 1066happens to have been deleted from the symbol table. C<$hasargs> is true 1067if a new instance of L<C<@_>|perlvar/@_> was set up for the frame. 1068C<$hints> and C<$bitmask> contain pragmatic hints that the caller was 1069compiled with. C<$hints> corresponds to L<C<$^H>|perlvar/$^H>, and 1070C<$bitmask> corresponds to 1071L<C<${^WARNING_BITS}>|perlvar/${^WARNING_BITS}>. The C<$hints> and 1072C<$bitmask> values are subject to change between versions of Perl, and 1073are not meant for external use. 1074 1075C<$hinthash> is a reference to a hash containing the value of 1076L<C<%^H>|perlvar/%^H> when the caller was compiled, or 1077L<C<undef>|/undef EXPR> if L<C<%^H>|perlvar/%^H> was empty. Do not 1078modify the values of this hash, as they are the actual values stored in 1079the optree. 1080 1081Note that the only types of call frames that are visible are subroutine 1082calls and C<eval>. Other forms of context, such as C<while> or C<foreach> 1083loops or C<try> blocks are not considered interesting to C<caller>, as they 1084do not alter the behaviour of the C<return> expression. 1085 1086Furthermore, when called from within the DB package in 1087list context, and with an argument, caller returns more 1088detailed information: it sets the list variable C<@DB::args> to be the 1089arguments with which the subroutine was invoked. 1090 1091Be aware that the optimizer might have optimized call frames away before 1092L<C<caller>|/caller EXPR> had a chance to get the information. That 1093means that C<caller(N)> might not return information about the call 1094frame you expect it to, for C<< N > 1 >>. In particular, C<@DB::args> 1095might have information from the previous time L<C<caller>|/caller EXPR> 1096was called. 1097 1098Be aware that setting C<@DB::args> is I<best effort>, intended for 1099debugging or generating backtraces, and should not be relied upon. In 1100particular, as L<C<@_>|perlvar/@_> contains aliases to the caller's 1101arguments, Perl does not take a copy of L<C<@_>|perlvar/@_>, so 1102C<@DB::args> will contain modifications the subroutine makes to 1103L<C<@_>|perlvar/@_> or its contents, not the original values at call 1104time. C<@DB::args>, like L<C<@_>|perlvar/@_>, does not hold explicit 1105references to its elements, so under certain cases its elements may have 1106become freed and reallocated for other variables or temporary values. 1107Finally, a side effect of the current implementation is that the effects 1108of C<shift @_> can I<normally> be undone (but not C<pop @_> or other 1109splicing, I<and> not if a reference to L<C<@_>|perlvar/@_> has been 1110taken, I<and> subject to the caveat about reallocated elements), so 1111C<@DB::args> is actually a hybrid of the current state and initial state 1112of L<C<@_>|perlvar/@_>. Buyer beware. 1113 1114=item chdir EXPR 1115X<chdir> 1116X<cd> 1117X<directory, change> 1118 1119=item chdir FILEHANDLE 1120 1121=item chdir DIRHANDLE 1122 1123=item chdir 1124 1125=for Pod::Functions change your current working directory 1126 1127Changes the working directory to EXPR, if possible. If EXPR is omitted, 1128changes to the directory specified by C<$ENV{HOME}>, if set; if not, 1129changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the 1130variable C<$ENV{'SYS$LOGIN'}> is also checked, and used if it is set.) If 1131neither is set, L<C<chdir>|/chdir EXPR> does nothing and fails. It 1132returns true on success, false otherwise. See the example under 1133L<C<die>|/die LIST>. 1134 1135On systems that support L<fchdir(2)>, you may pass a filehandle or 1136directory handle as the argument. On systems that don't support L<fchdir(2)>, 1137passing handles raises an exception. 1138 1139=item chmod LIST 1140X<chmod> X<permission> X<mode> 1141 1142=for Pod::Functions changes the permissions on a list of files 1143 1144Changes the permissions of a list of files. The first element of the 1145list must be the numeric mode, which should probably be an octal 1146number, and which definitely should I<not> be a string of octal digits: 1147C<0644> is okay, but C<"0644"> is not. Returns the number of files 1148successfully changed. See also L<C<oct>|/oct EXPR> if all you have is a 1149string. 1150 1151 my $cnt = chmod 0755, "foo", "bar"; 1152 chmod 0755, @executables; 1153 my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to 1154 # --w----r-T 1155 my $mode = "0644"; chmod oct($mode), "foo"; # this is better 1156 my $mode = 0644; chmod $mode, "foo"; # this is best 1157 1158On systems that support L<fchmod(2)>, you may pass filehandles among the 1159files. On systems that don't support L<fchmod(2)>, passing filehandles raises 1160an exception. Filehandles must be passed as globs or glob references to be 1161recognized; barewords are considered filenames. 1162 1163 open(my $fh, "<", "foo"); 1164 my $perm = (stat $fh)[2] & 07777; 1165 chmod($perm | 0600, $fh); 1166 1167You can also import the symbolic C<S_I*> constants from the 1168L<C<Fcntl>|Fcntl> module: 1169 1170 use Fcntl qw( :mode ); 1171 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; 1172 # Identical to the chmod 0755 of the example above. 1173 1174Portability issues: L<perlport/chmod>. 1175 1176=item chomp VARIABLE 1177X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> 1178 1179=item chomp( LIST ) 1180 1181=item chomp 1182 1183=for Pod::Functions remove a trailing record separator from a string 1184 1185This safer version of L<C<chop>|/chop VARIABLE> removes any trailing 1186string that corresponds to the current value of 1187L<C<$E<sol>>|perlvar/$E<sol>> (also known as C<$INPUT_RECORD_SEPARATOR> 1188in the L<C<English>|English> module). It returns the total 1189number of characters removed from all its arguments. It's often used to 1190remove the newline from the end of an input record when you're worried 1191that the final record may be missing its newline. When in paragraph 1192mode (C<$/ = ''>), it removes all trailing newlines from the string. 1193When in slurp mode (C<$/ = undef>) or fixed-length record mode 1194(L<C<$E<sol>>|perlvar/$E<sol>> is a reference to an integer or the like; 1195see L<perlvar>), L<C<chomp>|/chomp VARIABLE> won't remove anything. 1196If VARIABLE is omitted, it chomps L<C<$_>|perlvar/$_>. Example: 1197 1198 while (<>) { 1199 chomp; # avoid \n on last field 1200 my @array = split(/:/); 1201 # ... 1202 } 1203 1204If VARIABLE is a hash, it chomps the hash's values, but not its keys, 1205resetting the L<C<each>|/each HASH> iterator in the process. 1206 1207You can actually chomp anything that's an lvalue, including an assignment: 1208 1209 chomp(my $cwd = `pwd`); 1210 chomp(my $answer = <STDIN>); 1211 1212If you chomp a list, each element is chomped, and the total number of 1213characters removed is returned. 1214 1215Note that parentheses are necessary when you're chomping anything 1216that is not a simple variable. This is because C<chomp $cwd = `pwd`;> 1217is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as 1218C<chomp( $cwd = `pwd` )> which you might expect. Similarly, 1219C<chomp $x, $y> is interpreted as C<chomp($x), $y> rather than 1220as C<chomp($x, $y)>. 1221 1222=item chop VARIABLE 1223X<chop> 1224 1225=item chop( LIST ) 1226 1227=item chop 1228 1229=for Pod::Functions remove the last character from a string 1230 1231Chops off the last character of a string and returns the character 1232chopped. It is much more efficient than C<s/.$//s> because it neither 1233scans nor copies the string. If VARIABLE is omitted, chops 1234L<C<$_>|perlvar/$_>. 1235If VARIABLE is a hash, it chops the hash's values, but not its keys, 1236resetting the L<C<each>|/each HASH> iterator in the process. 1237 1238You can actually chop anything that's an lvalue, including an assignment. 1239 1240If you chop a list, each element is chopped. Only the value of the 1241last L<C<chop>|/chop VARIABLE> is returned. 1242 1243Note that L<C<chop>|/chop VARIABLE> returns the last character. To 1244return all but the last character, use C<substr($string, 0, -1)>. 1245 1246See also L<C<chomp>|/chomp VARIABLE>. 1247 1248=item chown LIST 1249X<chown> X<owner> X<user> X<group> 1250 1251=for Pod::Functions change the ownership on a list of files 1252 1253Changes the owner (and group) of a list of files. The first two 1254elements of the list must be the I<numeric> uid and gid, in that 1255order. A value of -1 in either position is interpreted by most 1256systems to leave that value unchanged. Returns the number of files 1257successfully changed. 1258 1259 my $cnt = chown $uid, $gid, 'foo', 'bar'; 1260 chown $uid, $gid, @filenames; 1261 1262On systems that support L<fchown(2)>, you may pass filehandles among the 1263files. On systems that don't support L<fchown(2)>, passing filehandles raises 1264an exception. Filehandles must be passed as globs or glob references to be 1265recognized; barewords are considered filenames. 1266 1267Here's an example that looks up nonnumeric uids in the passwd file: 1268 1269 print "User: "; 1270 chomp(my $user = <STDIN>); 1271 print "Files: "; 1272 chomp(my $pattern = <STDIN>); 1273 1274 my ($login,$pass,$uid,$gid) = getpwnam($user) 1275 or die "$user not in passwd file"; 1276 1277 my @ary = glob($pattern); # expand filenames 1278 chown $uid, $gid, @ary; 1279 1280On most systems, you are not allowed to change the ownership of the 1281file unless you're the superuser, although you should be able to change 1282the group to any of your secondary groups. On insecure systems, these 1283restrictions may be relaxed, but this is not a portable assumption. 1284On POSIX systems, you can detect this condition this way: 1285 1286 use POSIX qw(pathconf _PC_CHOWN_RESTRICTED); 1287 my $can_chown_giveaway = 1288 ! pathconf($path_of_interest, _PC_CHOWN_RESTRICTED); 1289 1290Portability issues: L<perlport/chown>. 1291 1292=item chr NUMBER 1293X<chr> X<character> X<ASCII> X<Unicode> 1294 1295=item chr 1296 1297=for Pod::Functions get character this number represents 1298 1299Returns the character represented by that NUMBER in the character set. 1300For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and 1301C<chr(0x263a)> is a Unicode smiley face. 1302 1303Negative values give the Unicode replacement character (C<chr(0xfffd)>), 1304except under the L<bytes> pragma, where the low eight bits of the value 1305(truncated to an integer) are used. 1306 1307If NUMBER is omitted, uses L<C<$_>|perlvar/$_>. 1308 1309For the reverse, use L<C<ord>|/ord EXPR>. 1310 1311Note that characters from 128 to 255 (inclusive) are by default 1312internally not encoded as UTF-8 for backward compatibility reasons. 1313 1314See L<perlunicode> for more about Unicode. 1315 1316=item chroot FILENAME 1317X<chroot> X<root> 1318 1319=item chroot 1320 1321=for Pod::Functions make directory new root for path lookups 1322 1323This function works like the system call by the same name: it makes the 1324named directory the new root directory for all further pathnames that 1325begin with a C</> by your process and all its children. (It doesn't 1326change your current working directory, which is unaffected.) For security 1327reasons, this call is restricted to the superuser. If FILENAME is 1328omitted, does a L<C<chroot>|/chroot FILENAME> to L<C<$_>|perlvar/$_>. 1329 1330B<NOTE:> It is mandatory for security to C<chdir("/")> 1331(L<C<chdir>|/chdir EXPR> to the root directory) immediately after a 1332L<C<chroot>|/chroot FILENAME>, otherwise the current working directory 1333may be outside of the new root. 1334 1335Portability issues: L<perlport/chroot>. 1336 1337=item class NAMESPACE 1338 1339=item class NAMESPACE VERSION 1340 1341=item class NAMESPACE BLOCK 1342 1343=item class NAMESPACE VERSION BLOCK 1344 1345=for Pod::Functions declare a separate global namespace that is an object class 1346 1347Declares the BLOCK or the rest of the compilation unit as being in the given 1348namespace, which implements an object class. This behaves similarly to 1349L<C<package>|/package NAMESPACE>, except that the newly-created package behaves 1350as a class. 1351 1352=item close FILEHANDLE 1353X<close> 1354 1355=item close 1356 1357=for Pod::Functions close file (or pipe or socket) handle 1358 1359Closes the file or pipe associated with the filehandle, flushes the IO 1360buffers, and closes the system file descriptor. Returns true if those 1361operations succeed, and if no error was reported by any PerlIO layer, 1362and there was no existing error on the filehandle. 1363 1364If there was an existing error on the filehandle, close will return 1365false and L<C<$!>|perlvar/$!> will be set to the error from the 1366failing operation, so you can safely use its value when reporting the 1367error. 1368 1369Closes the currently selected filehandle if the argument is 1370omitted. 1371 1372You don't have to close FILEHANDLE if you are immediately going to do 1373another L<C<open>|/open FILEHANDLE,MODE,EXPR> on it, because 1374L<C<open>|/open FILEHANDLE,MODE,EXPR> closes it for you. (See 1375L<C<open>|/open FILEHANDLE,MODE,EXPR>.) However, an explicit 1376L<C<close>|/close FILEHANDLE> on an input file resets the line counter 1377(L<C<$.>|perlvar/$.>), while the implicit close done by 1378L<C<open>|/open FILEHANDLE,MODE,EXPR> does not. 1379 1380If the filehandle came from a piped open, L<C<close>|/close FILEHANDLE> 1381returns false if one of the other syscalls involved fails or if its 1382program exits with non-zero status. If the only problem was that the 1383program exited non-zero, L<C<$!>|perlvar/$!> will be set to C<0>. 1384Closing a pipe also waits for the process executing on the pipe to 1385exit--in case you wish to look at the output of the pipe afterwards--and 1386implicitly puts the exit status value of that command into 1387L<C<$?>|perlvar/$?> and 1388L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>. 1389 1390If there are multiple threads running, L<C<close>|/close FILEHANDLE> on 1391a filehandle from a piped open returns true without waiting for the 1392child process to terminate, if the filehandle is still open in another 1393thread. 1394 1395Closing the read end of a pipe before the process writing to it at the 1396other end is done writing results in the writer receiving a SIGPIPE. If 1397the other end can't handle that, be sure to read all the data before 1398closing the pipe. 1399 1400Example: 1401 1402 open(OUTPUT, '|sort >foo') # pipe to sort 1403 or die "Can't start sort: $!"; 1404 #... # print stuff to output 1405 close OUTPUT # wait for sort to finish 1406 or warn $! ? "Error closing sort pipe: $!" 1407 : "Exit status $? from sort"; 1408 open(INPUT, 'foo') # get sort's results 1409 or die "Can't open 'foo' for input: $!"; 1410 1411FILEHANDLE may be an expression whose value can be used as an indirect 1412filehandle, usually the real filehandle name or an autovivified handle. 1413 1414If an error occurs when perl implicitly closes a handle, perl will 1415produce a L<warning|perldiag/"Warning: unable to close filehandle %s 1416properly: %s">. Explicitly calling close on the handle prevents that 1417warning. 1418 1419=item closedir DIRHANDLE 1420X<closedir> 1421 1422=for Pod::Functions close directory handle 1423 1424Closes a directory opened by L<C<opendir>|/opendir DIRHANDLE,EXPR> and 1425returns the success of that system call. 1426 1427=item connect SOCKET,NAME 1428X<connect> 1429 1430=for Pod::Functions connect to a remote socket 1431 1432Attempts to connect to a remote socket, just like L<connect(2)>. 1433Returns true if it succeeded, false otherwise. NAME should be a 1434packed address of the appropriate type for the socket. See the examples in 1435L<perlipc/"Sockets: Client/Server Communication">. 1436 1437=item continue BLOCK 1438X<continue> 1439 1440=item continue 1441 1442=for Pod::Functions optional trailing block in a while or foreach 1443 1444When followed by a BLOCK, L<C<continue>|/continue BLOCK> is actually a 1445flow control statement rather than a function. If there is a 1446L<C<continue>|/continue BLOCK> BLOCK attached to a BLOCK (typically in a 1447C<while> or C<foreach>), it is always executed just before the 1448conditional is about to be evaluated again, just like the third part of 1449a C<for> loop in C. Thus it can be used to increment a loop variable, 1450even when the loop has been continued via the L<C<next>|/next LABEL> 1451statement (which is similar to the C L<C<continue>|/continue BLOCK> 1452statement). 1453 1454L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, or 1455L<C<redo>|/redo LABEL> may appear within a 1456L<C<continue>|/continue BLOCK> block; L<C<last>|/last LABEL> and 1457L<C<redo>|/redo LABEL> behave as if they had been executed within the 1458main block. So will L<C<next>|/next LABEL>, but since it will execute a 1459L<C<continue>|/continue BLOCK> block, it may be more entertaining. 1460 1461 while (EXPR) { 1462 ### redo always comes here 1463 do_something; 1464 } continue { 1465 ### next always comes here 1466 do_something_else; 1467 # then back to the top to re-check EXPR 1468 } 1469 ### last always comes here 1470 1471Omitting the L<C<continue>|/continue BLOCK> section is equivalent to 1472using an empty one, logically enough, so L<C<next>|/next LABEL> goes 1473directly back to check the condition at the top of the loop. 1474 1475When there is no BLOCK, L<C<continue>|/continue BLOCK> is a function 1476that falls through the current C<when> or C<default> block instead of 1477iterating a dynamically enclosing C<foreach> or exiting a lexically 1478enclosing C<given>. In Perl 5.14 and earlier, this form of 1479L<C<continue>|/continue BLOCK> was only available when the 1480L<C<"switch"> feature|feature/The 'switch' feature> was enabled. See 1481L<feature> and L<perlsyn/"Switch Statements"> for more information. 1482 1483=item cos EXPR 1484X<cos> X<cosine> X<acos> X<arccosine> 1485 1486=item cos 1487 1488=for Pod::Functions cosine function 1489 1490Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, 1491takes the cosine of L<C<$_>|perlvar/$_>. 1492 1493For the inverse cosine operation, you may use the 1494L<C<Math::Trig::acos>|Math::Trig> function, or use this relation: 1495 1496 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } 1497 1498=item crypt PLAINTEXT,SALT 1499X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> 1500X<decrypt> X<cryptography> X<passwd> X<encrypt> 1501 1502=for Pod::Functions one-way passwd-style encryption 1503 1504Creates a digest string exactly like the L<crypt(3)> function in the C 1505library (assuming that you actually have a version there that has not 1506been extirpated as a potential munition). 1507 1508L<C<crypt>|/crypt PLAINTEXT,SALT> is a one-way hash function. The 1509PLAINTEXT and SALT are turned 1510into a short string, called a digest, which is returned. The same 1511PLAINTEXT and SALT will always return the same string, but there is no 1512(known) way to get the original PLAINTEXT from the hash. Small 1513changes in the PLAINTEXT or SALT will result in large changes in the 1514digest. 1515 1516There is no decrypt function. This function isn't all that useful for 1517cryptography (for that, look for F<Crypt> modules on your nearby CPAN 1518mirror) and the name "crypt" is a bit of a misnomer. Instead it is 1519primarily used to check if two pieces of text are the same without 1520having to transmit or store the text itself. An example is checking 1521if a correct password is given. The digest of the password is stored, 1522not the password itself. The user types in a password that is 1523L<C<crypt>|/crypt PLAINTEXT,SALT>'d with the same salt as the stored 1524digest. If the two digests match, the password is correct. 1525 1526When verifying an existing digest string you should use the digest as 1527the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used 1528to create the digest is visible as part of the digest. This ensures 1529L<C<crypt>|/crypt PLAINTEXT,SALT> will hash the new string with the same 1530salt as the digest. This allows your code to work with the standard 1531L<C<crypt>|/crypt PLAINTEXT,SALT> and with more exotic implementations. 1532In other words, assume nothing about the returned string itself nor 1533about how many bytes of SALT may matter. 1534 1535Traditionally the result is a string of 13 bytes: two first bytes of 1536the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only 1537the first eight bytes of PLAINTEXT mattered. But alternative 1538hashing schemes (like MD5), higher level security schemes (like C2), 1539and implementations on non-Unix platforms may produce different 1540strings. 1541 1542When choosing a new salt create a random two character string whose 1543characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', 1544'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of 1545characters is just a recommendation; the characters allowed in 1546the salt depend solely on your system's crypt library, and Perl can't 1547restrict what salts L<C<crypt>|/crypt PLAINTEXT,SALT> accepts. 1548 1549Here's an example that makes sure that whoever runs this program knows 1550their password: 1551 1552 my $pwd = (getpwuid($<))[1]; 1553 1554 system "stty -echo"; 1555 print "Password: "; 1556 chomp(my $word = <STDIN>); 1557 print "\n"; 1558 system "stty echo"; 1559 1560 if (crypt($word, $pwd) ne $pwd) { 1561 die "Sorry...\n"; 1562 } else { 1563 print "ok\n"; 1564 } 1565 1566Of course, typing in your own password to whoever asks you 1567for it is unwise. 1568 1569The L<C<crypt>|/crypt PLAINTEXT,SALT> function is unsuitable for hashing 1570large quantities of data, not least of all because you can't get the 1571information back. Look at the L<Digest> module for more robust 1572algorithms. 1573 1574If using L<C<crypt>|/crypt PLAINTEXT,SALT> on a Unicode string (which 1575I<potentially> has characters with codepoints above 255), Perl tries to 1576make sense of the situation by trying to downgrade (a copy of) the 1577string back to an eight-bit byte string before calling 1578L<C<crypt>|/crypt PLAINTEXT,SALT> (on that copy). If that works, good. 1579If not, L<C<crypt>|/crypt PLAINTEXT,SALT> dies with 1580L<C<Wide character in crypt>|perldiag/Wide character in %s>. 1581 1582Portability issues: L<perlport/crypt>. 1583 1584=item dbmclose HASH 1585X<dbmclose> 1586 1587=for Pod::Functions breaks binding on a tied dbm file 1588 1589[This function has been largely superseded by the 1590L<C<untie>|/untie VARIABLE> function.] 1591 1592Breaks the binding between a DBM file and a hash. 1593 1594Portability issues: L<perlport/dbmclose>. 1595 1596=item dbmopen HASH,DBNAME,MASK 1597X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> 1598 1599=for Pod::Functions create binding on a tied dbm file 1600 1601[This function has been largely superseded by the 1602L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function.] 1603 1604This binds a L<dbm(3)>, L<ndbm(3)>, L<sdbm(3)>, L<gdbm(3)>, or Berkeley 1605DB file to a hash. HASH is the name of the hash. (Unlike normal 1606L<C<open>|/open FILEHANDLE,MODE,EXPR>, the first argument is I<not> a 1607filehandle, even though it looks like one). DBNAME is the name of the 1608database (without the F<.dir> or F<.pag> extension if any). If the 1609database does not exist, it is created with protection specified by MASK 1610(as modified by the L<C<umask>|/umask EXPR>). To prevent creation of 1611the database if it doesn't exist, you may specify a MASK of 0, and the 1612function will return a false value if it can't find an existing 1613database. If your system supports only the older DBM functions, you may 1614make only one L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> call in your 1615program. In older versions of Perl, if your system had neither DBM nor 1616ndbm, calling L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK> produced a fatal 1617error; it now falls back to L<sdbm(3)>. 1618 1619If you don't have write access to the DBM file, you can only read hash 1620variables, not set them. If you want to test whether you can write, 1621either use file tests or try setting a dummy hash entry inside an 1622L<C<eval>|/eval EXPR> to trap the error. 1623 1624Note that functions such as L<C<keys>|/keys HASH> and 1625L<C<values>|/values HASH> may return huge lists when used on large DBM 1626files. You may prefer to use the L<C<each>|/each HASH> function to 1627iterate over large DBM files. Example: 1628 1629 # print out history file offsets 1630 dbmopen(%HIST,'/usr/lib/news/history',0666); 1631 while (($key,$val) = each %HIST) { 1632 print $key, ' = ', unpack('L',$val), "\n"; 1633 } 1634 dbmclose(%HIST); 1635 1636See also L<AnyDBM_File> for a more general description of the pros and 1637cons of the various dbm approaches, as well as L<DB_File> for a particularly 1638rich implementation. 1639 1640You can control which DBM library you use by loading that library 1641before you call L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>: 1642 1643 use DB_File; 1644 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") 1645 or die "Can't open netscape history file: $!"; 1646 1647Portability issues: L<perlport/dbmopen>. 1648 1649=item defined EXPR 1650X<defined> X<undef> X<undefined> 1651 1652=item defined 1653 1654=for Pod::Functions test whether a value, variable, or function is defined 1655 1656Returns a Boolean value telling whether EXPR has a value other than the 1657undefined value L<C<undef>|/undef EXPR>. If EXPR is not present, 1658L<C<$_>|perlvar/$_> is checked. 1659 1660Many operations return L<C<undef>|/undef EXPR> to indicate failure, end 1661of file, system error, uninitialized variable, and other exceptional 1662conditions. This function allows you to distinguish 1663L<C<undef>|/undef EXPR> from other values. (A simple Boolean test will 1664not distinguish among L<C<undef>|/undef EXPR>, zero, the empty string, 1665and C<"0">, which are all equally false.) Note that since 1666L<C<undef>|/undef EXPR> is a valid scalar, its presence doesn't 1667I<necessarily> indicate an exceptional condition: L<C<pop>|/pop ARRAY> 1668returns L<C<undef>|/undef EXPR> when its argument is an empty array, 1669I<or> when the element to return happens to be L<C<undef>|/undef EXPR>. 1670 1671You may also use C<defined(&func)> to check whether subroutine C<func> 1672has ever been defined. The return value is unaffected by any forward 1673declarations of C<func>. A subroutine that is not defined 1674may still be callable: its package may have an C<AUTOLOAD> method that 1675makes it spring into existence the first time that it is called; see 1676L<perlsub>. 1677 1678Use of L<C<defined>|/defined EXPR> on aggregates (hashes and arrays) is 1679no longer supported. It used to report whether memory for that 1680aggregate had ever been allocated. You should instead use a simple 1681test for size: 1682 1683 if (@an_array) { print "has array elements\n" } 1684 if (%a_hash) { print "has hash members\n" } 1685 1686When used on a hash element, it tells you whether the value is defined, 1687not whether the key exists in the hash. Use L<C<exists>|/exists EXPR> 1688for the latter purpose. 1689 1690Examples: 1691 1692 print if defined $switch{D}; 1693 print "$val\n" while defined($val = pop(@ary)); 1694 die "Can't readlink $sym: $!" 1695 unless defined($value = readlink $sym); 1696 sub foo { defined &$bar ? $bar->(@_) : die "No bar"; } 1697 $debugging = 0 unless defined $debugging; 1698 1699Note: Many folks tend to overuse L<C<defined>|/defined EXPR> and are 1700then surprised to discover that the number C<0> and C<""> (the 1701zero-length string) are, in fact, defined values. For example, if you 1702say 1703 1704 "ab" =~ /a(.*)b/; 1705 1706The pattern match succeeds and C<$1> is defined, although it 1707matched "nothing". It didn't really fail to match anything. Rather, it 1708matched something that happened to be zero characters long. This is all 1709very above-board and honest. When a function returns an undefined value, 1710it's an admission that it couldn't give you an honest answer. So you 1711should use L<C<defined>|/defined EXPR> only when questioning the 1712integrity of what you're trying to do. At other times, a simple 1713comparison to C<0> or C<""> is what you want. 1714 1715See also L<C<undef>|/undef EXPR>, L<C<exists>|/exists EXPR>, 1716L<C<ref>|/ref EXPR>. 1717 1718=item delete EXPR 1719X<delete> 1720 1721=for Pod::Functions deletes a value from a hash 1722 1723Given an expression that specifies an element or slice of a hash, 1724L<C<delete>|/delete EXPR> deletes the specified elements from that hash 1725so that L<C<exists>|/exists EXPR> on that element no longer returns 1726true. Setting a hash element to the undefined value does not remove its 1727key, but deleting it does; see L<C<exists>|/exists EXPR>. 1728 1729In list context, usually returns the value or values deleted, or the last such 1730element in scalar context. The return list's length corresponds to that of 1731the argument list: deleting non-existent elements returns the undefined value 1732in their corresponding positions. Since Perl 5.28, a 1733L<keyE<sol>value hash slice|perldata/KeyE<sol>Value Hash Slices> can be passed 1734to C<delete>, and the return value is a list of key/value pairs (two elements 1735for each item deleted from the hash). 1736 1737L<C<delete>|/delete EXPR> may also be used on arrays and array slices, 1738but its behavior is less straightforward. Although 1739L<C<exists>|/exists EXPR> will return false for deleted entries, 1740deleting array elements never changes indices of existing values; use 1741L<C<shift>|/shift ARRAY> or L<C<splice>|/splice 1742ARRAY,OFFSET,LENGTH,LIST> for that. However, if any deleted elements 1743fall at the end of an array, the array's size shrinks to the position of 1744the highest element that still tests true for L<C<exists>|/exists EXPR>, 1745or to 0 if none do. In other words, an array won't have trailing 1746nonexistent elements after a delete. 1747 1748B<WARNING:> Calling L<C<delete>|/delete EXPR> on array values is 1749strongly discouraged. The 1750notion of deleting or checking the existence of Perl array elements is not 1751conceptually coherent, and can lead to surprising behavior. 1752 1753Deleting from L<C<%ENV>|perlvar/%ENV> modifies the environment. 1754Deleting from a hash tied to a DBM file deletes the entry from the DBM 1755file. Deleting from a L<C<tied>|/tied VARIABLE> hash or array may not 1756necessarily return anything; it depends on the implementation of the 1757L<C<tied>|/tied VARIABLE> package's DELETE method, which may do whatever 1758it pleases. 1759 1760The C<delete local EXPR> construct localizes the deletion to the current 1761block at run time. Until the block exits, elements locally deleted 1762temporarily no longer exist. See L<perlsub/"Localized deletion of elements 1763of composite types">. 1764 1765 my %hash = (foo => 11, bar => 22, baz => 33); 1766 my $scalar = delete $hash{foo}; # $scalar is 11 1767 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 1768 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33) 1769 1770The following (inefficiently) deletes all the values of %HASH and @ARRAY: 1771 1772 foreach my $key (keys %HASH) { 1773 delete $HASH{$key}; 1774 } 1775 1776 foreach my $index (0 .. $#ARRAY) { 1777 delete $ARRAY[$index]; 1778 } 1779 1780And so do these: 1781 1782 delete @HASH{keys %HASH}; 1783 1784 delete @ARRAY[0 .. $#ARRAY]; 1785 1786But both are slower than assigning the empty list 1787or undefining %HASH or @ARRAY, which is the customary 1788way to empty out an aggregate: 1789 1790 %HASH = (); # completely empty %HASH 1791 undef %HASH; # forget %HASH ever existed 1792 1793 @ARRAY = (); # completely empty @ARRAY 1794 undef @ARRAY; # forget @ARRAY ever existed 1795 1796The EXPR can be arbitrarily complicated provided its 1797final operation is an element or slice of an aggregate: 1798 1799 delete $ref->[$x][$y]{$key}; 1800 delete $ref->[$x][$y]->@{$key1, $key2, @morekeys}; 1801 1802 delete $ref->[$x][$y][$index]; 1803 delete $ref->[$x][$y]->@[$index1, $index2, @moreindices]; 1804 1805=item die LIST 1806X<die> X<throw> X<exception> X<raise> X<$@> X<abort> 1807 1808=for Pod::Functions raise an exception or bail out 1809 1810L<C<die>|/die LIST> raises an exception. Inside an L<C<eval>|/eval EXPR> 1811the exception is stuffed into L<C<$@>|perlvar/$@> and the L<C<eval>|/eval 1812EXPR> is terminated with the undefined value. If the exception is 1813outside of all enclosing L<C<eval>|/eval EXPR>s, then the uncaught 1814exception is printed to C<STDERR> and perl exits with an exit code 1815indicating failure. If you need to exit the process with a specific 1816exit code, see L<C<exit>|/exit EXPR>. 1817 1818Equivalent examples: 1819 1820 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; 1821 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 1822 1823Most of the time, C<die> is called with a string to use as the exception. 1824You may either give a single non-reference operand to serve as the 1825exception, or a list of two or more items, which will be stringified 1826and concatenated to make the exception. 1827 1828If the string exception does not end in a newline, the current 1829script line number and input line number (if any) and a newline 1830are appended to it. Note that the "input line number" (also 1831known as "chunk") is subject to whatever notion of "line" happens to 1832be currently in effect, and is also available as the special variable 1833L<C<$.>|perlvar/$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. 1834 1835Hint: sometimes appending C<", stopped"> to your message will cause it 1836to make better sense when the string C<"at foo line 123"> is appended. 1837Suppose you are running script "canasta". 1838 1839 die "/etc/games is no good"; 1840 die "/etc/games is no good, stopped"; 1841 1842produce, respectively 1843 1844 /etc/games is no good at canasta line 123. 1845 /etc/games is no good, stopped at canasta line 123. 1846 1847If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@> 1848already contains an exception value (typically from a previous 1849L<C<eval>|/eval EXPR>), then that value is reused after 1850appending C<"\t...propagated">. This is useful for propagating exceptions: 1851 1852 eval { ... }; 1853 die unless $@ =~ /Expected exception/; 1854 1855If LIST was empty or made an empty string, 1856and L<C<$@>|perlvar/$@> contains an object 1857reference that has a C<PROPAGATE> method, that method will be called 1858with additional file and line number parameters. The return value 1859replaces the value in L<C<$@>|perlvar/$@>; i.e., as if 1860C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> were called. 1861 1862If LIST was empty or made an empty string, and L<C<$@>|perlvar/$@> 1863is also empty, then the string C<"Died"> is used. 1864 1865You can also call L<C<die>|/die LIST> with a reference argument, and if 1866this is trapped within an L<C<eval>|/eval EXPR>, L<C<$@>|perlvar/$@> 1867contains that reference. This permits more elaborate exception handling 1868using objects that maintain arbitrary state about the exception. Such a 1869scheme is sometimes preferable to matching particular string values of 1870L<C<$@>|perlvar/$@> with regular expressions. 1871 1872Because Perl stringifies uncaught exception messages before display, 1873you'll probably want to overload stringification operations on 1874exception objects. See L<overload> for details about that. 1875The stringified message should be non-empty, and should end in a newline, 1876in order to fit in with the treatment of string exceptions. 1877Also, because an exception object reference cannot be stringified 1878without destroying it, Perl doesn't attempt to append location or other 1879information to a reference exception. If you want location information 1880with a complex exception object, you'll have to arrange to put the 1881location information into the object yourself. 1882 1883Because L<C<$@>|perlvar/$@> is a global variable, be careful that 1884analyzing an exception caught by C<eval> doesn't replace the reference 1885in the global variable. It's 1886easiest to make a local copy of the reference before any manipulations. 1887Here's an example: 1888 1889 use Scalar::Util "blessed"; 1890 1891 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; 1892 if (my $ev_err = $@) { 1893 if (blessed($ev_err) 1894 && $ev_err->isa("Some::Module::Exception")) { 1895 # handle Some::Module::Exception 1896 } 1897 else { 1898 # handle all other possible exceptions 1899 } 1900 } 1901 1902If an uncaught exception results in interpreter exit, the exit code is 1903determined from the values of L<C<$!>|perlvar/$!> and 1904L<C<$?>|perlvar/$?> with this pseudocode: 1905 1906 exit $! if $!; # errno 1907 exit $? >> 8 if $? >> 8; # child exit status 1908 exit 255; # last resort 1909 1910As with L<C<exit>|/exit EXPR>, L<C<$?>|perlvar/$?> is set prior to 1911unwinding the call stack; any C<DESTROY> or C<END> handlers can then 1912alter this value, and thus Perl's exit code. 1913 1914The intent is to squeeze as much possible information about the likely cause 1915into the limited space of the system exit code. However, as 1916L<C<$!>|perlvar/$!> is the value of C's C<errno>, which can be set by 1917any system call, this means that the value of the exit code used by 1918L<C<die>|/die LIST> can be non-predictable, so should not be relied 1919upon, other than to be non-zero. 1920 1921You can arrange for a callback to be run just before the 1922L<C<die>|/die LIST> does its deed, by setting the 1923L<C<$SIG{__DIE__}>|perlvar/%SIG> hook. The associated handler is called 1924with the exception as an argument, and can change the exception, 1925if it sees fit, by 1926calling L<C<die>|/die LIST> again. See L<perlvar/%SIG> for details on 1927setting L<C<%SIG>|perlvar/%SIG> entries, and L<C<eval>|/eval EXPR> for some 1928examples. Although this feature was to be run only right before your 1929program was to exit, this is not currently so: the 1930L<C<$SIG{__DIE__}>|perlvar/%SIG> hook is currently called even inside 1931L<C<eval>|/eval EXPR>ed blocks/strings! If one wants the hook to do 1932nothing in such situations, put 1933 1934 die @_ if $^S; 1935 1936as the first line of the handler (see L<perlvar/$^S>). Because 1937this promotes strange action at a distance, this counterintuitive 1938behavior may be fixed in a future release. 1939 1940See also L<C<exit>|/exit EXPR>, L<C<warn>|/warn LIST>, and the L<Carp> 1941module. 1942 1943=item do BLOCK 1944X<do> X<block> 1945 1946=for Pod::Functions turn a BLOCK into a TERM 1947 1948Not really a function. Returns the value of the last command in the 1949sequence of commands indicated by BLOCK. When modified by the C<while> or 1950C<until> loop modifier, executes the BLOCK once before testing the loop 1951condition. (On other statements the loop modifiers test the conditional 1952first.) 1953 1954C<do BLOCK> does I<not> count as a loop, so the loop control statements 1955L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or 1956L<C<redo>|/redo LABEL> cannot be used to leave or restart the block. 1957See L<perlsyn> for alternative strategies. 1958 1959=item do EXPR 1960X<do> 1961 1962Uses the value of EXPR as a filename and executes the contents of the 1963file as a Perl script: 1964 1965 # load the exact specified file (./ and ../ special-cased) 1966 do '/foo/stat.pl'; 1967 do './stat.pl'; 1968 do '../foo/stat.pl'; 1969 1970 # search for the named file within @INC 1971 do 'stat.pl'; 1972 do 'foo/stat.pl'; 1973 1974C<do './stat.pl'> is largely like 1975 1976 eval `cat stat.pl`; 1977 1978except that it's more concise, runs no external processes, and keeps 1979track of the current filename for error messages. It also differs in that 1980code evaluated with C<do FILE> cannot see lexicals in the enclosing 1981scope; C<eval STRING> does. It's the same, however, in that it does 1982reparse the file every time you call it, so you probably don't want 1983to do this inside a loop. 1984 1985Using C<do> with a relative path (except for F<./> and F<../>), like 1986 1987 do 'foo/stat.pl'; 1988 1989will search the L<C<@INC>|perlvar/@INC> directories, and update 1990L<C<%INC>|perlvar/%INC> if the file is found. See L<perlvar/@INC> 1991and L<perlvar/%INC> for these variables. In particular, note that 1992whilst historically L<C<@INC>|perlvar/@INC> contained '.' (the 1993current directory) making these two cases equivalent, that is no 1994longer necessarily the case, as '.' is not included in C<@INC> by default 1995in perl versions 5.26.0 onwards. Instead, perl will now warn: 1996 1997 do "stat.pl" failed, '.' is no longer in @INC; 1998 did you mean do "./stat.pl"? 1999 2000If L<C<do>|/do EXPR> can read the file but cannot compile it, it 2001returns L<C<undef>|/undef EXPR> and sets an error message in 2002L<C<$@>|perlvar/$@>. If L<C<do>|/do EXPR> cannot read the file, it 2003returns undef and sets L<C<$!>|perlvar/$!> to the error. Always check 2004L<C<$@>|perlvar/$@> first, as compilation could fail in a way that also 2005sets L<C<$!>|perlvar/$!>. If the file is successfully compiled, 2006L<C<do>|/do EXPR> returns the value of the last expression evaluated. 2007 2008Inclusion of library modules is better done with the 2009L<C<use>|/use Module VERSION LIST> and L<C<require>|/require VERSION> 2010operators, which also do automatic error checking and raise an exception 2011if there's a problem. 2012 2013You might like to use L<C<do>|/do EXPR> to read in a program 2014configuration file. Manual error checking can be done this way: 2015 2016 # Read in config files: system first, then user. 2017 # Beware of using relative pathnames here. 2018 for $file ("/share/prog/defaults.rc", 2019 "$ENV{HOME}/.someprogrc") 2020 { 2021 unless ($return = do $file) { 2022 warn "couldn't parse $file: $@" if $@; 2023 warn "couldn't do $file: $!" unless defined $return; 2024 warn "couldn't run $file" unless $return; 2025 } 2026 } 2027 2028=item dump LABEL 2029X<dump> X<core> X<undump> 2030 2031=item dump EXPR 2032 2033=item dump 2034 2035=for Pod::Functions create an immediate core dump 2036 2037This function causes an immediate core dump. See also the B<-u> 2038command-line switch in L<perlrun|perlrun/-u>, which does the same thing. 2039Primarily this is so that you can use the B<undump> program (not 2040supplied) to turn your core dump into an executable binary after 2041having initialized all your variables at the beginning of the 2042program. When the new binary is executed it will begin by executing 2043a C<goto LABEL> (with all the restrictions that L<C<goto>|/goto LABEL> 2044suffers). 2045Think of it as a goto with an intervening core dump and reincarnation. 2046If C<LABEL> is omitted, restarts the program from the top. The 2047C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be 2048computed at run time, being otherwise identical to C<dump LABEL>. 2049 2050B<WARNING>: Any files opened at the time of the dump will I<not> 2051be open any more when the program is reincarnated, with possible 2052resulting confusion by Perl. 2053 2054This function is now largely obsolete, mostly because it's very hard to 2055convert a core file into an executable. As of Perl 5.30, it must be invoked 2056as C<CORE::dump()>. 2057 2058Unlike most named operators, this has the same precedence as assignment. 2059It is also exempt from the looks-like-a-function rule, so 2060C<dump ("foo")."bar"> will cause "bar" to be part of the argument to 2061L<C<dump>|/dump LABEL>. 2062 2063Portability issues: L<perlport/dump>. 2064 2065=item each HASH 2066X<each> X<hash, iterator> 2067 2068=item each ARRAY 2069X<array, iterator> 2070 2071=for Pod::Functions retrieve the next key/value pair from a hash 2072 2073When called on a hash in list context, returns a 2-element list 2074consisting of the key and value for the next element of a hash. In Perl 20755.12 and later only, it will also return the index and value for the next 2076element of an array so that you can iterate over it; older Perls consider 2077this a syntax error. When called in scalar context, returns only the key 2078(not the value) in a hash, or the index in an array. 2079 2080Hash entries are returned in an apparently random order. The actual random 2081order is specific to a given hash; the exact same series of operations 2082on two hashes may result in a different order for each hash. Any insertion 2083into the hash may change the order, as will any deletion, with the exception 2084that the most recent key returned by L<C<each>|/each HASH> or 2085L<C<keys>|/keys HASH> may be deleted without changing the order. So 2086long as a given hash is unmodified you may rely on 2087L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and 2088L<C<each>|/each HASH> to repeatedly return the same order 2089as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for 2090details on why hash order is randomized. Aside from the guarantees 2091provided here the exact details of Perl's hash algorithm and the hash 2092traversal order are subject to change in any release of Perl. 2093 2094After L<C<each>|/each HASH> has returned all entries from the hash or 2095array, the next call to L<C<each>|/each HASH> returns the empty list in 2096list context and L<C<undef>|/undef EXPR> in scalar context; the next 2097call following I<that> one restarts iteration. Each hash or array has 2098its own internal iterator, accessed by L<C<each>|/each HASH>, 2099L<C<keys>|/keys HASH>, and L<C<values>|/values HASH>. The iterator is 2100implicitly reset when L<C<each>|/each HASH> has reached the end as just 2101described; it can be explicitly reset by calling L<C<keys>|/keys HASH> 2102or L<C<values>|/values HASH> on the hash or array, or by referencing 2103the hash (but not array) in list context. If you add or delete 2104a hash's elements while iterating over it, the effect on the iterator is 2105unspecified; for example, entries may be skipped or duplicated--so don't 2106do that. Exception: It is always safe to delete the item most recently 2107returned by L<C<each>|/each HASH>, so the following code works properly: 2108 2109 while (my ($key, $value) = each %hash) { 2110 print $key, "\n"; 2111 delete $hash{$key}; # This is safe 2112 } 2113 2114Tied hashes may have a different ordering behaviour to perl's hash 2115implementation. 2116 2117The iterator used by C<each> is attached to the hash or array, and is 2118shared between all iteration operations applied to the same hash or array. 2119Thus all uses of C<each> on a single hash or array advance the same 2120iterator location. All uses of C<each> are also subject to having the 2121iterator reset by any use of C<keys> or C<values> on the same hash or 2122array, or by the hash (but not array) being referenced in list context. 2123This makes C<each>-based loops quite fragile: it is easy to arrive at 2124such a loop with the iterator already part way through the object, or to 2125accidentally clobber the iterator state during execution of the loop body. 2126It's easy enough to explicitly reset the iterator before starting a loop, 2127but there is no way to insulate the iterator state used by a loop from 2128the iterator state used by anything else that might execute during the 2129loop body. To avoid these problems, use a C<foreach> loop rather than 2130C<while>-C<each>. 2131 2132This extends to using C<each> on the result of an anonymous hash or 2133array constructor. A new underlying array or hash is created each 2134time so each will always start iterating from scratch, eg: 2135 2136 # loops forever 2137 while (my ($key, $value) = each @{ +{ a => 1 } }) { 2138 print "$key=$value\n"; 2139 } 2140 2141This prints out your environment like the L<printenv(1)> program, 2142but in a different order: 2143 2144 while (my ($key,$value) = each %ENV) { 2145 print "$key=$value\n"; 2146 } 2147 2148Starting with Perl 5.14, an experimental feature allowed 2149L<C<each>|/each HASH> to take a scalar expression. This experiment has 2150been deemed unsuccessful, and was removed as of Perl 5.24. 2151 2152As of Perl 5.18 you can use a bare L<C<each>|/each HASH> in a C<while> 2153loop, which will set L<C<$_>|perlvar/$_> on every iteration. 2154If either an C<each> expression or an explicit assignment of an C<each> 2155expression to a scalar is used as a C<while>/C<for> condition, then 2156the condition actually tests for definedness of the expression's value, 2157not for its regular truth value. 2158 2159 while (each %ENV) { 2160 print "$_=$ENV{$_}\n"; 2161 } 2162 2163To avoid confusing would-be users of your code who are running earlier 2164versions of Perl with mysterious syntax errors, put this sort of thing at 2165the top of your file to signal that your code will work I<only> on Perls of 2166a recent vintage: 2167 2168 use v5.12; # so keys/values/each work on arrays 2169 use v5.18; # so each assigns to $_ in a lone while test 2170 2171See also L<C<keys>|/keys HASH>, L<C<values>|/values HASH>, and 2172L<C<sort>|/sort SUBNAME LIST>. 2173 2174=item eof FILEHANDLE 2175X<eof> 2176X<end of file> 2177X<end-of-file> 2178 2179=item eof () 2180 2181=item eof 2182 2183=for Pod::Functions test a filehandle for its end 2184 2185Returns 1 if the next read on FILEHANDLE will return end of file I<or> if 2186FILEHANDLE is not open. FILEHANDLE may be an expression whose value 2187gives the real filehandle. (Note that this function actually 2188reads a character and then C<ungetc>s it, so isn't useful in an 2189interactive context.) Do not read from a terminal file (or call 2190C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such 2191as terminals may lose the end-of-file condition if you do. 2192 2193An L<C<eof>|/eof FILEHANDLE> without an argument uses the last file 2194read. Using L<C<eof()>|/eof FILEHANDLE> with empty parentheses is 2195different. It refers to the pseudo file formed from the files listed on 2196the command line and accessed via the C<< <> >> operator. Since 2197C<< <> >> isn't explicitly opened, as a normal filehandle is, an 2198L<C<eof()>|/eof FILEHANDLE> before C<< <> >> has been used will cause 2199L<C<@ARGV>|perlvar/@ARGV> to be examined to determine if input is 2200available. Similarly, an L<C<eof()>|/eof FILEHANDLE> after C<< <> >> 2201has returned end-of-file will assume you are processing another 2202L<C<@ARGV>|perlvar/@ARGV> list, and if you haven't set 2203L<C<@ARGV>|perlvar/@ARGV>, will read input from C<STDIN>; see 2204L<perlop/"I/O Operators">. 2205 2206In a C<< while (<>) >> loop, L<C<eof>|/eof FILEHANDLE> or C<eof(ARGV)> 2207can be used to detect the end of each file, whereas 2208L<C<eof()>|/eof FILEHANDLE> will detect the end of the very last file 2209only. Examples: 2210 2211 # reset line numbering on each input file 2212 while (<>) { 2213 next if /^\s*#/; # skip comments 2214 print "$.\t$_"; 2215 } continue { 2216 close ARGV if eof; # Not eof()! 2217 } 2218 2219 # insert dashes just before last line of last file 2220 while (<>) { 2221 if (eof()) { # check for end of last file 2222 print "--------------\n"; 2223 } 2224 print; 2225 last if eof(); # needed if we're reading from a terminal 2226 } 2227 2228Practical hint: you almost never need to use L<C<eof>|/eof FILEHANDLE> 2229in Perl, because the input operators typically return L<C<undef>|/undef 2230EXPR> when they run out of data or encounter an error. 2231 2232=item eval EXPR 2233X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> 2234X<error, handling> X<exception, handling> 2235 2236=item eval BLOCK 2237 2238=item eval 2239 2240=for Pod::Functions catch exceptions or compile and run code 2241 2242C<eval> in all its forms is used to execute a little Perl program, 2243trapping any errors encountered so they don't crash the calling program. 2244 2245Plain C<eval> with no argument is just C<eval EXPR>, where the 2246expression is understood to be contained in L<C<$_>|perlvar/$_>. Thus 2247there are only two real C<eval> forms; the one with an EXPR is often 2248called "string eval". In a string eval, the value of the expression 2249(which is itself determined within scalar context) is first parsed, and 2250if there were no errors, executed as a block within the lexical context 2251of the current Perl program. This form is typically used to delay 2252parsing and subsequent execution of the text of EXPR until run time. 2253Note that the value is parsed every time the C<eval> executes. 2254 2255The other form is called "block eval". It is less general than string 2256eval, but the code within the BLOCK is parsed only once (at the same 2257time the code surrounding the C<eval> itself was parsed) and executed 2258within the context of the current Perl program. This form is typically 2259used to trap exceptions more efficiently than the first, while also 2260providing the benefit of checking the code within BLOCK at compile time. 2261BLOCK is parsed and compiled just once. Since errors are trapped, it 2262often is used to check if a given feature is available. 2263 2264In both forms, the value returned is the value of the last expression 2265evaluated inside the mini-program; a return statement may also be used, just 2266as with subroutines. The expression providing the return value is evaluated 2267in void, scalar, or list context, depending on the context of the 2268C<eval> itself. See L<C<wantarray>|/wantarray> for more 2269on how the evaluation context can be determined. 2270 2271If there is a syntax error or runtime error, or a L<C<die>|/die LIST> 2272statement is executed, C<eval> returns 2273L<C<undef>|/undef EXPR> in scalar context, or an empty list in list 2274context, and L<C<$@>|perlvar/$@> is set to the error message. (Prior to 22755.16, a bug caused L<C<undef>|/undef EXPR> to be returned in list 2276context for syntax errors, but not for runtime errors.) If there was no 2277error, L<C<$@>|perlvar/$@> is set to the empty string. A control flow 2278operator like L<C<last>|/last LABEL> or L<C<goto>|/goto LABEL> can 2279bypass the setting of L<C<$@>|perlvar/$@>. Beware that using 2280C<eval> neither silences Perl from printing warnings to 2281STDERR, nor does it stuff the text of warning messages into 2282L<C<$@>|perlvar/$@>. To do either of those, you have to use the 2283L<C<$SIG{__WARN__}>|perlvar/%SIG> facility, or turn off warnings inside 2284the BLOCK or EXPR using S<C<no warnings 'all'>>. See 2285L<C<warn>|/warn LIST>, L<perlvar>, and L<warnings>. 2286 2287Note that, because C<eval> traps otherwise-fatal errors, 2288it is useful for determining whether a particular feature (such as 2289L<C<socket>|/socket SOCKET,DOMAIN,TYPE,PROTOCOL> or 2290L<C<symlink>|/symlink OLDFILE,NEWFILE>) is implemented. It is also 2291Perl's exception-trapping mechanism, where the L<C<die>|/die LIST> 2292operator is used to raise exceptions. 2293 2294Before Perl 5.14, the assignment to L<C<$@>|perlvar/$@> occurred before 2295restoration 2296of localized variables, which means that for your code to run on older 2297versions, a temporary is required if you want to mask some, but not all 2298errors: 2299 2300 # alter $@ on nefarious repugnancy only 2301 { 2302 my $e; 2303 { 2304 local $@; # protect existing $@ 2305 eval { test_repugnancy() }; 2306 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only 2307 $@ =~ /nefarious/ and $e = $@; 2308 } 2309 die $e if defined $e 2310 } 2311 2312There are some different considerations for each form: 2313 2314=over 4 2315 2316=item String eval 2317 2318Since the return value of EXPR is executed as a block within the lexical 2319context of the current Perl program, any outer lexical variables are 2320visible to it, and any package variable settings or subroutine and 2321format definitions remain afterwards. 2322 2323Note that when C<BEGIN {}> blocks are embedded inside of an eval block 2324the contents of the block will be executed immediately and before the rest 2325of the eval code is executed. You can disable this entirely by 2326 2327 local ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} = 0; 2328 eval $string; 2329 2330which will cause any embedded C<BEGIN> blocks in C<$string> to throw an 2331exception. 2332 2333=over 4 2334 2335=item Under the L<C<"unicode_eval"> feature|feature/The 'unicode_eval' and 'evalbytes' features> 2336 2337If this feature is enabled (which is the default under a C<use 5.16> or 2338higher declaration), Perl assumes that EXPR is a character string. 2339Any S<C<use utf8>> or S<C<no utf8>> declarations within 2340the string thus have no effect. Source filters are forbidden as well. 2341(C<unicode_strings>, however, can appear within the string.) 2342 2343See also the L<C<evalbytes>|/evalbytes EXPR> operator, which works properly 2344with source filters. 2345 2346=item Outside the C<"unicode_eval"> feature 2347 2348In this case, the behavior is problematic and is not so easily 2349described. Here are two bugs that cannot easily be fixed without 2350breaking existing programs: 2351 2352=over 4 2353 2354=item * 2355 2356Perl's internal storage of EXPR affects the behavior of the executed code. 2357For example: 2358 2359 my $v = eval "use utf8; '$expr'"; 2360 2361If $expr is C<"\xc4\x80"> (U+0100 in UTF-8), then the value stored in C<$v> 2362will depend on whether Perl stores $expr "upgraded" (cf. L<utf8>) or 2363not: 2364 2365=over 2366 2367=item * If upgraded, C<$v> will be C<"\xc4\x80"> (i.e., the 2368C<use utf8> has no effect.) 2369 2370=item * If non-upgraded, C<$v> will be C<"\x{100}">. 2371 2372=back 2373 2374This is undesirable since being 2375upgraded or not should not affect a string's behavior. 2376 2377=item * 2378 2379Source filters activated within C<eval> leak out into whichever file 2380scope is currently being compiled. To give an example with the CPAN module 2381L<Semi::Semicolons>: 2382 2383 BEGIN { eval "use Semi::Semicolons; # not filtered" } 2384 # filtered here! 2385 2386L<C<evalbytes>|/evalbytes EXPR> fixes that to work the way one would 2387expect: 2388 2389 use feature "evalbytes"; 2390 BEGIN { evalbytes "use Semi::Semicolons; # filtered" } 2391 # not filtered 2392 2393=back 2394 2395=back 2396 2397Problems can arise if the string expands a scalar containing a floating 2398point number. That scalar can expand to letters, such as C<"NaN"> or 2399C<"Infinity">; or, within the scope of a L<C<use locale>|locale>, the 2400decimal point character may be something other than a dot (such as a 2401comma). None of these are likely to parse as you are likely expecting. 2402 2403You should be especially careful to remember what's being looked at 2404when: 2405 2406 eval $x; # CASE 1 2407 eval "$x"; # CASE 2 2408 2409 eval '$x'; # CASE 3 2410 eval { $x }; # CASE 4 2411 2412 eval "\$$x++"; # CASE 5 2413 $$x++; # CASE 6 2414 2415Cases 1 and 2 above behave identically: they run the code contained in 2416the variable $x. (Although case 2 has misleading double quotes making 2417the reader wonder what else might be happening (nothing is).) Cases 3 2418and 4 likewise behave in the same way: they run the code C<'$x'>, which 2419does nothing but return the value of $x. (Case 4 is preferred for 2420purely visual reasons, but it also has the advantage of compiling at 2421compile-time instead of at run-time.) Case 5 is a place where 2422normally you I<would> like to use double quotes, except that in this 2423particular situation, you can just use symbolic references instead, as 2424in case 6. 2425 2426An C<eval ''> executed within a subroutine defined 2427in the C<DB> package doesn't see the usual 2428surrounding lexical scope, but rather the scope of the first non-DB piece 2429of code that called it. You don't normally need to worry about this unless 2430you are writing a Perl debugger. 2431 2432The final semicolon, if any, may be omitted from the value of EXPR. 2433 2434=item Block eval 2435 2436If the code to be executed doesn't vary, you may use the eval-BLOCK 2437form to trap run-time errors without incurring the penalty of 2438recompiling each time. The error, if any, is still returned in 2439L<C<$@>|perlvar/$@>. 2440Examples: 2441 2442 # make divide-by-zero nonfatal 2443 eval { $answer = $x / $y; }; warn $@ if $@; 2444 2445 # same thing, but less efficient 2446 eval '$answer = $x / $y'; warn $@ if $@; 2447 2448 # a compile-time error 2449 eval { $answer = }; # WRONG 2450 2451 # a run-time error 2452 eval '$answer ='; # sets $@ 2453 2454If you want to trap errors when loading an XS module, some problems with 2455the binary interface (such as Perl version skew) may be fatal even with 2456C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See 2457L<perlrun|perlrun/PERL_DL_NONLAZY>. 2458 2459Using the C<eval {}> form as an exception trap in libraries does have some 2460issues. Due to the current arguably broken state of C<__DIE__> hooks, you 2461may wish not to trigger any C<__DIE__> hooks that user code may have installed. 2462You can use the C<local $SIG{__DIE__}> construct for this purpose, 2463as this example shows: 2464 2465 # a private exception trap for divide-by-zero 2466 eval { local $SIG{'__DIE__'}; $answer = $x / $y; }; 2467 warn $@ if $@; 2468 2469This is especially significant, given that C<__DIE__> hooks can call 2470L<C<die>|/die LIST> again, which has the effect of changing their error 2471messages: 2472 2473 # __DIE__ hooks may modify error messages 2474 { 2475 local $SIG{'__DIE__'} = 2476 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; 2477 eval { die "foo lives here" }; 2478 print $@ if $@; # prints "bar lives here" 2479 } 2480 2481Because this promotes action at a distance, this counterintuitive behavior 2482may be fixed in a future release. 2483 2484C<eval BLOCK> does I<not> count as a loop, so the loop control statements 2485L<C<next>|/next LABEL>, L<C<last>|/last LABEL>, or 2486L<C<redo>|/redo LABEL> cannot be used to leave or restart the block. 2487 2488The final semicolon, if any, may be omitted from within the BLOCK. 2489 2490=back 2491 2492=item evalbytes EXPR 2493X<evalbytes> 2494 2495=item evalbytes 2496 2497=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream 2498 2499This function is similar to a L<string eval|/eval EXPR>, except it 2500always parses its argument (or L<C<$_>|perlvar/$_> if EXPR is omitted) 2501as a byte string. If the string contains any code points above 255, then 2502it cannot be a byte string, and the C<evalbytes> will fail with the error 2503stored in C<$@>. 2504 2505C<use utf8> and C<no utf8> within the string have their usual effect. 2506 2507Source filters activated within the evaluated code apply to the code 2508itself. 2509 2510L<C<evalbytes>|/evalbytes EXPR> is available starting in Perl v5.16. To 2511access it, you must say C<CORE::evalbytes>, but you can omit the 2512C<CORE::> if the 2513L<C<"evalbytes"> feature|feature/The 'unicode_eval' and 'evalbytes' features> 2514is enabled. This is enabled automatically with a C<use v5.16> (or 2515higher) declaration in the current scope. 2516 2517=item exec LIST 2518X<exec> X<execute> 2519 2520=item exec PROGRAM LIST 2521 2522=for Pod::Functions abandon this program to run another 2523 2524The L<C<exec>|/exec LIST> function executes a system command I<and never 2525returns>; use L<C<system>|/system LIST> instead of L<C<exec>|/exec LIST> 2526if you want it to return. It fails and 2527returns false only if the command does not exist I<and> it is executed 2528directly instead of via your system's command shell (see below). 2529 2530Since it's a common mistake to use L<C<exec>|/exec LIST> instead of 2531L<C<system>|/system LIST>, Perl warns you if L<C<exec>|/exec LIST> is 2532called in void context and if there is a following statement that isn't 2533L<C<die>|/die LIST>, L<C<warn>|/warn LIST>, or L<C<exit>|/exit EXPR> (if 2534L<warnings> are enabled--but you always do that, right?). If you 2535I<really> want to follow an L<C<exec>|/exec LIST> with some other 2536statement, you can use one of these styles to avoid the warning: 2537 2538 exec ('foo') or print STDERR "couldn't exec foo: $!"; 2539 { exec ('foo') }; print STDERR "couldn't exec foo: $!"; 2540 2541If there is more than one argument in LIST, this calls L<execvp(3)> with the 2542arguments in LIST. If there is only one element in LIST, the argument is 2543checked for shell metacharacters, and if there are any, the entire 2544argument is passed to the system's command shell for parsing (this is 2545C</bin/sh -c> on Unix platforms, but varies on other platforms). If 2546there are no shell metacharacters in the argument, it is split into words 2547and passed directly to C<execvp>, which is more efficient. Examples: 2548 2549 exec '/bin/echo', 'Your arguments are: ', @ARGV; 2550 exec "sort $outfile | uniq"; 2551 2552If you don't really want to execute the first argument, but want to lie 2553to the program you are executing about its own name, you can specify 2554the program you actually want to run as an "indirect object" (without a 2555comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always 2556forces interpretation of the LIST as a multivalued list, even if there 2557is only a single scalar in the list.) Example: 2558 2559 my $shell = '/bin/csh'; 2560 exec $shell '-sh'; # pretend it's a login shell 2561 2562or, more directly, 2563 2564 exec {'/bin/csh'} '-sh'; # pretend it's a login shell 2565 2566When the arguments get executed via the system shell, results are 2567subject to its quirks and capabilities. See L<perlop/"`STRING`"> 2568for details. 2569 2570Using an indirect object with L<C<exec>|/exec LIST> or 2571L<C<system>|/system LIST> is also more secure. This usage (which also 2572works fine with L<C<system>|/system LIST>) forces 2573interpretation of the arguments as a multivalued list, even if the 2574list had just one argument. That way you're safe from the shell 2575expanding wildcards or splitting up words with whitespace in them. 2576 2577 my @args = ( "echo surprise" ); 2578 2579 exec @args; # subject to shell escapes 2580 # if @args == 1 2581 exec { $args[0] } @args; # safe even with one-arg list 2582 2583The first version, the one without the indirect object, ran the I<echo> 2584program, passing it C<"surprise"> an argument. The second version didn't; 2585it tried to run a program named I<"echo surprise">, didn't find it, and set 2586L<C<$?>|perlvar/$?> to a non-zero value indicating failure. 2587 2588On Windows, only the C<exec PROGRAM LIST> indirect object syntax will 2589reliably avoid using the shell; C<exec LIST>, even with more than one 2590element, will fall back to the shell if the first spawn fails. 2591 2592Perl attempts to flush all files opened for output before the exec, 2593but this may not be supported on some platforms (see L<perlport>). 2594To be safe, you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>> 2595(C<$AUTOFLUSH> in L<English>) or call the C<autoflush> method of 2596L<C<IO::Handle>|IO::Handle/METHODS> on any open handles to avoid lost 2597output. 2598 2599Note that L<C<exec>|/exec LIST> will not call your C<END> blocks, nor 2600will it invoke C<DESTROY> methods on your objects. 2601 2602Portability issues: L<perlport/exec>. 2603 2604=item exists EXPR 2605X<exists> X<autovivification> 2606 2607=for Pod::Functions test whether a hash key is present 2608 2609Given an expression that specifies an element of a hash, returns true if the 2610specified element in the hash has ever been initialized, even if the 2611corresponding value is undefined. 2612 2613 print "Exists\n" if exists $hash{$key}; 2614 print "Defined\n" if defined $hash{$key}; 2615 print "True\n" if $hash{$key}; 2616 2617exists may also be called on array elements, but its behavior is much less 2618obvious and is strongly tied to the use of L<C<delete>|/delete EXPR> on 2619arrays. 2620 2621B<WARNING:> Calling L<C<exists>|/exists EXPR> on array values is 2622strongly discouraged. The 2623notion of deleting or checking the existence of Perl array elements is not 2624conceptually coherent, and can lead to surprising behavior. 2625 2626 print "Exists\n" if exists $array[$index]; 2627 print "Defined\n" if defined $array[$index]; 2628 print "True\n" if $array[$index]; 2629 2630A hash or array element can be true only if it's defined and defined only if 2631it exists, but the reverse doesn't necessarily hold true. 2632 2633Given an expression that specifies the name of a subroutine, 2634returns true if the specified subroutine has ever been declared, even 2635if it is undefined. Mentioning a subroutine name for exists or defined 2636does not count as declaring it. Note that a subroutine that does not 2637exist may still be callable: its package may have an C<AUTOLOAD> 2638method that makes it spring into existence the first time that it is 2639called; see L<perlsub>. 2640 2641 print "Exists\n" if exists &subroutine; 2642 print "Defined\n" if defined &subroutine; 2643 2644Note that the EXPR can be arbitrarily complicated as long as the final 2645operation is a hash or array key lookup or subroutine name: 2646 2647 if (exists $ref->{A}->{B}->{$key}) { } 2648 if (exists $hash{A}{B}{$key}) { } 2649 2650 if (exists $ref->{A}->{B}->[$ix]) { } 2651 if (exists $hash{A}{B}[$ix]) { } 2652 2653 if (exists &{$ref->{A}{B}{$key}}) { } 2654 2655Although the most deeply nested array or hash element will not spring into 2656existence just because its existence was tested, any intervening ones will. 2657Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring 2658into existence due to the existence test for the C<$key> element above. 2659This happens anywhere the arrow operator is used, including even here: 2660 2661 undef $ref; 2662 if (exists $ref->{"Some key"}) { } 2663 print $ref; # prints HASH(0x80d3d5c) 2664 2665Use of a subroutine call, rather than a subroutine name, as an argument 2666to L<C<exists>|/exists EXPR> is an error. 2667 2668 exists ⊂ # OK 2669 exists &sub(); # Error 2670 2671=item exit EXPR 2672X<exit> X<terminate> X<abort> 2673 2674=item exit 2675 2676=for Pod::Functions terminate this program 2677 2678Evaluates EXPR and exits immediately with that value. Example: 2679 2680 my $ans = <STDIN>; 2681 exit 0 if $ans =~ /^[Xx]/; 2682 2683See also L<C<die>|/die LIST>. If EXPR is omitted, exits with C<0> 2684status. The only 2685universally recognized values for EXPR are C<0> for success and C<1> 2686for error; other values are subject to interpretation depending on the 2687environment in which the Perl program is running. For example, exiting 268869 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause 2689the mailer to return the item undelivered, but that's not true everywhere. 2690 2691Don't use L<C<exit>|/exit EXPR> to abort a subroutine if there's any 2692chance that someone might want to trap whatever error happened. Use 2693L<C<die>|/die LIST> instead, which can be trapped by an 2694L<C<eval>|/eval EXPR>. 2695 2696The L<C<exit>|/exit EXPR> function does not always exit immediately. It 2697calls any defined C<END> routines first, but these C<END> routines may 2698not themselves abort the exit. Likewise any object destructors that 2699need to be called are called before the real exit. C<END> routines and 2700destructors can change the exit status by modifying L<C<$?>|perlvar/$?>. 2701If this is a problem, you can call 2702L<C<POSIX::_exit($status)>|POSIX/C<_exit>> to avoid C<END> and destructor 2703processing. See L<perlmod> for details. 2704 2705Portability issues: L<perlport/exit>. 2706 2707=item exp EXPR 2708X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> 2709 2710=item exp 2711 2712=for Pod::Functions raise I<e> to a power 2713 2714Returns I<e> (the natural logarithm base) to the power of EXPR. 2715If EXPR is omitted, gives C<exp($_)>. 2716 2717=item fc EXPR 2718X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold> 2719 2720=item fc 2721 2722=for Pod::Functions +fc return casefolded version of a string 2723 2724Returns the casefolded version of EXPR. This is the internal function 2725implementing the C<\F> escape in double-quoted strings. 2726 2727Casefolding is the process of mapping strings to a form where case 2728differences are erased; comparing two strings in their casefolded 2729form is effectively a way of asking if two strings are equal, 2730regardless of case. 2731 2732Roughly, if you ever found yourself writing this 2733 2734 lc($this) eq lc($that) # Wrong! 2735 # or 2736 uc($this) eq uc($that) # Also wrong! 2737 # or 2738 $this =~ /^\Q$that\E\z/i # Right! 2739 2740Now you can write 2741 2742 fc($this) eq fc($that) 2743 2744And get the correct results. 2745 2746Perl only implements the full form of casefolding, but you can access 2747the simple folds using L<Unicode::UCD/B<casefold()>> and 2748L<Unicode::UCD/B<prop_invmap()>>. 2749For further information on casefolding, refer to 2750the Unicode Standard, specifically sections 3.13 C<Default Case Operations>, 27514.2 C<Case-Normative>, and 5.18 C<Case Mappings>, 2752available at L<https://www.unicode.org/versions/latest/>, as well as the 2753Case Charts available at L<https://www.unicode.org/charts/case/>. 2754 2755If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 2756 2757This function behaves the same way under various pragmas, such as within 2758L<S<C<"use feature 'unicode_strings">>|feature/The 'unicode_strings' feature>, 2759as L<C<lc>|/lc EXPR> does, with the single exception of 2760L<C<fc>|/fc EXPR> of I<LATIN CAPITAL LETTER SHARP S> (U+1E9E) within the 2761scope of L<S<C<use locale>>|locale>. The foldcase of this character 2762would normally be C<"ss">, but as explained in the L<C<lc>|/lc EXPR> 2763section, case 2764changes that cross the 255/256 boundary are problematic under locales, 2765and are hence prohibited. Therefore, this function under locale returns 2766instead the string C<"\x{17F}\x{17F}">, which is the I<LATIN SMALL LETTER 2767LONG S>. Since that character itself folds to C<"s">, the string of two 2768of them together should be equivalent to a single U+1E9E when foldcased. 2769 2770While the Unicode Standard defines two additional forms of casefolding, 2771one for Turkic languages and one that never maps one character into multiple 2772characters, these are not provided by the Perl core. However, the CPAN module 2773L<C<Unicode::Casing>|Unicode::Casing> may be used to provide an implementation. 2774 2775L<C<fc>|/fc EXPR> is available only if the 2776L<C<"fc"> feature|feature/The 'fc' feature> is enabled or if it is 2777prefixed with C<CORE::>. The 2778L<C<"fc"> feature|feature/The 'fc' feature> is enabled automatically 2779with a C<use v5.16> (or higher) declaration in the current scope. 2780 2781=item fcntl FILEHANDLE,FUNCTION,SCALAR 2782X<fcntl> 2783 2784=for Pod::Functions file control system call 2785 2786Implements the L<fcntl(2)> function. You'll probably have to say 2787 2788 use Fcntl; 2789 2790first to get the correct constant definitions. Argument processing and 2791value returned work just like L<C<ioctl>|/ioctl 2792FILEHANDLE,FUNCTION,SCALAR> below. For example: 2793 2794 use Fcntl; 2795 my $flags = fcntl($filehandle, F_GETFL, 0) 2796 or die "Can't fcntl F_GETFL: $!"; 2797 2798You don't have to check for L<C<defined>|/defined EXPR> on the return 2799from L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>. Like 2800L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>, it maps a C<0> return 2801from the system call into C<"0 but true"> in Perl. This string is true 2802in boolean context and C<0> in numeric context. It is also exempt from 2803the normal 2804L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s> 2805L<warnings> on improper numeric conversions. 2806 2807Note that L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> raises an 2808exception if used on a machine that doesn't implement L<fcntl(2)>. See 2809the L<Fcntl> module or your L<fcntl(2)> manpage to learn what functions 2810are available on your system. 2811 2812Here's an example of setting a filehandle named C<$REMOTE> to be 2813non-blocking at the system level. You'll have to negotiate 2814L<C<$E<verbar>>|perlvar/$E<verbar>> on your own, though. 2815 2816 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); 2817 2818 my $flags = fcntl($REMOTE, F_GETFL, 0) 2819 or die "Can't get flags for the socket: $!\n"; 2820 2821 fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK) 2822 or die "Can't set flags for the socket: $!\n"; 2823 2824Portability issues: L<perlport/fcntl>. 2825 2826=item __FILE__ 2827X<__FILE__> 2828 2829=for Pod::Functions the name of the current source file 2830 2831A special token that returns the name of the file in which it occurs. 2832It can be altered by the mechanism described at 2833L<perlsyn/"Plain Old Comments (Not!)">. 2834 2835=item field VARNAME 2836X<field> 2837 2838=for Pod::Functions declare a field variable of the current class 2839 2840Declares a new field variable within the current class. Methods and 2841C<ADJUST> blocks of the class will have access to this variable as if it 2842was a lexical in scope at that point. 2843 2844=item fileno FILEHANDLE 2845X<fileno> 2846 2847=item fileno DIRHANDLE 2848 2849=for Pod::Functions return file descriptor from filehandle 2850 2851Returns the file descriptor for a filehandle or directory handle, 2852or undefined if the 2853filehandle is not open. If there is no real file descriptor at the OS 2854level, as can happen with filehandles connected to memory objects via 2855L<C<open>|/open FILEHANDLE,MODE,EXPR> with a reference for the third 2856argument, -1 is returned. 2857 2858This is mainly useful for constructing bitmaps for 2859L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> and low-level POSIX 2860tty-handling operations. 2861If FILEHANDLE is an expression, the value is taken as an indirect 2862filehandle, generally its name. 2863 2864You can use this to find out whether two handles refer to the 2865same underlying descriptor: 2866 2867 if (fileno($this) != -1 && fileno($this) == fileno($that)) { 2868 print "\$this and \$that are dups\n"; 2869 } elsif (fileno($this) != -1 && fileno($that) != -1) { 2870 print "\$this and \$that have different " . 2871 "underlying file descriptors\n"; 2872 } else { 2873 print "At least one of \$this and \$that does " . 2874 "not have a real file descriptor\n"; 2875 } 2876 2877The behavior of L<C<fileno>|/fileno FILEHANDLE> on a directory handle 2878depends on the operating system. On a system with L<dirfd(3)> or 2879similar, L<C<fileno>|/fileno FILEHANDLE> on a directory 2880handle returns the underlying file descriptor associated with the 2881handle; on systems with no such support, it returns the undefined value, 2882and sets L<C<$!>|perlvar/$!> (errno). 2883 2884=item flock FILEHANDLE,OPERATION 2885X<flock> X<lock> X<locking> 2886 2887=for Pod::Functions lock an entire file with an advisory lock 2888 2889Calls L<flock(2)>, or an emulation of it, on FILEHANDLE. Returns true 2890for success, false on failure. Produces a fatal error if used on a 2891machine that doesn't implement L<flock(2)>, L<fcntl(2)> locking, or 2892L<lockf(3)>. L<C<flock>|/flock FILEHANDLE,OPERATION> is Perl's portable 2893file-locking interface, although it locks entire files only, not 2894records. 2895 2896Two potentially non-obvious but traditional L<C<flock>|/flock 2897FILEHANDLE,OPERATION> semantics are 2898that it waits indefinitely until the lock is granted, and that its locks 2899are B<merely advisory>. Such discretionary locks are more flexible, but 2900offer fewer guarantees. This means that programs that do not also use 2901L<C<flock>|/flock FILEHANDLE,OPERATION> may modify files locked with 2902L<C<flock>|/flock FILEHANDLE,OPERATION>. See L<perlport>, 2903your port's specific documentation, and your system-specific local manpages 2904for details. It's best to assume traditional behavior if you're writing 2905portable programs. (But if you're not, you should as always feel perfectly 2906free to write for your own system's idiosyncrasies (sometimes called 2907"features"). Slavish adherence to portability concerns shouldn't get 2908in the way of your getting your job done.) 2909 2910OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with 2911LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but 2912you can use the symbolic names if you import them from the L<Fcntl> module, 2913either individually, or as a group using the C<:flock> tag. LOCK_SH 2914requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN 2915releases a previously requested lock. If LOCK_NB is bitwise-or'ed with 2916LOCK_SH or LOCK_EX, then L<C<flock>|/flock FILEHANDLE,OPERATION> returns 2917immediately rather than blocking waiting for the lock; check the return 2918status to see if you got it. 2919 2920To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE 2921before locking or unlocking it. 2922 2923Note that the emulation built with L<lockf(3)> doesn't provide shared 2924locks, and it requires that FILEHANDLE be open with write intent. These 2925are the semantics that L<lockf(3)> implements. Most if not all systems 2926implement L<lockf(3)> in terms of L<fcntl(2)> locking, though, so the 2927differing semantics shouldn't bite too many people. 2928 2929Note that the L<fcntl(2)> emulation of L<flock(3)> requires that FILEHANDLE 2930be open with read intent to use LOCK_SH and requires that it be open 2931with write intent to use LOCK_EX. 2932 2933Note also that some versions of L<C<flock>|/flock FILEHANDLE,OPERATION> 2934cannot lock things over the network; you would need to use the more 2935system-specific L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR> for 2936that. If you like you can force Perl to ignore your system's L<flock(2)> 2937function, and so provide its own L<fcntl(2)>-based emulation, by passing 2938the switch C<-Ud_flock> to the F<Configure> program when you configure 2939and build a new Perl. 2940 2941Here's a mailbox appender for BSD systems. 2942 2943 # import LOCK_* and SEEK_END constants 2944 use Fcntl qw(:flock SEEK_END); 2945 2946 sub lock { 2947 my ($fh) = @_; 2948 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; 2949 # and, in case we're running on a very old UNIX 2950 # variant without the modern O_APPEND semantics... 2951 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; 2952 } 2953 2954 sub unlock { 2955 my ($fh) = @_; 2956 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; 2957 } 2958 2959 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") 2960 or die "Can't open mailbox: $!"; 2961 2962 lock($mbox); 2963 print $mbox $msg,"\n\n"; 2964 unlock($mbox); 2965 2966On systems that support a real L<flock(2)>, locks are inherited across 2967L<C<fork>|/fork> calls, whereas those that must resort to the more 2968capricious L<fcntl(2)> function lose their locks, making it seriously 2969harder to write servers. 2970 2971See also L<DB_File> for other L<C<flock>|/flock FILEHANDLE,OPERATION> 2972examples. 2973 2974Portability issues: L<perlport/flock>. 2975 2976=item fork 2977X<fork> X<child> X<parent> 2978 2979=for Pod::Functions create a new process just like this one 2980 2981Does a L<fork(2)> system call to create a new process running the 2982same program at the same point. It returns the child pid to the 2983parent process, C<0> to the child process, or L<C<undef>|/undef EXPR> if 2984the fork is 2985unsuccessful. File descriptors (and sometimes locks on those descriptors) 2986are shared, while everything else is copied. On most systems supporting 2987L<fork(2)>, great care has gone into making it extremely efficient (for 2988example, using copy-on-write technology on data pages), making it the 2989dominant paradigm for multitasking over the last few decades. 2990 2991Perl attempts to flush all files opened for output before forking the 2992child process, but this may not be supported on some platforms (see 2993L<perlport>). To be safe, you may need to set 2994L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) or 2995call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> on 2996any open handles to avoid duplicate output. 2997 2998If you L<C<fork>|/fork> without ever waiting on your children, you will 2999accumulate zombies. On some systems, you can avoid this by setting 3000L<C<$SIG{CHLD}>|perlvar/%SIG> to C<"IGNORE">. See also L<perlipc> for 3001more examples of forking and reaping moribund children. 3002 3003Note that if your forked child inherits system file descriptors like 3004STDIN and STDOUT that are actually connected by a pipe or socket, even 3005if you exit, then the remote server (such as, say, a CGI script or a 3006backgrounded job launched from a remote shell) won't think you're done. 3007You should reopen those to F</dev/null> if it's any issue. 3008 3009On some platforms such as Windows, where the L<fork(2)> system call is 3010not available, Perl can be built to emulate L<C<fork>|/fork> in the Perl 3011interpreter. The emulation is designed, at the level of the Perl 3012program, to be as compatible as possible with the "Unix" L<fork(2)>. 3013However it has limitations that have to be considered in code intended 3014to be portable. See L<perlfork> for more details. 3015 3016Portability issues: L<perlport/fork>. 3017 3018=item format 3019X<format> 3020 3021=for Pod::Functions declare a picture format with use by the write() function 3022 3023Declare a picture format for use by the L<C<write>|/write FILEHANDLE> 3024function. For example: 3025 3026 format Something = 3027 Test: @<<<<<<<< @||||| @>>>>> 3028 $str, $%, '$' . int($num) 3029 . 3030 3031 $str = "widget"; 3032 $num = $cost/$quantity; 3033 $~ = 'Something'; 3034 write; 3035 3036See L<perlform> for many details and examples. 3037 3038=item formline PICTURE,LIST 3039X<formline> 3040 3041=for Pod::Functions internal function used for formats 3042 3043This is an internal function used by L<C<format>|/format>s, though you 3044may call it, too. It formats (see L<perlform>) a list of values 3045according to the contents of PICTURE, placing the output into the format 3046output accumulator, L<C<$^A>|perlvar/$^A> (or C<$ACCUMULATOR> in 3047L<English>). Eventually, when a L<C<write>|/write FILEHANDLE> is done, 3048the contents of L<C<$^A>|perlvar/$^A> are written to some filehandle. 3049You could also read L<C<$^A>|perlvar/$^A> and then set 3050L<C<$^A>|perlvar/$^A> back to C<"">. Note that a format typically does 3051one L<C<formline>|/formline PICTURE,LIST> per line of form, but the 3052L<C<formline>|/formline PICTURE,LIST> function itself doesn't care how 3053many newlines are embedded in the PICTURE. This means that the C<~> and 3054C<~~> tokens treat the entire PICTURE as a single line. You may 3055therefore need to use multiple formlines to implement a single record 3056format, just like the L<C<format>|/format> compiler. 3057 3058Be careful if you put double quotes around the picture, because an C<@> 3059character may be taken to mean the beginning of an array name. 3060L<C<formline>|/formline PICTURE,LIST> always returns true. See 3061L<perlform> for other examples. 3062 3063If you are trying to use this instead of L<C<write>|/write FILEHANDLE> 3064to capture the output, you may find it easier to open a filehandle to a 3065scalar (C<< open my $fh, ">", \$output >>) and write to that instead. 3066 3067=item getc FILEHANDLE 3068X<getc> X<getchar> X<character> X<file, read> 3069 3070=item getc 3071 3072=for Pod::Functions get the next character from the filehandle 3073 3074Returns the next character from the input file attached to FILEHANDLE, 3075or the undefined value at end of file or if there was an error (in 3076the latter case L<C<$!>|perlvar/$!> is set). If FILEHANDLE is omitted, 3077reads from 3078STDIN. This is not particularly efficient. However, it cannot be 3079used by itself to fetch single characters without waiting for the user 3080to hit enter. For that, try something more like: 3081 3082 if ($BSD_STYLE) { 3083 system "stty cbreak </dev/tty >/dev/tty 2>&1"; 3084 } 3085 else { 3086 system "stty", '-icanon', 'eol', "\001"; 3087 } 3088 3089 my $key = getc(STDIN); 3090 3091 if ($BSD_STYLE) { 3092 system "stty -cbreak </dev/tty >/dev/tty 2>&1"; 3093 } 3094 else { 3095 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL 3096 } 3097 print "\n"; 3098 3099Determination of whether C<$BSD_STYLE> should be set is left as an 3100exercise to the reader. 3101 3102The L<C<POSIX::getattr>|POSIX/C<getattr>> function can do this more 3103portably on systems purporting POSIX compliance. See also the 3104L<C<Term::ReadKey>|Term::ReadKey> module on CPAN. 3105 3106=item getlogin 3107X<getlogin> X<login> 3108 3109=for Pod::Functions return who logged in at this tty 3110 3111This implements the C library function of the same name, which on most 3112systems returns the current login from F</etc/utmp>, if any. If it 3113returns the empty string, use L<C<getpwuid>|/getpwuid UID>. 3114 3115 my $login = getlogin || getpwuid($<) || "Kilroy"; 3116 3117Do not consider L<C<getlogin>|/getlogin> for authentication: it is not 3118as secure as L<C<getpwuid>|/getpwuid UID>. 3119 3120Portability issues: L<perlport/getlogin>. 3121 3122=item getpeername SOCKET 3123X<getpeername> X<peer> 3124 3125=for Pod::Functions find the other end of a socket connection 3126 3127Returns the packed sockaddr address of the other end of the SOCKET 3128connection. 3129 3130 use Socket; 3131 my $hersockaddr = getpeername($sock); 3132 my ($port, $iaddr) = sockaddr_in($hersockaddr); 3133 my $herhostname = gethostbyaddr($iaddr, AF_INET); 3134 my $herstraddr = inet_ntoa($iaddr); 3135 3136=item getpgrp PID 3137X<getpgrp> X<group> 3138 3139=for Pod::Functions get process group 3140 3141Returns the current process group for the specified PID. Use 3142a PID of C<0> to get the current process group for the 3143current process. Will raise an exception if used on a machine that 3144doesn't implement L<getpgrp(2)>. If PID is omitted, returns the process 3145group of the current process. 3146 3147Some very old machines may not support C<PID != 0> and will throw an 3148exception if C<PID != 0>. 3149 3150Portability issues: L<perlport/getpgrp>. 3151 3152=item getppid 3153X<getppid> X<parent> X<pid> 3154 3155=for Pod::Functions get parent process ID 3156 3157Returns the process id of the parent process. 3158 3159Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work 3160around non-POSIX thread semantics the minority of Linux systems (and 3161Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation 3162has since been removed. See the documentation for L<$$|perlvar/$$> for 3163details. 3164 3165Portability issues: L<perlport/getppid>. 3166 3167=item getpriority WHICH,WHO 3168X<getpriority> X<priority> X<nice> 3169 3170=for Pod::Functions get current nice value 3171 3172Returns the current priority for a process, a process group, or a user. 3173(See L<getpriority(2)>.) Will raise a fatal exception if used on a 3174machine that doesn't implement L<getpriority(2)>. 3175 3176C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER> 3177imported from L<POSIX/RESOURCE CONSTANTS>. 3178 3179Portability issues: L<perlport/getpriority>. 3180 3181=item getpwnam NAME 3182X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> 3183X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> 3184X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> 3185X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> 3186X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> 3187X<endnetent> X<endprotoent> X<endservent> 3188 3189=for Pod::Functions get passwd record given user login name 3190 3191=item getgrnam NAME 3192 3193=for Pod::Functions get group record given group name 3194 3195=item gethostbyname NAME 3196 3197=for Pod::Functions get host record given name 3198 3199=item getnetbyname NAME 3200 3201=for Pod::Functions get networks record given name 3202 3203=item getprotobyname NAME 3204 3205=for Pod::Functions get protocol record given name 3206 3207=item getpwuid UID 3208 3209=for Pod::Functions get passwd record given user ID 3210 3211=item getgrgid GID 3212 3213=for Pod::Functions get group record given group user ID 3214 3215=item getservbyname NAME,PROTO 3216 3217=for Pod::Functions get services record given its name 3218 3219=item gethostbyaddr ADDR,ADDRTYPE 3220 3221=for Pod::Functions get host record given its address 3222 3223=item getnetbyaddr ADDR,ADDRTYPE 3224 3225=for Pod::Functions get network record given its address 3226 3227=item getprotobynumber NUMBER 3228 3229=for Pod::Functions get protocol record numeric protocol 3230 3231=item getservbyport PORT,PROTO 3232 3233=for Pod::Functions get services record given numeric port 3234 3235=item getpwent 3236 3237=for Pod::Functions get next passwd record 3238 3239=item getgrent 3240 3241=for Pod::Functions get next group record 3242 3243=item gethostent 3244 3245=for Pod::Functions get next hosts record 3246 3247=item getnetent 3248 3249=for Pod::Functions get next networks record 3250 3251=item getprotoent 3252 3253=for Pod::Functions get next protocols record 3254 3255=item getservent 3256 3257=for Pod::Functions get next services record 3258 3259=item setpwent 3260 3261=for Pod::Functions prepare passwd file for use 3262 3263=item setgrent 3264 3265=for Pod::Functions prepare group file for use 3266 3267=item sethostent STAYOPEN 3268 3269=for Pod::Functions prepare hosts file for use 3270 3271=item setnetent STAYOPEN 3272 3273=for Pod::Functions prepare networks file for use 3274 3275=item setprotoent STAYOPEN 3276 3277=for Pod::Functions prepare protocols file for use 3278 3279=item setservent STAYOPEN 3280 3281=for Pod::Functions prepare services file for use 3282 3283=item endpwent 3284 3285=for Pod::Functions be done using passwd file 3286 3287=item endgrent 3288 3289=for Pod::Functions be done using group file 3290 3291=item endhostent 3292 3293=for Pod::Functions be done using hosts file 3294 3295=item endnetent 3296 3297=for Pod::Functions be done using networks file 3298 3299=item endprotoent 3300 3301=for Pod::Functions be done using protocols file 3302 3303=item endservent 3304 3305=for Pod::Functions be done using services file 3306 3307These routines are the same as their counterparts in the 3308system C library. In list context, the return values from the 3309various get routines are as follows: 3310 3311 # 0 1 2 3 4 3312 my ( $name, $passwd, $gid, $members ) = getgr* 3313 my ( $name, $aliases, $addrtype, $net ) = getnet* 3314 my ( $name, $aliases, $port, $proto ) = getserv* 3315 my ( $name, $aliases, $proto ) = getproto* 3316 my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost* 3317 my ( $name, $passwd, $uid, $gid, $quota, 3318 $comment, $gcos, $dir, $shell, $expire ) = getpw* 3319 # 5 6 7 8 9 3320 3321(If the entry doesn't exist, the return value is a single meaningless true 3322value.) 3323 3324The exact meaning of the $gcos field varies but it usually contains 3325the real name of the user (as opposed to the login name) and other 3326information pertaining to the user. Beware, however, that in many 3327system users are able to change this information and therefore it 3328cannot be trusted and therefore the $gcos is tainted (see 3329L<perlsec>). The $passwd and $shell, user's encrypted password and 3330login shell, are also tainted, for the same reason. 3331 3332In scalar context, you get the name, unless the function was a 3333lookup by name, in which case you get the other thing, whatever it is. 3334(If the entry doesn't exist you get the undefined value.) For example: 3335 3336 my $uid = getpwnam($name); 3337 my $name = getpwuid($num); 3338 my $name = getpwent(); 3339 my $gid = getgrnam($name); 3340 my $name = getgrgid($num); 3341 my $name = getgrent(); 3342 # etc. 3343 3344In I<getpw*()> the fields $quota, $comment, and $expire are special 3345in that they are unsupported on many systems. If the 3346$quota is unsupported, it is an empty scalar. If it is supported, it 3347usually encodes the disk quota. If the $comment field is unsupported, 3348it is an empty scalar. If it is supported it usually encodes some 3349administrative comment about the user. In some systems the $quota 3350field may be $change or $age, fields that have to do with password 3351aging. In some systems the $comment field may be $class. The $expire 3352field, if present, encodes the expiration period of the account or the 3353password. For the availability and the exact meaning of these fields 3354in your system, please consult L<getpwnam(3)> and your system's 3355F<pwd.h> file. You can also find out from within Perl what your 3356$quota and $comment fields mean and whether you have the $expire field 3357by using the L<C<Config>|Config> module and the values C<d_pwquota>, C<d_pwage>, 3358C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password 3359files are supported only if your vendor has implemented them in the 3360intuitive fashion that calling the regular C library routines gets the 3361shadow versions if you're running under privilege or if there exists 3362the L<shadow(3)> functions as found in System V (this includes Solaris 3363and Linux). Those systems that implement a proprietary shadow password 3364facility are unlikely to be supported. 3365 3366The $members value returned by I<getgr*()> is a space-separated list of 3367the login names of the members of the group. 3368 3369For the I<gethost*()> functions, if the C<h_errno> variable is supported in 3370C, it will be returned to you via L<C<$?>|perlvar/$?> if the function 3371call fails. The 3372C<@addrs> value returned by a successful call is a list of raw 3373addresses returned by the corresponding library call. In the 3374Internet domain, each address is four bytes long; you can unpack it 3375by saying something like: 3376 3377 my ($w,$x,$y,$z) = unpack('W4',$addr[0]); 3378 3379The Socket library makes this slightly easier: 3380 3381 use Socket; 3382 my $iaddr = inet_aton("127.1"); # or whatever address 3383 my $name = gethostbyaddr($iaddr, AF_INET); 3384 3385 # or going the other way 3386 my $straddr = inet_ntoa($iaddr); 3387 3388In the opposite way, to resolve a hostname to the IP address 3389you can write this: 3390 3391 use Socket; 3392 my $packed_ip = gethostbyname("www.perl.org"); 3393 my $ip_address; 3394 if (defined $packed_ip) { 3395 $ip_address = inet_ntoa($packed_ip); 3396 } 3397 3398Make sure L<C<gethostbyname>|/gethostbyname NAME> is called in SCALAR 3399context and that its return value is checked for definedness. 3400 3401The L<C<getprotobynumber>|/getprotobynumber NUMBER> function, even 3402though it only takes one argument, has the precedence of a list 3403operator, so beware: 3404 3405 getprotobynumber $number eq 'icmp' # WRONG 3406 getprotobynumber($number eq 'icmp') # actually means this 3407 getprotobynumber($number) eq 'icmp' # better this way 3408 3409If you get tired of remembering which element of the return list 3410contains which return value, by-name interfaces are provided in standard 3411modules: L<C<File::stat>|File::stat>, L<C<Net::hostent>|Net::hostent>, 3412L<C<Net::netent>|Net::netent>, L<C<Net::protoent>|Net::protoent>, 3413L<C<Net::servent>|Net::servent>, L<C<Time::gmtime>|Time::gmtime>, 3414L<C<Time::localtime>|Time::localtime>, and 3415L<C<User::grent>|User::grent>. These override the normal built-ins, 3416supplying versions that return objects with the appropriate names for 3417each field. For example: 3418 3419 use File::stat; 3420 use User::pwent; 3421 my $is_theirs = (stat($filename)->uid == getpwnam($whoever)->uid); 3422 3423Even though it looks as though they're the same method calls (uid), 3424they aren't, because a C<File::stat> object is different from 3425a C<User::pwent> object. 3426 3427Many of these functions are not safe in a multi-threaded environment 3428where more than one thread can be using them. In particular, functions 3429like C<getpwent()> iterate per-process and not per-thread, so if two 3430threads are simultaneously iterating, neither will get all the records. 3431 3432Some systems have thread-safe versions of some of the functions, such as 3433C<getpwnam_r()> instead of C<getpwnam()>. There, Perl automatically and 3434invisibly substitutes the thread-safe version, without notice. This 3435means that code that safely runs on some systems can fail on others that 3436lack the thread-safe versions. 3437 3438Portability issues: L<perlport/getpwnam> to L<perlport/endservent>. 3439 3440=item getsockname SOCKET 3441X<getsockname> 3442 3443=for Pod::Functions retrieve the sockaddr for a given socket 3444 3445Returns the packed sockaddr address of this end of the SOCKET connection, 3446in case you don't know the address because you have several different 3447IPs that the connection might have come in on. 3448 3449 use Socket; 3450 my $mysockaddr = getsockname($sock); 3451 my ($port, $myaddr) = sockaddr_in($mysockaddr); 3452 printf "Connect to %s [%s]\n", 3453 scalar gethostbyaddr($myaddr, AF_INET), 3454 inet_ntoa($myaddr); 3455 3456=item getsockopt SOCKET,LEVEL,OPTNAME 3457X<getsockopt> 3458 3459=for Pod::Functions get socket options on a given socket 3460 3461Queries the option named OPTNAME associated with SOCKET at a given LEVEL. 3462Options may exist at multiple protocol levels depending on the socket 3463type, but at least the uppermost socket level SOL_SOCKET (defined in the 3464L<C<Socket>|Socket> module) will exist. To query options at another 3465level the protocol number of the appropriate protocol controlling the 3466option should be supplied. For example, to indicate that an option is 3467to be interpreted by the TCP protocol, LEVEL should be set to the 3468protocol number of TCP, which you can get using 3469L<C<getprotobyname>|/getprotobyname NAME>. 3470 3471The function returns a packed string representing the requested socket 3472option, or L<C<undef>|/undef EXPR> on error, with the reason for the 3473error placed in L<C<$!>|perlvar/$!>. Just what is in the packed string 3474depends on LEVEL and OPTNAME; consult L<getsockopt(2)> for details. A 3475common case is that the option is an integer, in which case the result 3476is a packed integer, which you can decode using 3477L<C<unpack>|/unpack TEMPLATE,EXPR> with the C<i> (or C<I>) format. 3478 3479Here's an example to test whether Nagle's algorithm is enabled on a socket: 3480 3481 use Socket qw(:all); 3482 3483 defined(my $tcp = getprotobyname("tcp")) 3484 or die "Could not determine the protocol number for tcp"; 3485 # my $tcp = IPPROTO_TCP; # Alternative 3486 my $packed = getsockopt($socket, $tcp, TCP_NODELAY) 3487 or die "getsockopt TCP_NODELAY: $!"; 3488 my $nodelay = unpack("I", $packed); 3489 print "Nagle's algorithm is turned ", 3490 $nodelay ? "off\n" : "on\n"; 3491 3492Portability issues: L<perlport/getsockopt>. 3493 3494=item glob EXPR 3495X<glob> X<wildcard> X<filename, expansion> X<expand> 3496 3497=item glob 3498 3499=for Pod::Functions expand filenames using wildcards 3500 3501In list context, returns a (possibly empty) list of filename expansions on 3502the value of EXPR such as the Unix shell Bash would do. In 3503scalar context, glob iterates through such filename expansions, returning 3504L<C<undef>|/undef EXPR> when the list is exhausted. If EXPR is omitted, 3505L<C<$_>|perlvar/$_> is used. 3506 3507 # List context 3508 my @txt_files = glob("*.txt"); 3509 my @perl_files = glob("*.pl *.pm"); 3510 3511 # Scalar context 3512 while (my $file = glob("*.mp3")) { 3513 # Do stuff 3514 } 3515 3516Glob also supports an alternate syntax using C<< < >> C<< > >> as 3517delimiters. While this syntax is supported, it is recommended that you 3518use C<glob> instead as it is more readable and searchable. 3519 3520 my @txt_files = <"*.txt">; 3521 3522If you need case insensitive file globbing that can be achieved using the 3523C<:nocase> parameter of the L<C<bsd_glob>|File::Glob/C<bsd_glob>> module. 3524 3525 use File::Glob qw(:globally :nocase); 3526 3527 my @txt = glob("readme*"); # README readme.txt Readme.md 3528 3529Note that L<C<glob>|/glob EXPR> splits its arguments on whitespace and 3530treats 3531each segment as separate pattern. As such, C<glob("*.c *.h")> 3532matches all files with a F<.c> or F<.h> extension. The expression 3533C<glob(".* *")> matches all files in the current working directory. 3534If you want to glob filenames that might contain whitespace, you'll 3535have to use extra quotes around the spacey filename to protect it. 3536For example, to glob filenames that have an C<e> followed by a space 3537followed by an C<f>, use one of: 3538 3539 my @spacies = <"*e f*">; 3540 my @spacies = glob('"*e f*"'); 3541 my @spacies = glob(q("*e f*")); 3542 3543If you had to get a variable through, you could do this: 3544 3545 my @spacies = glob("'*${var}e f*'"); 3546 my @spacies = glob(qq("*${var}e f*")); 3547 3548If non-empty braces are the only wildcard characters used in the 3549L<C<glob>|/glob EXPR>, no filenames are matched, but potentially many 3550strings are returned. For example, this produces nine strings, one for 3551each pairing of fruits and colors: 3552 3553 my @many = glob("{apple,tomato,cherry}={green,yellow,red}"); 3554 3555This operator is implemented using the standard C<File::Glob> extension. 3556See L<C<bsd_glob>|File::Glob/C<bsd_glob>> for details, including 3557L<C<bsd_glob>|File::Glob/C<bsd_glob>>, which does not treat whitespace 3558as a pattern separator. 3559 3560If a C<glob> expression is used as the condition of a C<while> or C<for> 3561loop, then it will be implicitly assigned to C<$_>. If either a C<glob> 3562expression or an explicit assignment of a C<glob> expression to a scalar 3563is used as a C<while>/C<for> condition, then the condition actually 3564tests for definedness of the expression's value, not for its regular 3565truth value. 3566 3567Internal implemenation details: 3568 3569This is the internal function implementing the C<< <*.c> >> operator, 3570but you can use it directly. The C<< <*.c> >> operator is discussed in 3571more detail in L<perlop/"I/O Operators">. 3572 3573Portability issues: L<perlport/glob>. 3574 3575=item gmtime EXPR 3576X<gmtime> X<UTC> X<Greenwich> 3577 3578=item gmtime 3579 3580=for Pod::Functions convert UNIX time into record or string using Greenwich time 3581 3582Works just like L<C<localtime>|/localtime EXPR>, but the returned values 3583are localized for the standard Greenwich time zone. 3584 3585Note: When called in list context, $isdst, the last value 3586returned by gmtime, is always C<0>. There is no 3587Daylight Saving Time in GMT. 3588 3589Portability issues: L<perlport/gmtime>. 3590 3591=item goto LABEL 3592X<goto> X<jump> X<jmp> 3593 3594=item goto EXPR 3595 3596=item goto &NAME 3597 3598=for Pod::Functions create spaghetti code 3599 3600The C<goto LABEL> form finds the statement labeled with LABEL and 3601resumes execution there. It can't be used to get out of a block or 3602subroutine given to L<C<sort>|/sort SUBNAME LIST>. It can be used to go 3603almost anywhere else within the dynamic scope, including out of 3604subroutines, but it's usually better to use some other construct such as 3605L<C<last>|/last LABEL> or L<C<die>|/die LIST>. The author of Perl has 3606never felt the need to use this form of L<C<goto>|/goto LABEL> (in Perl, 3607that is; C is another matter). (The difference is that C does not offer 3608named loops combined with loop control. Perl does, and this replaces 3609most structured uses of L<C<goto>|/goto LABEL> in other languages.) 3610 3611The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or 3612a label name. If it evaluates to a code reference, it will be handled 3613like C<goto &NAME>, below. This is especially useful for implementing 3614tail recursion via C<goto __SUB__>. 3615 3616If the expression evaluates to a label name, its scope will be resolved 3617dynamically. This allows for computed L<C<goto>|/goto LABEL>s per 3618FORTRAN, but isn't necessarily recommended if you're optimizing for 3619maintainability: 3620 3621 goto ("FOO", "BAR", "GLARCH")[$i]; 3622 3623As shown in this example, C<goto EXPR> is exempt from the "looks like a 3624function" rule. A pair of parentheses following it does not (necessarily) 3625delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>. 3626Also, unlike most named operators, this has the same precedence as 3627assignment. 3628 3629Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is 3630deprecated and will issue a warning; it will become a fatal error in 3631Perl 5.42. While still available, it may not be used to 3632go into any construct that requires initialization, such as a 3633subroutine, a C<foreach> loop, or a C<given> 3634block. In general, it may not be used to jump into the parameter 3635of a binary or list operator, but it may be used to jump into the 3636I<first> parameter of a binary operator. (The C<=> 3637assignment operator's "first" operand is its right-hand 3638operand.) It also can't be used to go into a 3639construct that is optimized away. 3640 3641The C<goto &NAME> form is quite different from the other forms of 3642L<C<goto>|/goto LABEL>. In fact, it isn't a goto in the normal sense at 3643all, and doesn't have the stigma associated with other gotos. Instead, 3644it exits the current subroutine (losing any changes set by 3645L<C<local>|/local EXPR>) and immediately calls in its place the named 3646subroutine using the current value of L<C<@_>|perlvar/@_>. This is used 3647by C<AUTOLOAD> subroutines that wish to load another subroutine and then 3648pretend that the other subroutine had been called in the first place 3649(except that any modifications to L<C<@_>|perlvar/@_> in the current 3650subroutine are propagated to the other subroutine.) After the 3651L<C<goto>|/goto LABEL>, not even L<C<caller>|/caller EXPR> will be able 3652to tell that this routine was called first. 3653 3654NAME needn't be the name of a subroutine; it can be a scalar variable 3655containing a code reference or a block that evaluates to a code 3656reference. 3657 3658=item grep BLOCK LIST 3659X<grep> 3660 3661=item grep EXPR,LIST 3662 3663=for Pod::Functions locate elements in a list test true against a given criterion 3664 3665This is similar in spirit to, but not the same as, L<grep(1)> and its 3666relatives. In particular, it is not limited to using regular expressions. 3667 3668Evaluates the BLOCK or EXPR for each element of LIST (locally setting 3669L<C<$_>|perlvar/$_> to each element) and returns the list value 3670consisting of those 3671elements for which the expression evaluated to true. In scalar 3672context, returns the number of times the expression was true. 3673 3674 my @foo = grep(!/^#/, @bar); # weed out comments 3675 3676or equivalently, 3677 3678 my @foo = grep {!/^#/} @bar; # weed out comments 3679 3680Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can 3681be used to 3682modify the elements of the LIST. While this is useful and supported, 3683it can cause bizarre results if the elements of LIST are not variables. 3684Similarly, grep returns aliases into the original list, much as a for 3685loop's index variable aliases the list elements. That is, modifying an 3686element of a list returned by grep (for example, in a C<foreach>, 3687L<C<map>|/map BLOCK LIST> or another L<C<grep>|/grep BLOCK LIST>) 3688actually modifies the element in the original list. 3689This is usually something to be avoided when writing clear code. 3690 3691See also L<C<map>|/map BLOCK LIST> for a list composed of the results of 3692the BLOCK or EXPR. 3693 3694=item hex EXPR 3695X<hex> X<hexadecimal> 3696 3697=item hex 3698 3699=for Pod::Functions convert a hexadecimal string to a number 3700 3701Interprets EXPR as a hex string and returns the corresponding numeric value. 3702If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 3703 3704 print hex '0xAf'; # prints '175' 3705 print hex 'aF'; # same 3706 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/ 3707 3708A hex string consists of hex digits and an optional C<0x> or C<x> prefix. 3709Each hex digit may be preceded by a single underscore, which will be ignored. 3710Any other character triggers a warning and causes the rest of the string 3711to be ignored (even leading whitespace, unlike L<C<oct>|/oct EXPR>). 3712Only integers can be represented, and integer overflow triggers a warning. 3713 3714To convert strings that might start with any of C<0>, C<0x>, or C<0b>, 3715see L<C<oct>|/oct EXPR>. To present something as hex, look into 3716L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, 3717L<C<sprintf>|/sprintf FORMAT, LIST>, and 3718L<C<unpack>|/unpack TEMPLATE,EXPR>. 3719 3720=item import LIST 3721X<import> 3722 3723=for Pod::Functions patch a module's namespace into your own 3724 3725There is no builtin L<C<import>|/import LIST> function. It is just an 3726ordinary method (subroutine) defined (or inherited) by modules that wish 3727to export names to another module. The 3728L<C<use>|/use Module VERSION LIST> function calls the 3729L<C<import>|/import LIST> method for the package used. See also 3730L<C<use>|/use Module VERSION LIST>, L<perlmod>, and L<Exporter>. 3731 3732=item index STR,SUBSTR,POSITION 3733X<index> X<indexOf> X<InStr> 3734 3735=item index STR,SUBSTR 3736 3737=for Pod::Functions find a substring within a string 3738 3739The index function searches for one string within another, but without 3740the wildcard-like behavior of a full regular-expression pattern match. 3741It returns the position of the first occurrence of SUBSTR in STR at 3742or after POSITION. If POSITION is omitted, starts searching from the 3743beginning of the string. POSITION before the beginning of the string 3744or after its end is treated as if it were the beginning or the end, 3745respectively. POSITION and the return value are based at zero. 3746If the substring is not found, L<C<index>|/index STR,SUBSTR,POSITION> 3747returns -1. 3748 3749Find characters or strings: 3750 3751 index("Perl is great", "P"); # Returns 0 3752 index("Perl is great", "g"); # Returns 8 3753 index("Perl is great", "great"); # Also returns 8 3754 3755Attempting to find something not there: 3756 3757 index("Perl is great", "Z"); # Returns -1 (not found) 3758 3759Using an offset to find the I<second> occurrence: 3760 3761 index("Perl is great", "e", 5); # Returns 10 3762 3763=item int EXPR 3764X<int> X<integer> X<truncate> X<trunc> X<floor> 3765 3766=item int 3767 3768=for Pod::Functions get the integer portion of a number 3769 3770Returns the integer portion of EXPR. If EXPR is omitted, uses 3771L<C<$_>|perlvar/$_>. 3772You should not use this function for rounding: one because it truncates 3773towards C<0>, and two because machine representations of floating-point 3774numbers can sometimes produce counterintuitive results. For example, 3775C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's 3776because it's really more like -268.99999999999994315658 instead. Usually, 3777the L<C<sprintf>|/sprintf FORMAT, LIST>, 3778L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, or the 3779L<C<POSIX::floor>|POSIX/C<floor>> and L<C<POSIX::ceil>|POSIX/C<ceil>> 3780functions will serve you better than will L<C<int>|/int EXPR>. 3781 3782=item ioctl FILEHANDLE,FUNCTION,SCALAR 3783X<ioctl> 3784 3785=for Pod::Functions system-dependent device control system call 3786 3787Implements the L<ioctl(2)> function. You'll probably first have to say 3788 3789 require "sys/ioctl.ph"; # probably in 3790 # $Config{archlib}/sys/ioctl.ph 3791 3792to get the correct function definitions. If F<sys/ioctl.ph> doesn't 3793exist or doesn't have the correct definitions you'll have to roll your 3794own, based on your C header files such as F<< <sys/ioctl.h> >>. 3795(There is a Perl script called B<h2ph> that comes with the Perl kit that 3796may help you in this, but it's nontrivial.) SCALAR will be read and/or 3797written depending on the FUNCTION; a C pointer to the string value of SCALAR 3798will be passed as the third argument of the actual 3799L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> call. (If SCALAR 3800has no string value but does have a numeric value, that value will be 3801passed rather than a pointer to the string value. To guarantee this to be 3802true, add a C<0> to the scalar before using it.) The 3803L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR> 3804functions may be needed to manipulate the values of structures used by 3805L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>. 3806 3807The return value of L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR> (and 3808L<C<fcntl>|/fcntl FILEHANDLE,FUNCTION,SCALAR>) is as follows: 3809 3810 if OS returns: then Perl returns: 3811 -1 undefined value 3812 0 string "0 but true" 3813 anything else that number 3814 3815Thus Perl returns true on success and false on failure, yet you can 3816still easily determine the actual value returned by the operating 3817system: 3818 3819 my $retval = ioctl(...) || -1; 3820 printf "System returned %d\n", $retval; 3821 3822The special string C<"0 but true"> is exempt from 3823L<C<Argument "..." isn't numeric>|perldiag/Argument "%s" isn't numeric%s> 3824L<warnings> on improper numeric conversions. 3825 3826Portability issues: L<perlport/ioctl>. 3827 3828=item join EXPR,LIST 3829X<join> 3830 3831=for Pod::Functions join a list into a string using a separator 3832 3833Joins the separate strings of LIST into a single string with fields 3834separated by the value of EXPR, and returns that new string. Example: 3835 3836 my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); 3837 3838Beware that unlike L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>, 3839L<C<join>|/join EXPR,LIST> doesn't take a pattern as its first argument. 3840Compare L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>. 3841 3842=item keys HASH 3843X<keys> X<key> 3844 3845=item keys ARRAY 3846 3847=for Pod::Functions retrieve list of indices from a hash 3848 3849Called in list context, returns a list consisting of all the keys of the 3850named hash, or in Perl 5.12 or later only, the indices of an array. Perl 3851releases prior to 5.12 will produce a syntax error if you try to use an 3852array argument. In scalar context, returns the number of keys or indices. 3853 3854Hash entries are returned in an apparently random order. The actual random 3855order is specific to a given hash; the exact same series of operations 3856on two hashes may result in a different order for each hash. Any insertion 3857into the hash may change the order, as will any deletion, with the exception 3858that the most recent key returned by L<C<each>|/each HASH> or 3859L<C<keys>|/keys HASH> may be deleted without changing the order. So 3860long as a given hash is unmodified you may rely on 3861L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and L<C<each>|/each 3862HASH> to repeatedly return the same order 3863as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for 3864details on why hash order is randomized. Aside from the guarantees 3865provided here the exact details of Perl's hash algorithm and the hash 3866traversal order are subject to change in any release of Perl. Tied hashes 3867may behave differently to Perl's hashes with respect to changes in order on 3868insertion and deletion of items. 3869 3870As a side effect, calling L<C<keys>|/keys HASH> resets the internal 3871iterator of the HASH or ARRAY (see L<C<each>|/each HASH>) before 3872yielding the keys. In 3873particular, calling L<C<keys>|/keys HASH> in void context resets the 3874iterator with no other overhead. 3875 3876Here is yet another way to print your environment: 3877 3878 my @keys = keys %ENV; 3879 my @values = values %ENV; 3880 while (@keys) { 3881 print pop(@keys), '=', pop(@values), "\n"; 3882 } 3883 3884or how about sorted by key: 3885 3886 foreach my $key (sort(keys %ENV)) { 3887 print $key, '=', $ENV{$key}, "\n"; 3888 } 3889 3890The returned values are copies of the original keys in the hash, so 3891modifying them will not affect the original hash. Compare 3892L<C<values>|/values HASH>. 3893 3894To sort a hash by value, you'll need to use a 3895L<C<sort>|/sort SUBNAME LIST> function. Here's a descending numeric 3896sort of a hash by its values: 3897 3898 foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { 3899 printf "%4d %s\n", $hash{$key}, $key; 3900 } 3901 3902Used as an lvalue, L<C<keys>|/keys HASH> allows you to increase the 3903number of hash buckets 3904allocated for the given hash. This can gain you a measure of efficiency if 3905you know the hash is going to get big. (This is similar to pre-extending 3906an array by assigning a larger number to $#array.) If you say 3907 3908 keys %hash = 200; 3909 3910then C<%hash> will have at least 200 buckets allocated for it--256 of them, 3911in fact, since it rounds up to the next power of two. These 3912buckets will be retained even if you do C<%hash = ()>, use C<undef 3913%hash> if you want to free the storage while C<%hash> is still in scope. 3914You can't shrink the number of buckets allocated for the hash using 3915L<C<keys>|/keys HASH> in this way (but you needn't worry about doing 3916this by accident, as trying has no effect). C<keys @array> in an lvalue 3917context is a syntax error. 3918 3919Starting with Perl 5.14, an experimental feature allowed 3920L<C<keys>|/keys HASH> to take a scalar expression. This experiment has 3921been deemed unsuccessful, and was removed as of Perl 5.24. 3922 3923To avoid confusing would-be users of your code who are running earlier 3924versions of Perl with mysterious syntax errors, put this sort of thing at 3925the top of your file to signal that your code will work I<only> on Perls of 3926a recent vintage: 3927 3928 use v5.12; # so keys/values/each work on arrays 3929 3930See also L<C<each>|/each HASH>, L<C<values>|/values HASH>, and 3931L<C<sort>|/sort SUBNAME LIST>. 3932 3933=item kill SIGNAL, LIST 3934 3935=item kill SIGNAL 3936X<kill> X<signal> 3937 3938=for Pod::Functions send a signal to a process or process group 3939 3940Sends a signal to a list of processes. Returns the number of arguments 3941that were successfully used to signal (which is not necessarily the same 3942as the number of processes actually killed, e.g. where a process group is 3943killed). 3944 3945 my $cnt = kill 'HUP', $child1, $child2; 3946 kill 'KILL', @goners; 3947 3948SIGNAL may be either a signal name (a string) or a signal number. A signal 3949name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the 3950same signal. The string form of SIGNAL is recommended for portability because 3951the same signal may have different numbers in different operating systems. 3952 3953A list of signal names supported by the current platform can be found in 3954C<$Config{sig_name}>, which is provided by the L<C<Config>|Config> 3955module. See L<Config> for more details. 3956 3957A negative signal name is the same as a negative signal number, killing process 3958groups instead of processes. For example, C<kill '-KILL', $pgrp> and 3959C<kill -9, $pgrp> will send C<SIGKILL> to 3960the entire process group specified. That 3961means you usually want to use positive not negative signals. 3962 3963If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>), 3964no signal is sent to the process, but L<C<kill>|/kill SIGNAL, LIST> 3965checks whether it's I<possible> to send a signal to it 3966(that means, to be brief, that the process is owned by the same user, or we are 3967the super-user). This is useful to check that a child process is still 3968alive (even if only as a zombie) and hasn't changed its UID. See 3969L<perlport> for notes on the portability of this construct. 3970 3971The behavior of kill when a I<PROCESS> number is zero or negative depends on 3972the operating system. For example, on POSIX-conforming systems, zero will 3973signal the current process group, -1 will signal all processes, and any 3974other negative PROCESS number will act as a negative signal number and 3975kill the entire process group specified. 3976 3977If both the SIGNAL and the PROCESS are negative, the results are undefined. 3978A warning may be produced in a future version. 3979 3980See L<perlipc/"Signals"> for more details. 3981 3982On some platforms such as Windows where the L<fork(2)> system call is not 3983available, Perl can be built to emulate L<C<fork>|/fork> at the 3984interpreter level. 3985This emulation has limitations related to kill that have to be considered, 3986for code running on Windows and in code intended to be portable. 3987 3988See L<perlfork> for more details. 3989 3990If there is no I<LIST> of processes, no signal is sent, and the return 3991value is 0. This form is sometimes used, however, because it causes 3992tainting checks to be run, if your perl support taint checks. But see 3993L<perlsec/Laundering and Detecting Tainted Data>. 3994 3995Portability issues: L<perlport/kill>. 3996 3997=item last LABEL 3998X<last> X<break> 3999 4000=item last EXPR 4001 4002=item last 4003 4004=for Pod::Functions exit a block prematurely 4005 4006The L<C<last>|/last LABEL> command is like the C<break> statement in C 4007(as used in 4008loops); it immediately exits the loop in question. If the LABEL is 4009omitted, the command refers to the innermost enclosing 4010loop. The C<last EXPR> form, available starting in Perl 40115.18.0, allows a label name to be computed at run time, 4012and is otherwise identical to C<last LABEL>. The 4013L<C<continue>|/continue BLOCK> block, if any, is not executed: 4014 4015 LINE: while (<STDIN>) { 4016 last LINE if /^$/; # exit when done with header 4017 #... 4018 } 4019 4020L<C<last>|/last LABEL> cannot return a value from a block that typically 4021returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform 4022its flow control behavior, which precludes any return value. It should not be 4023used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST> 4024operation. 4025 4026Note that a block by itself is semantically identical to a loop 4027that executes once. Thus L<C<last>|/last LABEL> can be used to effect 4028an early exit out of such a block. 4029 4030See also L<C<continue>|/continue BLOCK> for an illustration of how 4031L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and 4032L<C<redo>|/redo LABEL> work. 4033 4034Unlike most named operators, this has the same precedence as assignment. 4035It is also exempt from the looks-like-a-function rule, so 4036C<last ("foo")."bar"> will cause "bar" to be part of the argument to 4037L<C<last>|/last LABEL>. 4038 4039=item lc EXPR 4040X<lc> X<lowercase> 4041 4042=item lc 4043 4044=for Pod::Functions return lower-case version of a string 4045 4046Returns a lowercased version of EXPR. If EXPR is omitted, uses 4047L<C<$_>|perlvar/$_>. 4048 4049 my $str = lc("Perl is GREAT"); # "perl is great" 4050 4051What gets returned depends on several factors: 4052 4053=over 4054 4055=item If C<use bytes> is in effect: 4056 4057The results follow ASCII rules. Only the characters C<A-Z> change, 4058to C<a-z> respectively. 4059 4060=item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect: 4061 4062Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode 4063rules for the remaining code points (this last can only happen if 4064the UTF8 flag is also set). See L<perllocale>. 4065 4066Starting in v5.20, Perl uses full Unicode rules if the locale is 4067UTF-8. Otherwise, there is a deficiency in this scheme, which is that 4068case changes that cross the 255/256 4069boundary are not well-defined. For example, the lower case of LATIN CAPITAL 4070LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII 4071platforms). But under C<use locale> (prior to v5.20 or not a UTF-8 4072locale), the lower case of U+1E9E is 4073itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the 4074current locale, and Perl has no way of knowing if that character even 4075exists in the locale, much less what code point it is. Perl returns 4076a result that is above 255 (almost always the input character unchanged), 4077for all instances (and there aren't many) where the 255/256 boundary 4078would otherwise be crossed; and starting in v5.22, it raises a 4079L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning. 4080 4081=item Otherwise, If EXPR has the UTF8 flag set: 4082 4083Unicode rules are used for the case change. 4084 4085=item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect: 4086 4087Unicode rules are used for the case change. 4088 4089=item Otherwise: 4090 4091ASCII rules are used for the case change. The lowercase of any character 4092outside the ASCII range is the character itself. 4093 4094=back 4095 4096B<Note:> This is the internal function implementing the 4097L<C<\L>|perlop/"Quote and Quote-like Operators"> escape in double-quoted 4098strings. 4099 4100 my $str = "Perl is \LGREAT\E"; # "Perl is great" 4101 4102=item lcfirst EXPR 4103X<lcfirst> X<lowercase> 4104 4105=item lcfirst 4106 4107=for Pod::Functions return a string with just the next letter in lower case 4108 4109Returns the value of EXPR with the first character lowercased. This 4110is the internal function implementing the C<\l> escape in 4111double-quoted strings. 4112 4113If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 4114 4115This function behaves the same way under various pragmas, such as in a locale, 4116as L<C<lc>|/lc EXPR> does. 4117 4118=item length EXPR 4119X<length> X<size> 4120 4121=item length 4122 4123=for Pod::Functions return the number of characters in a string 4124 4125Returns the length in I<characters> of the value of EXPR. If EXPR is 4126omitted, returns the length of L<C<$_>|perlvar/$_>. If EXPR is 4127undefined, returns L<C<undef>|/undef EXPR>. 4128 4129This function cannot be used on an entire array or hash to find out how 4130many elements these have. For that, use C<scalar @array> and C<scalar keys 4131%hash>, respectively. 4132 4133Like all Perl character operations, L<C<length>|/length EXPR> normally 4134deals in logical 4135characters, not physical bytes. For how many bytes a string encoded as 4136UTF-8 would take up, use C<length(Encode::encode('UTF-8', EXPR))> 4137(you'll have to C<use Encode> first). See L<Encode> and L<perlunicode>. 4138 4139=item __LINE__ 4140X<__LINE__> 4141 4142=for Pod::Functions the current source line number 4143 4144A special token that compiles to the current line number. 4145It can be altered by the mechanism described at 4146L<perlsyn/"Plain Old Comments (Not!)">. 4147 4148=item link OLDFILE,NEWFILE 4149X<link> 4150 4151=for Pod::Functions create a hard link in the filesystem 4152 4153Creates a new filename linked to the old filename. Returns true for 4154success, false otherwise. 4155 4156Portability issues: L<perlport/link>. 4157 4158=item listen SOCKET,QUEUESIZE 4159X<listen> 4160 4161=for Pod::Functions register your socket as a server 4162 4163Does the same thing that the L<listen(2)> system call does. Returns true if 4164it succeeded, false otherwise. See the example in 4165L<perlipc/"Sockets: Client/Server Communication">. 4166 4167=item local EXPR 4168X<local> 4169 4170=for Pod::Functions create a temporary value for a global variable (dynamic scoping) 4171 4172You really probably want to be using L<C<my>|/my VARLIST> instead, 4173because L<C<local>|/local EXPR> isn't what most people think of as 4174"local". See L<perlsub/"Private Variables via my()"> for details. 4175 4176A local modifies the listed variables to be local to the enclosing 4177block, file, or eval. If more than one value is listed, the list must 4178be placed in parentheses. See L<perlsub/"Temporary Values via local()"> 4179for details, including issues with tied arrays and hashes. 4180 4181Like L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and 4182L<C<our>|/our VARLIST>, L<C<local>|/local EXPR> can operate on a variable 4183anywhere it appears in an expression (aside from interpolation in strings). 4184Unlike the other declarations, the effect of L<C<local>|/local EXPR> happens 4185at runtime, and so it will apply to additional uses of the same variable 4186executed after the declaration, even within the same statement. Note that 4187this does not include uses within an expression assigned to the variable 4188when it is localized, because the assigned expression is evaluated before 4189the localization. 4190 4191 package main; 4192 our $x = 2; 4193 { 4194 foo($x, local $x = $x + 1, $x); # foo() receives (2, 3, 3) 4195 # $main::x is 3 within the call to foo() 4196 } 4197 foo($x); # foo() receives (2) and $main::x is 2 4198 4199The C<delete local EXPR> construct can also be used to localize the deletion 4200of array/hash elements to the current block. 4201See L<perlsub/"Localized deletion of elements of composite types">. 4202 4203=item localtime EXPR 4204X<localtime> X<ctime> 4205 4206=item localtime 4207 4208=for Pod::Functions convert UNIX time into record or string using local time 4209 4210Converts a time as returned by the time function to a 9-element list 4211with the time analyzed for the local time zone. Typically used as 4212follows: 4213 4214 # 0 1 2 3 4 5 6 7 8 4215 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 4216 localtime(time); 4217 4218All list elements are numeric and come straight out of the C 'struct 4219tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours 4220of the specified time. 4221 4222C<$mday> is the day of the month and C<$mon> the month in 4223the range C<0..11>, with 0 indicating January and 11 indicating December. 4224This makes it easy to get a month name from a list: 4225 4226 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec); 4227 print "$abbr[$mon] $mday"; 4228 # $mon=9, $mday=18 gives "Oct 18" 4229 4230C<$year> contains the number of years since 1900. To get the full 4231year write: 4232 4233 $year += 1900; 4234 4235To get the last two digits of the year (e.g., "01" in 2001) do: 4236 4237 $year = sprintf("%02d", $year % 100); 4238 4239C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating 4240Wednesday. C<$yday> is the day of the year, in the range C<0..364> 4241(or C<0..365> in leap years.) 4242 4243C<$isdst> is true if the specified time occurs when Daylight Saving 4244Time is in effect, false otherwise. 4245 4246If EXPR is omitted, L<C<localtime>|/localtime EXPR> uses the current 4247time (as returned by L<C<time>|/time>). 4248 4249In scalar context, L<C<localtime>|/localtime EXPR> returns the 4250L<ctime(3)> value: 4251 4252 my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" 4253 4254This scalar value is always in English, and is B<not> locale-dependent. 4255To get similar but locale-dependent date strings, try for example: 4256 4257 use POSIX qw(strftime); 4258 my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; 4259 # or for GMT formatted appropriately for your locale: 4260 my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; 4261 4262C<$now_string> will be formatted according to the current LC_TIME locale 4263the program or thread is running in. See L<perllocale> for how to set 4264up and change that locale. Note that C<%a> and C<%b>, the short forms 4265of the day of the week and the month of the year, may not necessarily be 4266three characters wide. 4267 4268The L<Time::gmtime> and L<Time::localtime> modules provide a convenient, 4269by-name access mechanism to the L<C<gmtime>|/gmtime EXPR> and 4270L<C<localtime>|/localtime EXPR> functions, respectively. 4271 4272For a comprehensive date and time representation look at the 4273L<DateTime> module on CPAN. 4274 4275For GMT instead of local time use the L<C<gmtime>|/gmtime EXPR> builtin. 4276 4277See also the L<C<Time::Local>|Time::Local> module (for converting 4278seconds, minutes, hours, and such back to the integer value returned by 4279L<C<time>|/time>), and the L<POSIX> module's 4280L<C<mktime>|POSIX/C<mktime>> function. 4281 4282Portability issues: L<perlport/localtime>. 4283 4284=item lock THING 4285X<lock> 4286 4287=for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method 4288 4289This function places an advisory lock on a shared variable or referenced 4290object contained in I<THING> until the lock goes out of scope. 4291 4292The value returned is the scalar itself, if the argument is a scalar, or a 4293reference, if the argument is a hash, array or subroutine. 4294 4295L<C<lock>|/lock THING> is a "weak keyword"; this means that if you've 4296defined a function 4297by this name (before any calls to it), that function will be called 4298instead. If you are not under C<use threads::shared> this does nothing. 4299See L<threads::shared>. 4300 4301=item log EXPR 4302X<log> X<logarithm> X<e> X<ln> X<base> 4303 4304=item log 4305 4306=for Pod::Functions retrieve the natural logarithm for a number 4307 4308Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, 4309returns the log of L<C<$_>|perlvar/$_>. To get the 4310log of another base, use basic algebra: 4311The base-N log of a number is equal to the natural log of that number 4312divided by the natural log of N. For example: 4313 4314 sub log10 { 4315 my $n = shift; 4316 return log($n)/log(10); 4317 } 4318 4319See also L<C<exp>|/exp EXPR> for the inverse operation. 4320 4321=item lstat FILEHANDLE 4322X<lstat> 4323 4324=item lstat EXPR 4325 4326=item lstat DIRHANDLE 4327 4328=item lstat 4329 4330=for Pod::Functions stat a symbolic link 4331 4332Does the same thing as the L<C<stat>|/stat FILEHANDLE> function 4333(including setting the special C<_> filehandle) but stats a symbolic 4334link instead of the file the symbolic link points to. If symbolic links 4335are unimplemented on your system, a normal L<C<stat>|/stat FILEHANDLE> 4336is done. For much more detailed information, please see the 4337documentation for L<C<stat>|/stat FILEHANDLE>. 4338 4339If EXPR is omitted, stats L<C<$_>|perlvar/$_>. 4340 4341Portability issues: L<perlport/lstat>. 4342 4343=item m// 4344 4345=for Pod::Functions match a string with a regular expression pattern 4346 4347The match operator. See L<perlop/"Regexp Quote-Like Operators">. 4348 4349=item map BLOCK LIST 4350X<map> 4351 4352=item map EXPR,LIST 4353 4354=for Pod::Functions apply a change to a list to get back a new list with the changes 4355 4356Evaluates the BLOCK or EXPR for each element of LIST (locally setting 4357L<C<$_>|perlvar/$_> to each element) and composes a list of the results of 4358each such evaluation. Each element of LIST may produce zero, one, or more 4359elements in the generated list, so the number of elements in the generated 4360list may differ from that in LIST. In scalar context, returns the total 4361number of elements so generated. In list context, returns the generated list. 4362 4363 my @chars = map(chr, @numbers); 4364 4365translates a list of numbers to the corresponding characters. 4366 4367 my @squares = map { $_ * $_ } @numbers; 4368 4369translates a list of numbers to their squared values. 4370 4371 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers; 4372 4373shows that number of returned elements can differ from the number of 4374input elements. To omit an element, return an empty list (). 4375This could also be achieved by writing 4376 4377 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers; 4378 4379which makes the intention more clear. 4380 4381Map always returns a list, which can be 4382assigned to a hash such that the elements 4383become key/value pairs. See L<perldata> for more details. 4384 4385 my %hash = map { get_a_key_for($_) => $_ } @array; 4386 4387is just a funny way to write 4388 4389 my %hash; 4390 foreach (@array) { 4391 $hash{get_a_key_for($_)} = $_; 4392 } 4393 4394Note that L<C<$_>|perlvar/$_> is an alias to the list value, so it can 4395be used to modify the elements of the LIST. While this is useful and 4396supported, it can cause bizarre results if the elements of LIST are not 4397variables. Using a regular C<foreach> loop for this purpose would be 4398clearer in most cases. See also L<C<grep>|/grep BLOCK LIST> for a 4399list composed of those items of the original list for which the BLOCK 4400or EXPR evaluates to true. 4401 4402C<{> starts both hash references and blocks, so C<map { ...> could be either 4403the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look 4404ahead for the closing C<}> it has to take a guess at which it's dealing with 4405based on what it finds just after the 4406C<{>. Usually it gets it right, but if it 4407doesn't it won't realize something is wrong until it gets to the C<}> and 4408encounters the missing (or unexpected) comma. The syntax error will be 4409reported close to the C<}>, but you'll need to change something near the C<{> 4410such as using a unary C<+> or semicolon to give Perl some help: 4411 4412 my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong 4413 my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right 4414 my %hash = map {; "\L$_" => 1 } @array # this also works 4415 my %hash = map { ("\L$_" => 1) } @array # as does this 4416 my %hash = map { lc($_) => 1 } @array # and this. 4417 my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! 4418 4419 my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) 4420 4421or to force an anon hash constructor use C<+{>: 4422 4423 my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs 4424 # comma at end 4425 4426to get a list of anonymous hashes each with only one entry apiece. 4427 4428=item method NAME BLOCK 4429X<method> 4430 4431=item method NAME : ATTRS BLOCK 4432 4433=for Pod::Functions declare a method of a class 4434 4435Creates a new named method in the scope of the class that it appears within. 4436This is only valid inside a L<C<class>|/class NAMESPACE> declaration. 4437 4438=item mkdir FILENAME,MODE 4439X<mkdir> X<md> X<directory, create> 4440 4441=item mkdir FILENAME 4442 4443=item mkdir 4444 4445=for Pod::Functions create a directory 4446 4447Creates the directory specified by FILENAME, with permissions 4448specified by MODE (as modified by L<C<umask>|/umask EXPR>). If it 4449succeeds it returns true; otherwise it returns false and sets 4450L<C<$!>|perlvar/$!> (errno). 4451MODE defaults to 0777 if omitted, and FILENAME defaults 4452to L<C<$_>|perlvar/$_> if omitted. 4453 4454In general, it is better to create directories with a permissive MODE 4455and let the user modify that with their L<C<umask>|/umask EXPR> than it 4456is to supply 4457a restrictive MODE and give the user no way to be more permissive. 4458The exceptions to this rule are when the file or directory should be 4459kept private (mail files, for instance). The documentation for 4460L<C<umask>|/umask EXPR> discusses the choice of MODE in more detail. 4461If bits in MODE other than the permission bits are set, the result may 4462be implementation defined, per POSIX 1003.1-2008. 4463 4464Note that according to the POSIX 1003.1-1996 the FILENAME may have any 4465number of trailing slashes. Some operating and filesystems do not get 4466this right, so Perl automatically removes all trailing slashes to keep 4467everyone happy. 4468 4469To recursively create a directory structure, look at 4470the L<C<make_path>|File::Path/make_path( $dir1, $dir2, .... )> function 4471of the L<File::Path> module. 4472 4473=item msgctl ID,CMD,ARG 4474X<msgctl> 4475 4476=for Pod::Functions SysV IPC message control operations 4477 4478Calls the System V IPC function L<msgctl(2)>. You'll probably have to say 4479 4480 use IPC::SysV; 4481 4482first to get the correct constant definitions. If CMD is C<IPC_STAT>, 4483then ARG must be a variable that will hold the returned C<msqid_ds> 4484structure. Returns like L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>: 4485the undefined value for error, C<"0 but true"> for zero, or the actual 4486return value otherwise. See also L<perlipc/"SysV IPC"> and the 4487documentation for L<C<IPC::SysV>|IPC::SysV> and 4488L<C<IPC::Semaphore>|IPC::Semaphore>. 4489 4490Portability issues: L<perlport/msgctl>. 4491 4492=item msgget KEY,FLAGS 4493X<msgget> 4494 4495=for Pod::Functions get SysV IPC message queue 4496 4497Calls the System V IPC function L<msgget(2)>. Returns the message queue 4498id, or L<C<undef>|/undef EXPR> on error. See also L<perlipc/"SysV IPC"> 4499and the documentation for L<C<IPC::SysV>|IPC::SysV> and 4500L<C<IPC::Msg>|IPC::Msg>. 4501 4502Portability issues: L<perlport/msgget>. 4503 4504=item msgrcv ID,VAR,SIZE,TYPE,FLAGS 4505X<msgrcv> 4506 4507=for Pod::Functions receive a SysV IPC message from a message queue 4508 4509Calls the System V IPC function msgrcv to receive a message from 4510message queue ID into variable VAR with a maximum message size of 4511SIZE. Note that when a message is received, the message type as a 4512native long integer will be the first thing in VAR, followed by the 4513actual message. This packing may be opened with C<unpack("l! a*")>. 4514Taints the variable. Returns true if successful, false 4515on error. See also L<perlipc/"SysV IPC"> and the documentation for 4516L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>. 4517 4518Portability issues: L<perlport/msgrcv>. 4519 4520=item msgsnd ID,MSG,FLAGS 4521X<msgsnd> 4522 4523=for Pod::Functions send a SysV IPC message to a message queue 4524 4525Calls the System V IPC function msgsnd to send the message MSG to the 4526message queue ID. MSG must begin with the native long integer message 4527type, followed by the message itself. This kind of packing can be achieved 4528with C<pack("l! a*", $type, $message)>. Returns true if successful, 4529false on error. See also L<perlipc/"SysV IPC"> and the documentation 4530for L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Msg>|IPC::Msg>. 4531 4532Portability issues: L<perlport/msgsnd>. 4533 4534=item my VARLIST 4535X<my> 4536 4537=item my TYPE VARLIST 4538 4539=item my VARLIST : ATTRS 4540 4541=item my TYPE VARLIST : ATTRS 4542 4543=for Pod::Functions declare and assign a local variable (lexical scoping) 4544 4545A L<C<my>|/my VARLIST> declares the listed variables to be local 4546(lexically) to the enclosing block, file, or L<C<eval>|/eval EXPR>. If 4547more than one variable is listed, the list must be placed in 4548parentheses. 4549 4550Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used 4551as a dummy placeholder, for example to skip assignment of initial 4552values: 4553 4554 my ( undef, $min, $hour ) = localtime; 4555 4556Like L<C<state>|/state VARLIST>, L<C<local>|/local EXPR>, and 4557L<C<our>|/our VARLIST>, L<C<my>|/my VARLIST> can operate on a variable 4558anywhere it appears in an expression (aside from interpolation in strings). 4559The declaration will not apply to additional uses of the same variable until 4560the next statement. This means additional uses of that variable within the 4561same statement will act as they would have before that declaration occurred, 4562or result in a strict 'vars' error, as appropriate. 4563 4564 package main; 4565 our $x = 2; 4566 foo($x, my $x = $x + 1, $x); # foo() receives (2, 3, 2) 4567 foo($x, $main::x); # foo() receives (3, 2) 4568 4569Redeclaring a variable in the same scope or statement will "shadow" the 4570previous declaration, creating a new instance and preventing access to 4571the previous one. This is usually undesired and, if warnings are enabled, 4572will result in a warning in the C<shadow> category. 4573 4574The exact semantics and interface of TYPE and ATTRS are still 4575evolving. TYPE may be a bareword, a constant declared 4576with L<C<use constant>|constant>, or L<C<__PACKAGE__>|/__PACKAGE__>. It 4577is 4578currently bound to the use of the L<fields> pragma, 4579and attributes are handled using the L<attributes> pragma, or starting 4580from Perl 5.8.0 also via the L<Attribute::Handlers> module. See 4581L<perlsub/"Private Variables via my()"> for details. 4582 4583=item next LABEL 4584X<next> X<continue> 4585 4586=item next EXPR 4587 4588=item next 4589 4590=for Pod::Functions iterate a block prematurely 4591 4592The L<C<next>|/next LABEL> command is like the C<continue> statement in 4593C; it starts the next iteration of the loop: 4594 4595 LINE: while (<STDIN>) { 4596 next LINE if /^#/; # discard comments 4597 #... 4598 } 4599 4600Note that if there were a L<C<continue>|/continue BLOCK> block on the 4601above, it would get 4602executed even on discarded lines. If LABEL is omitted, the command 4603refers to the innermost enclosing loop. The C<next EXPR> form, available 4604as of Perl 5.18.0, allows a label name to be computed at run time, being 4605otherwise identical to C<next LABEL>. 4606 4607L<C<next>|/next LABEL> cannot return a value from a block that typically 4608returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform 4609its flow control behavior, which precludes any return value. It should not be 4610used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST> 4611operation. 4612 4613Note that a block by itself is semantically identical to a loop 4614that executes once. Thus L<C<next>|/next LABEL> will exit such a block 4615early. 4616 4617See also L<C<continue>|/continue BLOCK> for an illustration of how 4618L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and 4619L<C<redo>|/redo LABEL> work. 4620 4621Unlike most named operators, this has the same precedence as assignment. 4622It is also exempt from the looks-like-a-function rule, so 4623C<next ("foo")."bar"> will cause "bar" to be part of the argument to 4624L<C<next>|/next LABEL>. 4625 4626=item no MODULE VERSION LIST 4627X<no declarations> 4628X<unimporting> 4629 4630=item no MODULE VERSION 4631 4632=item no MODULE LIST 4633 4634=item no MODULE 4635 4636=item no VERSION 4637 4638=for Pod::Functions unimport some module symbols or semantics at compile time 4639 4640See the L<C<use>|/use Module VERSION LIST> function, of which 4641L<C<no>|/no MODULE VERSION LIST> is the opposite. 4642 4643=item oct EXPR 4644X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> 4645 4646=item oct 4647 4648=for Pod::Functions convert a string to an octal number 4649 4650Interprets EXPR as an octal string and returns the corresponding 4651value. An octal string consists of octal digits and, as of Perl 5.33.5, 4652an optional C<0o> or C<o> prefix. Each octal digit may be preceded by 4653a single underscore, which will be ignored. 4654(If EXPR happens to start off with C<0x> or C<x>, interprets it as a 4655hex string. If EXPR starts off with C<0b> or C<b>, it is interpreted as a 4656binary string. Leading whitespace is ignored in all three cases.) 4657The following will handle decimal, binary, octal, and hex in standard 4658Perl notation: 4659 4660 $val = oct($val) if $val =~ /^0/; 4661 4662If EXPR is omitted, uses L<C<$_>|perlvar/$_>. To go the other way 4663(produce a number in octal), use L<C<sprintf>|/sprintf FORMAT, LIST> or 4664L<C<printf>|/printf FILEHANDLE FORMAT, LIST>: 4665 4666 my $dec_perms = (stat("filename"))[2] & 07777; 4667 my $oct_perm_str = sprintf "%o", $perms; 4668 4669The L<C<oct>|/oct EXPR> function is commonly used when a string such as 4670C<644> needs 4671to be converted into a file mode, for example. Although Perl 4672automatically converts strings into numbers as needed, this automatic 4673conversion assumes base 10. 4674 4675Leading white space is ignored without warning, as too are any trailing 4676non-digits, such as a decimal point (L<C<oct>|/oct EXPR> only handles 4677non-negative integers, not negative integers or floating point). 4678 4679=item open FILEHANDLE,MODE,EXPR 4680X<open> X<pipe> X<file, open> X<fopen> 4681 4682=item open FILEHANDLE,MODE,EXPR,LIST 4683 4684=item open FILEHANDLE,MODE,REFERENCE 4685 4686=item open FILEHANDLE,EXPR 4687 4688=item open FILEHANDLE 4689 4690=for Pod::Functions open a file, pipe, or descriptor 4691 4692Associates an internal FILEHANDLE with the external file specified by 4693EXPR. That filehandle will subsequently allow you to perform 4694I/O operations on that file, such as reading from it or writing to it. 4695 4696Instead of a filename, you may specify an external command 4697(plus an optional argument list) or a scalar reference, in order to open 4698filehandles on commands or in-memory scalars, respectively. 4699 4700A thorough reference to C<open> follows. For a gentler introduction to 4701the basics of C<open>, see also the L<perlopentut> manual page. 4702 4703=over 4704 4705=item Working with files 4706 4707Most often, C<open> gets invoked with three arguments: the required 4708FILEHANDLE (usually an empty scalar variable), followed by MODE (usually 4709a literal describing the I/O mode the filehandle will use), and then the 4710filename that the new filehandle will refer to. 4711 4712=over 4713 4714=item Simple examples 4715 4716Reading from a file: 4717 4718 open(my $fh, "<", "input.txt") 4719 or die "Can't open < input.txt: $!"; 4720 4721 # Process every line in input.txt 4722 while (my $line = readline($fh)) { 4723 # 4724 # ... do something interesting with $line here ... 4725 # 4726 } 4727 4728or writing to one: 4729 4730 open(my $fh, ">", "output.txt") 4731 or die "Can't open > output.txt: $!"; 4732 4733 print $fh "This line gets printed into output.txt.\n"; 4734 4735For a summary of common filehandle operations such as these, see 4736L<perlintro/Files and I/O>. 4737 4738=item About filehandles 4739 4740The first argument to C<open>, labeled FILEHANDLE in this reference, is 4741usually a scalar variable. (Exceptions exist, described in "Other 4742considerations", below.) If the call to C<open> succeeds, then the 4743expression provided as FILEHANDLE will get assigned an open 4744I<filehandle>. That filehandle provides an internal reference to the 4745specified external file, conveniently stored in a Perl variable, and 4746ready for I/O operations such as reading and writing. 4747 4748=item About modes 4749 4750When calling C<open> with three or more arguments, the second argument 4751-- labeled MODE here -- defines the I<open mode>. MODE is usually a 4752literal string comprising special characters that define the intended 4753I/O role of the filehandle being created: whether it's read-only, or 4754read-and-write, and so on. 4755 4756If MODE is C<< < >>, the file is opened for input (read-only). 4757If MODE is C<< > >>, the file is opened for output, with existing files 4758first being truncated ("clobbered") and nonexisting files newly created. 4759If MODE is C<<< >> >>>, the file is opened for appending, again being 4760created if necessary. 4761 4762You can put a C<+> in front of the C<< > >> or C<< < >> to 4763indicate that you want both read and write access to the file; thus 4764C<< +< >> is almost always preferred for read/write updates--the 4765C<< +> >> mode would clobber the file first. You can't usually use 4766either read-write mode for updating textfiles, since they have 4767variable-length records. See the B<-i> switch in 4768L<perlrun|perlrun/-i[extension]> for a better approach. The file is 4769created with permissions of C<0666> modified by the process's 4770L<C<umask>|/umask EXPR> value. 4771 4772These various prefixes correspond to the L<fopen(3)> modes of C<r>, 4773C<r+>, C<w>, C<w+>, C<a>, and C<a+>. 4774 4775More examples of different modes in action: 4776 4777 # Open a file for concatenation 4778 open(my $log, ">>", "/usr/spool/news/twitlog") 4779 or warn "Couldn't open log file; discarding input"; 4780 4781 # Open a file for reading and writing 4782 open(my $dbase, "+<", "dbase.mine") 4783 or die "Can't open 'dbase.mine' for update: $!"; 4784 4785=item Checking the return value 4786 4787Open returns nonzero on success, the undefined value otherwise. If the 4788C<open> involved a pipe, the return value happens to be the pid of the 4789subprocess. 4790 4791When opening a file, it's seldom a good idea to continue if the request 4792failed, so C<open> is frequently used with L<C<die>|/die LIST>. Even if 4793you want your code to do something other than C<die> on a failed open, 4794you should still always check the return value from opening a file. 4795 4796=back 4797 4798=item Specifying I/O layers in MODE 4799 4800You can use the three-argument form of open to specify 4801I/O layers (sometimes referred to as "disciplines") to apply to the new 4802filehandle. These affect how the input and output are processed (see 4803L<open> and 4804L<PerlIO> for more details). For example: 4805 4806 # loads PerlIO::encoding automatically 4807 open(my $fh, "<:encoding(UTF-8)", $filename) 4808 || die "Can't open UTF-8 encoded $filename: $!"; 4809 4810This opens the UTF8-encoded file containing Unicode characters; 4811see L<perluniintro>. Note that if layers are specified in the 4812three-argument form, then default layers stored in 4813L<C<${^OPEN}>|perlvar/${^OPEN}> 4814(usually set by the L<open> pragma or the switch C<-CioD>) are ignored. 4815Those layers will also be ignored if you specify a colon with no name 4816following it. In that case the default layer for the operating system 4817(:raw on Unix, :crlf on Windows) is used. 4818 4819On some systems (in general, DOS- and Windows-based systems) 4820L<C<binmode>|/binmode FILEHANDLE, LAYER> is necessary when you're not 4821working with a text file. For the sake of portability it is a good idea 4822always to use it when appropriate, and never to use it when it isn't 4823appropriate. Also, people can set their I/O to be by default 4824UTF8-encoded Unicode, not bytes. 4825 4826=item Using C<undef> for temporary files 4827 4828As a special case the three-argument form with a read/write mode and the third 4829argument being L<C<undef>|/undef EXPR>: 4830 4831 open(my $tmp, "+>", undef) or die ... 4832 4833opens a filehandle to a newly created empty anonymous temporary file. 4834(This happens under any mode, which makes C<< +> >> the only useful and 4835sensible mode to use.) You will need to 4836L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading. 4837 4838 4839=item Opening a filehandle into an in-memory scalar 4840 4841You can open filehandles directly to Perl scalars instead of a file or 4842other resource external to the program. To do so, provide a reference to 4843that scalar as the third argument to C<open>, like so: 4844 4845 open(my $memory, ">", \$var) 4846 or die "Can't open memory file: $!"; 4847 print $memory "foo!\n"; # output will appear in $var 4848 4849To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first: 4850 4851 close STDOUT; 4852 open(STDOUT, ">", \$variable) 4853 or die "Can't open STDOUT: $!"; 4854 4855The scalars for in-memory files are treated as octet strings: unless 4856the file is being opened with truncation the scalar may not contain 4857any code points over 0xFF. 4858 4859Opening in-memory files I<can> fail for a variety of reasons. As with 4860any other C<open>, check the return value for success. 4861 4862I<Technical note>: This feature works only when Perl is built with 4863PerlIO -- the default, except with older (pre-5.16) Perl installations 4864that were configured to not include it (e.g. via C<Configure 4865-Uuseperlio>). You can see whether your Perl was built with PerlIO by 4866running C<perl -V:useperlio>. If it says C<'define'>, you have PerlIO; 4867otherwise you don't. 4868 4869See L<perliol> for detailed info on PerlIO. 4870 4871=item Opening a filehandle into a command 4872 4873If MODE is C<|->, then the filename is 4874interpreted as a command to which output is to be piped, and if MODE 4875is C<-|>, the filename is interpreted as a command that pipes 4876output to us. In the two-argument (and one-argument) form, one should 4877replace dash (C<->) with the command. 4878See L<perlipc/"Using open() for IPC"> for more examples of this. 4879(You are not allowed to L<C<open>|/open FILEHANDLE,MODE,EXPR> to a command 4880that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, and 4881L<perlipc/"Bidirectional Communication with Another Process"> for 4882alternatives.) 4883 4884 4885 open(my $article_fh, "-|", "caesar <$article") # decrypt 4886 # article 4887 or die "Can't start caesar: $!"; 4888 4889 open(my $article_fh, "caesar <$article |") # ditto 4890 or die "Can't start caesar: $!"; 4891 4892 open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id 4893 or die "Can't start sort: $!"; 4894 4895 4896In the form of pipe opens taking three or more arguments, if LIST is specified 4897(extra arguments after the command name) then LIST becomes arguments 4898to the command invoked if the platform supports it. The meaning of 4899L<C<open>|/open FILEHANDLE,MODE,EXPR> with more than three arguments for 4900non-pipe modes is not yet defined, but experimental "layers" may give 4901extra LIST arguments meaning. 4902 4903If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|> 4904with the one- or two-argument forms of 4905L<C<open>|/open FILEHANDLE,MODE,EXPR>), an implicit L<C<fork>|/fork> is done, 4906so L<C<open>|/open FILEHANDLE,MODE,EXPR> returns twice: in the parent process 4907it returns the pid 4908of the child process, and in the child process it returns (a defined) C<0>. 4909Use C<defined($pid)> or C<//> to determine whether the open was successful. 4910 4911For example, use either 4912 4913 my $child_pid = open(my $from_kid, "-|") 4914 // die "Can't fork: $!"; 4915 4916or 4917 4918 my $child_pid = open(my $to_kid, "|-") 4919 // die "Can't fork: $!"; 4920 4921followed by 4922 4923 if ($child_pid) { 4924 # am the parent: 4925 # either write $to_kid or else read $from_kid 4926 ... 4927 waitpid $child_pid, 0; 4928 } else { 4929 # am the child; use STDIN/STDOUT normally 4930 ... 4931 exit; 4932 } 4933 4934The filehandle behaves normally for the parent, but I/O to that 4935filehandle is piped from/to the STDOUT/STDIN of the child process. 4936In the child process, the filehandle isn't opened--I/O happens from/to 4937the new STDOUT/STDIN. Typically this is used like the normal 4938piped open when you want to exercise more control over just how the 4939pipe command gets executed, such as when running setuid and 4940you don't want to have to scan shell commands for metacharacters. 4941 4942The following blocks are more or less equivalent: 4943 4944 open(my $fh, "|tr '[a-z]' '[A-Z]'"); 4945 open(my $fh, "|-", "tr '[a-z]' '[A-Z]'"); 4946 open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]'; 4947 open(my $fh, "|-", "tr", '[a-z]', '[A-Z]'); 4948 4949 open(my $fh, "cat -n '$file'|"); 4950 open(my $fh, "-|", "cat -n '$file'"); 4951 open(my $fh, "-|") || exec "cat", "-n", $file; 4952 open(my $fh, "-|", "cat", "-n", $file); 4953 4954The last two examples in each block show the pipe as "list form", which 4955is not yet supported on all platforms. (If your platform has a real 4956L<C<fork>|/fork>, such as Linux and macOS, you can use the list form; it 4957also works on Windows with Perl 5.22 or later.) You would want to use 4958the list form of the pipe so you can pass literal arguments to the 4959command without risk of the shell interpreting any shell metacharacters 4960in them. However, this also bars you from opening pipes to commands that 4961intentionally contain shell metacharacters, such as: 4962 4963 open(my $fh, "|cat -n | expand -4 | lpr") 4964 || die "Can't open pipeline to lpr: $!"; 4965 4966See L<perlipc/"Safe Pipe Opens"> for more examples of this. 4967 4968=item Duping filehandles 4969 4970You may also, in the Bourne shell tradition, specify an EXPR beginning 4971with C<< >& >>, in which case the rest of the string is interpreted 4972as the name of a filehandle (or file descriptor, if numeric) to be 4973duped (as in L<dup(2)>) and opened. You may use C<&> after C<< > >>, 4974C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. 4975The mode you specify should match the mode of the original filehandle. 4976(Duping a filehandle does not take into account any existing contents 4977of IO buffers.) If you use the three-argument 4978form, then you can pass either a 4979number, the name of a filehandle, or the normal "reference to a glob". 4980 4981Here is a script that saves, redirects, and restores C<STDOUT> and 4982C<STDERR> using various methods: 4983 4984 #!/usr/bin/perl 4985 open(my $oldout, ">&STDOUT") 4986 or die "Can't dup STDOUT: $!"; 4987 open(OLDERR, ">&", \*STDERR) 4988 or die "Can't dup STDERR: $!"; 4989 4990 open(STDOUT, '>', "foo.out") 4991 or die "Can't redirect STDOUT: $!"; 4992 open(STDERR, ">&STDOUT") 4993 or die "Can't dup STDOUT: $!"; 4994 4995 select STDERR; $| = 1; # make unbuffered 4996 select STDOUT; $| = 1; # make unbuffered 4997 4998 print STDOUT "stdout 1\n"; # this works for 4999 print STDERR "stderr 1\n"; # subprocesses too 5000 5001 open(STDOUT, ">&", $oldout) 5002 or die "Can't dup \$oldout: $!"; 5003 open(STDERR, ">&OLDERR") 5004 or die "Can't dup OLDERR: $!"; 5005 5006 print STDOUT "stdout 2\n"; 5007 print STDERR "stderr 2\n"; 5008 5009If you specify C<< '<&=X' >>, where C<X> is a file descriptor number 5010or a filehandle, then Perl will do an equivalent of C's L<fdopen(3)> of 5011that file descriptor (and not call L<dup(2)>); this is more 5012parsimonious of file descriptors. For example: 5013 5014 # open for input, reusing the fileno of $fd 5015 open(my $fh, "<&=", $fd) 5016 5017or 5018 5019 open(my $fh, "<&=$fd") 5020 5021or 5022 5023 # open for append, using the fileno of $oldfh 5024 open(my $fh, ">>&=", $oldfh) 5025 5026Being parsimonious on filehandles is also useful (besides being 5027parsimonious) for example when something is dependent on file 5028descriptors, like for example locking using 5029L<C<flock>|/flock FILEHANDLE,OPERATION>. If you do just 5030C<< open(my $A, ">>&", $B) >>, the filehandle C<$A> will not have the 5031same file descriptor as C<$B>, and therefore C<flock($A)> will not 5032C<flock($B)> nor vice versa. But with C<< open(my $A, ">>&=", $B) >>, 5033the filehandles will share the same underlying system file descriptor. 5034 5035Note that under Perls older than 5.8.0, Perl uses the standard C library's' 5036L<fdopen(3)> to implement the C<=> functionality. On many Unix systems, 5037L<fdopen(3)> fails when file descriptors exceed a certain value, typically 255. 5038For Perls 5.8.0 and later, PerlIO is (most often) the default. 5039 5040=item Legacy usage 5041 5042This section describes ways to call C<open> outside of best practices; 5043you may encounter these uses in older code. Perl does not consider their 5044use deprecated, exactly, but neither is it recommended in new code, for 5045the sake of clarity and readability. 5046 5047=over 5048 5049=item Specifying mode and filename as a single argument 5050 5051In the one- and two-argument forms of the call, the mode and filename 5052should be concatenated (in that order), preferably separated by white 5053space. You can--but shouldn't--omit the mode in these forms when that mode 5054is C<< < >>. It is safe to use the two-argument form of 5055L<C<open>|/open FILEHANDLE,MODE,EXPR> if the filename argument is a known literal. 5056 5057 open(my $dbase, "+<dbase.mine") # ditto 5058 or die "Can't open 'dbase.mine' for update: $!"; 5059 5060In the two-argument (and one-argument) form, opening C<< <- >> 5061or C<-> opens STDIN and opening C<< >- >> opens STDOUT. 5062 5063New code should favor the three-argument form of C<open> over this older 5064form. Declaring the mode and the filename as two distinct arguments 5065avoids any confusion between the two. 5066 5067=item Assigning a filehandle to a bareword 5068 5069An older style is to use a bareword as the filehandle, as 5070 5071 open(FH, "<", "input.txt") 5072 or die "Can't open < input.txt: $!"; 5073 5074Then you can use C<FH> as the filehandle, in C<< close FH >> and C<< 5075<FH> >> and so on. Note that it's a global variable, so this form is 5076not recommended when dealing with filehandles other than Perl's built-in ones 5077(e.g. STDOUT and STDIN). In fact, using a bareword for the filehandle is 5078an error when the 5079L<C<"bareword_filehandles"> feature|feature/"The 'bareword_filehandles' feature"> 5080has been disabled. This feature is disabled automatically when in the 5081scope of C<use v5.36.0> or later. 5082 5083=item Calling C<open> with one argument via global variables 5084 5085As a shortcut, a one-argument call takes the filename from the global 5086scalar variable of the same name as the bareword filehandle: 5087 5088 $ARTICLE = 100; 5089 open(ARTICLE) 5090 or die "Can't find article $ARTICLE: $!\n"; 5091 5092Here C<$ARTICLE> must be a global scalar variable in the same package 5093as the filehandle - not one declared with L<C<my>|/my VARLIST> or 5094L<C<state>|/state VARLIST>. 5095 5096=back 5097 5098=item Other considerations 5099 5100=over 5101 5102=item Automatic filehandle closure 5103 5104The filehandle will be closed when its reference count reaches zero. If 5105it is a lexically scoped variable declared with L<C<my>|/my VARLIST>, 5106that usually means the end of the enclosing scope. However, this 5107automatic close does not check for errors, so it is better to explicitly 5108close filehandles, especially those used for writing: 5109 5110 close($handle) 5111 || warn "close failed: $!"; 5112 5113=item Automatic pipe flushing 5114 5115Perl will attempt to flush all files opened for 5116output before any operation that may do a fork, but this may not be 5117supported on some platforms (see L<perlport>). To be safe, you may need 5118to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) 5119or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> 5120on any open handles. 5121 5122On systems that support a close-on-exec flag on files, the flag will 5123be set for the newly opened file descriptor as determined by the value 5124of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>. 5125 5126Closing any piped filehandle causes the parent process to wait for the 5127child to finish, then returns the status value in L<C<$?>|perlvar/$?> and 5128L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>. 5129 5130=item Direct versus by-reference assignment of filehandles 5131 5132If FILEHANDLE -- the first argument in a call to C<open> -- is an 5133undefined scalar variable (or array or hash element), a new filehandle 5134is autovivified, meaning that the variable is assigned a reference to a 5135newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an 5136expression, its value is the real filehandle. (This is considered a 5137symbolic reference, so C<use strict "refs"> should I<not> be in effect.) 5138 5139=item Whitespace and special characters in the filename argument 5140 5141The filename passed to the one- and two-argument forms of 5142L<C<open>|/open FILEHANDLE,MODE,EXPR> will 5143have leading and trailing whitespace deleted and normal 5144redirection characters honored. This property, known as "magic open", 5145can often be used to good effect. A user could specify a filename of 5146F<"rsh cat file |">, or you could change certain filenames as needed: 5147 5148 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; 5149 open(my $fh, $filename) 5150 or die "Can't open $filename: $!"; 5151 5152Use the three-argument form to open a file with arbitrary weird characters in it, 5153 5154 open(my $fh, "<", $file) 5155 || die "Can't open $file: $!"; 5156 5157otherwise it's necessary to protect any leading and trailing whitespace: 5158 5159 $file =~ s#^(\s)#./$1#; 5160 open(my $fh, "< $file\0") 5161 || die "Can't open $file: $!"; 5162 5163(this may not work on some bizarre filesystems). One should 5164conscientiously choose between the I<magic> and I<three-argument> form 5165of L<C<open>|/open FILEHANDLE,MODE,EXPR>: 5166 5167 open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!"; 5168 5169will allow the user to specify an argument of the form C<"rsh cat file |">, 5170but will not work on a filename that happens to have a trailing space, while 5171 5172 open(my $in, "<", $ARGV[0]) 5173 || die "Can't open $ARGV[0]: $!"; 5174 5175will have exactly the opposite restrictions. (However, some shells 5176support the syntax C<< perl your_program.pl <( rsh cat file ) >>, which 5177produces a filename that can be opened normally.) 5178 5179=item Invoking C-style C<open> 5180 5181If you want a "real" C L<open(2)>, then you should use the 5182L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> function, which involves 5183no such magic (but uses different filemodes than Perl 5184L<C<open>|/open FILEHANDLE,MODE,EXPR>, which corresponds to C L<fopen(3)>). 5185This is another way to protect your filenames from interpretation. For 5186example: 5187 5188 use IO::Handle; 5189 sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL) 5190 or die "Can't open $path: $!"; 5191 $fh->autoflush(1); 5192 print $fh "stuff $$\n"; 5193 seek($fh, 0, 0); 5194 print "File contains: ", readline($fh); 5195 5196See L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> for some details about 5197mixing reading and writing. 5198 5199=item Portability issues 5200 5201See L<perlport/open>. 5202 5203=back 5204 5205=back 5206 5207 5208=item opendir DIRHANDLE,EXPR 5209X<opendir> 5210 5211=for Pod::Functions open a directory 5212 5213Opens a directory named EXPR for processing by 5214L<C<readdir>|/readdir DIRHANDLE>, L<C<telldir>|/telldir DIRHANDLE>, 5215L<C<seekdir>|/seekdir DIRHANDLE,POS>, 5216L<C<rewinddir>|/rewinddir DIRHANDLE>, and 5217L<C<closedir>|/closedir DIRHANDLE>. Returns true if successful. 5218DIRHANDLE may be an expression whose value can be used as an indirect 5219dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined 5220scalar variable (or array or hash element), the variable is assigned a 5221reference to a new anonymous dirhandle; that is, it's autovivified. 5222Dirhandles are the same objects as filehandles; an I/O object can only 5223be open as one of these handle types at once. 5224 5225See the example at L<C<readdir>|/readdir DIRHANDLE>. 5226 5227=item ord EXPR 5228X<ord> X<encoding> 5229 5230=item ord 5231 5232=for Pod::Functions find a character's code point 5233 5234Returns the code point of the first character of EXPR. 5235If EXPR is an empty string, returns 0. If EXPR is omitted, uses 5236L<C<$_>|perlvar/$_>. 5237(Note I<character>, not byte.) 5238 5239For the reverse, see L<C<chr>|/chr NUMBER>. 5240See L<perlunicode> for more about Unicode. 5241 5242=item our VARLIST 5243X<our> X<global> 5244 5245=item our TYPE VARLIST 5246 5247=item our VARLIST : ATTRS 5248 5249=item our TYPE VARLIST : ATTRS 5250 5251=for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping) 5252 5253L<C<our>|/our VARLIST> makes a lexical alias to a package (i.e. global) 5254variable of the same name in the current package for use within the 5255current lexical scope. 5256 5257L<C<our>|/our VARLIST> has the same scoping rules as 5258L<C<my>|/my VARLIST> or L<C<state>|/state VARLIST>, meaning that it is 5259only valid within a lexical scope. Unlike L<C<my>|/my VARLIST> and 5260L<C<state>|/state VARLIST>, which both declare new (lexical) variables, 5261L<C<our>|/our VARLIST> only creates an alias to an existing variable: a 5262package variable of the same name. 5263 5264This means that when C<use strict 'vars'> is in effect, L<C<our>|/our 5265VARLIST> lets you use a package variable without qualifying it with the 5266package name, but only within the lexical scope of the 5267L<C<our>|/our VARLIST> declaration. This applies immediately--even 5268within the same statement. 5269 5270 package Foo; 5271 use v5.36; # which implies "use strict;" 5272 5273 $Foo::foo = 23; 5274 5275 { 5276 our $foo; # alias to $Foo::foo 5277 print $foo; # prints 23 5278 } 5279 5280 print $Foo::foo; # prints 23 5281 5282 print $foo; # ERROR: requires explicit package name 5283 5284This works even if the package variable has not been used before, as 5285package variables spring into existence when first used. 5286 5287 package Foo; 5288 use v5.36; 5289 5290 our $foo = 23; # just like $Foo::foo = 23 5291 5292 print $Foo::foo; # prints 23 5293 5294Because the variable becomes legal immediately under C<use strict 'vars'>, so 5295long as there is no variable with that name is already in scope, you can then 5296reference the package variable again even within the same statement. 5297 5298 package Foo; 5299 use v5.36; 5300 5301 my $foo = $foo; # error, undeclared $foo on right-hand side 5302 our $foo = $foo; # no errors 5303 5304If more than one variable is listed, the list must be placed 5305in parentheses. 5306 5307 our($bar, $baz); 5308 5309Like L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and 5310L<C<local>|/local EXPR>, L<C<our>|/our VARLIST> can operate on a variable 5311anywhere it appears in an expression (aside from interpolation in strings). 5312The declaration will not apply to additional uses of the same variable until 5313the next statement. This means additional uses of that variable within the 5314same statement will act as they would have before that declaration occurred, 5315with the exception that it will still satisfy strict 'vars' and interpret that 5316variable as the newly aliased package variable if it was not yet declared in 5317that scope. 5318 5319 package main; 5320 my $x = 2; 5321 foo($x, our $x = $x + 1, $x); # foo() receives (2, 3, 2) 5322 foo($x, our $z = 5, $z); # foo() receives (3, 5, 5) 5323 5324An L<C<our>|/our VARLIST> declaration declares an alias for a package 5325variable that will be visible 5326across its entire lexical scope, even across package boundaries. The 5327package in which the variable is entered is determined at the point 5328of the declaration, not at the point of use. This means the following 5329behavior holds: 5330 5331 package Foo; 5332 our $bar; # declares $Foo::bar for rest of lexical scope 5333 $bar = 20; 5334 5335 package Bar; 5336 print $bar; # prints 20, as it refers to $Foo::bar 5337 5338Multiple L<C<our>|/our VARLIST> declarations with the same name in the 5339same lexical 5340scope are allowed if they are in different packages. If they happen 5341to be in the same package, Perl will emit warnings if you have asked 5342for them, just like multiple L<C<my>|/my VARLIST> declarations. Unlike 5343a second L<C<my>|/my VARLIST> declaration, which will bind the name to a 5344fresh variable, a second L<C<our>|/our VARLIST> declaration in the same 5345package, in the same scope, is merely redundant. 5346 5347 use warnings; 5348 package Foo; 5349 our $bar; # declares $Foo::bar for rest of lexical scope 5350 $bar = 20; 5351 5352 package Bar; 5353 our $bar = 30; # declares $Bar::bar for rest of lexical scope 5354 print $bar; # prints 30 5355 5356 our $bar; # emits warning but has no other effect 5357 print $bar; # still prints 30 5358 5359An L<C<our>|/our VARLIST> declaration may also have a list of attributes 5360associated with it. 5361 5362The exact semantics and interface of TYPE and ATTRS are still 5363evolving. TYPE is currently bound to the use of the L<fields> pragma, 5364and attributes are handled using the L<attributes> pragma, or, starting 5365from Perl 5.8.0, also via the L<Attribute::Handlers> module. See 5366L<perlsub/"Private Variables via my()"> for details. 5367 5368Note that with a parenthesised list, L<C<undef>|/undef EXPR> can be used 5369as a dummy placeholder, for example to skip assignment of initial 5370values: 5371 5372 our ( undef, $min, $hour ) = localtime; 5373 5374L<C<our>|/our VARLIST> differs from L<C<use vars>|vars>, which allows 5375use of an unqualified name I<only> within the affected package, but 5376across scopes. 5377 5378=item pack TEMPLATE,LIST 5379X<pack> 5380 5381=for Pod::Functions convert a list into a binary representation 5382 5383Takes a LIST of values and converts it into a string using the rules 5384given by the TEMPLATE. The resulting string is the concatenation of 5385the converted values. Typically, each converted value looks 5386like its machine-level representation. For example, on 32-bit machines 5387an integer may be represented by a sequence of 4 bytes, which will in 5388Perl be presented as a string that's 4 characters long. 5389 5390See L<perlpacktut> for an introduction to this function. 5391 5392The TEMPLATE is a sequence of characters that give the order and type 5393of values, as follows: 5394 5395 a A string with arbitrary binary data, will be null padded. 5396 A A text (ASCII) string, will be space padded. 5397 Z A null-terminated (ASCIZ) string, will be null padded. 5398 5399 b A bit string (ascending bit order inside each byte, 5400 like vec()). 5401 B A bit string (descending bit order inside each byte). 5402 h A hex string (low nybble first). 5403 H A hex string (high nybble first). 5404 5405 c A signed char (8-bit) value. 5406 C An unsigned char (octet) value. 5407 W An unsigned char value (can be greater than 255). 5408 5409 s A signed short (16-bit) value. 5410 S An unsigned short value. 5411 5412 l A signed long (32-bit) value. 5413 L An unsigned long value. 5414 5415 q A signed quad (64-bit) value. 5416 Q An unsigned quad value. 5417 (Quads are available only if your system supports 64-bit 5418 integer values _and_ if Perl has been compiled to support 5419 those. Raises an exception otherwise.) 5420 5421 i A signed integer value. 5422 I An unsigned integer value. 5423 (This 'integer' is _at_least_ 32 bits wide. Its exact 5424 size depends on what a local C compiler calls 'int'.) 5425 5426 n An unsigned short (16-bit) in "network" (big-endian) order. 5427 N An unsigned long (32-bit) in "network" (big-endian) order. 5428 v An unsigned short (16-bit) in "VAX" (little-endian) order. 5429 V An unsigned long (32-bit) in "VAX" (little-endian) order. 5430 5431 j A Perl internal signed integer value (IV). 5432 J A Perl internal unsigned integer value (UV). 5433 5434 f A single-precision float in native format. 5435 d A double-precision float in native format. 5436 5437 F A Perl internal floating-point value (NV) in native format 5438 D A float of long-double precision in native format. 5439 (Long doubles are available only if your system supports 5440 long double values. Raises an exception otherwise. 5441 Note that there are different long double formats.) 5442 5443 p A pointer to a null-terminated string. 5444 P A pointer to a structure (fixed-length string). 5445 5446 u A uuencoded string. 5447 U A Unicode character number. Encodes to a character in char- 5448 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in 5449 byte mode. Also on EBCDIC platforms, the character number will 5450 be the native EBCDIC value for character numbers below 256. 5451 This allows most programs using this feature to not have to 5452 care which type of platform they are running on. 5453 5454 w A BER compressed integer (not an ASN.1 BER, see perlpacktut 5455 for details). Its bytes represent an unsigned integer in 5456 base 128, most significant digit first, with as few digits 5457 as possible. Bit eight (the high bit) is set on each byte 5458 except the last. 5459 5460 x A null byte (a.k.a ASCII NUL, "\000", chr(0)) 5461 X Back up a byte. 5462 @ Null-fill or truncate to absolute position, counted from the 5463 start of the innermost ()-group. 5464 . Null-fill or truncate to absolute position specified by 5465 the value. 5466 ( Start of a ()-group. 5467 5468One or more modifiers below may optionally follow certain letters in the 5469TEMPLATE (the second column lists letters for which the modifier is valid): 5470 5471 ! sSlLiI Forces native (short, long, int) sizes instead 5472 of fixed (16-/32-bit) sizes. 5473 5474 ! xX Make x and X act as alignment commands. 5475 5476 ! nNvV Treat integers as signed instead of unsigned. 5477 5478 ! @. Specify position as byte offset in the internal 5479 representation of the packed string. Efficient 5480 but dangerous. 5481 5482 > sSiIlLqQ Force big-endian byte-order on the type. 5483 jJfFdDpP (The "big end" touches the construct.) 5484 5485 < sSiIlLqQ Force little-endian byte-order on the type. 5486 jJfFdDpP (The "little end" touches the construct.) 5487 5488The C<< > >> and C<< < >> modifiers can also be used on C<()> groups 5489to force a particular byte-order on all components in that group, 5490including all its subgroups. 5491 5492=begin comment 5493 5494Larry recalls that the hex and bit string formats (H, h, B, b) were added to 5495pack for processing data from NASA's Magellan probe. Magellan was in an 5496elliptical orbit, using the antenna for the radar mapping when close to 5497Venus and for communicating data back to Earth for the rest of the orbit. 5498There were two transmission units, but one of these failed, and then the 5499other developed a fault whereby it would randomly flip the sense of all the 5500bits. It was easy to automatically detect complete records with the correct 5501sense, and complete records with all the bits flipped. However, this didn't 5502recover the records where the sense flipped midway. A colleague of Larry's 5503was able to pretty much eyeball where the records flipped, so they wrote an 5504editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to 5505manually correct the records and recover the data. For this purpose pack 5506gained the hex and bit string format specifiers. 5507 5508git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit 550927e2fb84680b9cc1), but the patch description makes no mention of their 5510addition, let alone the story behind them. 5511 5512=end comment 5513 5514The following rules apply: 5515 5516=over 5517 5518=item * 5519 5520Each letter may optionally be followed by a number indicating the repeat 5521count. A numeric repeat count may optionally be enclosed in brackets, as 5522in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from 5523the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>, 5524C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means 5525something else, described below. Supplying a C<*> for the repeat count 5526instead of a number means to use however many items are left, except for: 5527 5528=over 5529 5530=item * 5531 5532C<@>, C<x>, and C<X>, where it is equivalent to C<0>. 5533 5534=item * 5535 5536<.>, where it means relative to the start of the string. 5537 5538=item * 5539 5540C<u>, where it is equivalent to 1 (or 45, which here is equivalent). 5541 5542=back 5543 5544One can replace a numeric repeat count with a template letter enclosed in 5545brackets to use the packed byte length of the bracketed template for the 5546repeat count. 5547 5548For example, the template C<x[L]> skips as many bytes as in a packed long, 5549and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when 5550variable-expanded) unpacks. If the template in brackets contains alignment 5551commands (such as C<x![d]>), its packed length is calculated as if the 5552start of the template had the maximal possible alignment. 5553 5554When used with C<Z>, a C<*> as the repeat count is guaranteed to add a 5555trailing null byte, so the resulting string is always one byte longer than 5556the byte length of the item itself. 5557 5558When used with C<@>, the repeat count represents an offset from the start 5559of the innermost C<()> group. 5560 5561When used with C<.>, the repeat count determines the starting position to 5562calculate the value offset as follows: 5563 5564=over 5565 5566=item * 5567 5568If the repeat count is C<0>, it's relative to the current position. 5569 5570=item * 5571 5572If the repeat count is C<*>, the offset is relative to the start of the 5573packed string. 5574 5575=item * 5576 5577And if it's an integer I<n>, the offset is relative to the start of the 5578I<n>th innermost C<( )> group, or to the start of the string if I<n> is 5579bigger then the group level. 5580 5581=back 5582 5583The repeat count for C<u> is interpreted as the maximal number of bytes 5584to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat 5585count should not be more than 65. 5586 5587=item * 5588 5589The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a 5590string of length count, padding with nulls or spaces as needed. When 5591unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything 5592after the first null, and C<a> returns data with no stripping at all. 5593 5594If the value to pack is too long, the result is truncated. If it's too 5595long and an explicit count is provided, C<Z> packs only C<$count-1> bytes, 5596followed by a null byte. Thus C<Z> always packs a trailing null, except 5597when the count is 0. 5598 5599=item * 5600 5601Likewise, the C<b> and C<B> formats pack a string that's that many bits long. 5602Each such format generates 1 bit of the result. These are typically followed 5603by a repeat count like C<B8> or C<B64>. 5604 5605Each result bit is based on the least-significant bit of the corresponding 5606input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> 5607and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">. 5608 5609Starting from the beginning of the input string, each 8-tuple 5610of characters is converted to 1 character of output. With format C<b>, 5611the first character of the 8-tuple determines the least-significant bit of a 5612character; with format C<B>, it determines the most-significant bit of 5613a character. 5614 5615If the length of the input string is not evenly divisible by 8, the 5616remainder is packed as if the input string were padded by null characters 5617at the end. Similarly during unpacking, "extra" bits are ignored. 5618 5619If the input string is longer than needed, remaining characters are ignored. 5620 5621A C<*> for the repeat count uses all characters of the input field. 5622On unpacking, bits are converted to a string of C<0>s and C<1>s. 5623 5624=item * 5625 5626The C<h> and C<H> formats pack a string that many nybbles (4-bit groups, 5627representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long. 5628 5629For each such format, L<C<pack>|/pack TEMPLATE,LIST> generates 4 bits of result. 5630With non-alphabetical characters, the result is based on the 4 least-significant 5631bits of the input character, i.e., on C<ord($char)%16>. In particular, 5632characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes 5633C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result 5634is compatible with the usual hexadecimal digits, so that C<"a"> and 5635C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex 5636characters with this format. 5637 5638Starting from the beginning of the template to 5639L<C<pack>|/pack TEMPLATE,LIST>, each pair 5640of characters is converted to 1 character of output. With format C<h>, the 5641first character of the pair determines the least-significant nybble of the 5642output character; with format C<H>, it determines the most-significant 5643nybble. 5644 5645If the length of the input string is not even, it behaves as if padded by 5646a null character at the end. Similarly, "extra" nybbles are ignored during 5647unpacking. 5648 5649If the input string is longer than needed, extra characters are ignored. 5650 5651A C<*> for the repeat count uses all characters of the input field. For 5652L<C<unpack>|/unpack TEMPLATE,EXPR>, nybbles are converted to a string of 5653hexadecimal digits. 5654 5655=item * 5656 5657The C<p> format packs a pointer to a null-terminated string. You are 5658responsible for ensuring that the string is not a temporary value, as that 5659could potentially get deallocated before you got around to using the packed 5660result. The C<P> format packs a pointer to a structure of the size indicated 5661by the length. A null pointer is created if the corresponding value for 5662C<p> or C<P> is L<C<undef>|/undef EXPR>; similarly with 5663L<C<unpack>|/unpack TEMPLATE,EXPR>, where a null pointer unpacks into 5664L<C<undef>|/undef EXPR>. 5665 5666If your system has a strange pointer size--meaning a pointer is neither as 5667big as an int nor as big as a long--it may not be possible to pack or 5668unpack pointers in big- or little-endian byte order. Attempting to do 5669so raises an exception. 5670 5671=item * 5672 5673The C</> template character allows packing and unpacking of a sequence of 5674items where the packed structure contains a packed item count followed by 5675the packed items themselves. This is useful when the structure you're 5676unpacking has encoded the sizes or repeat counts for some of its fields 5677within the structure itself as separate fields. 5678 5679For L<C<pack>|/pack TEMPLATE,LIST>, you write 5680I<length-item>C</>I<sequence-item>, and the 5681I<length-item> describes how the length value is packed. Formats likely 5682to be of most use are integer-packing ones like C<n> for Java strings, 5683C<w> for ASN.1 or SNMP, and C<N> for Sun XDR. 5684 5685For L<C<pack>|/pack TEMPLATE,LIST>, I<sequence-item> may have a repeat 5686count, in which case 5687the minimum of that and the number of available items is used as the argument 5688for I<length-item>. If it has no repeat count or uses a '*', the number 5689of available items is used. 5690 5691For L<C<unpack>|/unpack TEMPLATE,EXPR>, an internal stack of integer 5692arguments unpacked so far is 5693used. You write C</>I<sequence-item> and the repeat count is obtained by 5694popping off the last element from the stack. The I<sequence-item> must not 5695have a repeat count. 5696 5697If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">), 5698the I<length-item> is the string length, not the number of strings. With 5699an explicit repeat count for pack, the packed string is adjusted to that 5700length. For example: 5701 5702 This code: gives this result: 5703 5704 unpack("W/a", "\004Gurusamy") ("Guru") 5705 unpack("a3/A A*", "007 Bond J ") (" Bond", "J") 5706 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".") 5707 5708 pack("n/a* w/a","hello,","world") "\000\006hello,\005world" 5709 pack("a/W2", ord("a") .. ord("z")) "2ab" 5710 5711The I<length-item> is not returned explicitly from 5712L<C<unpack>|/unpack TEMPLATE,EXPR>. 5713 5714Supplying a count to the I<length-item> format letter is only useful with 5715C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may 5716introduce C<"\000"> characters, which Perl does not regard as legal in 5717numeric strings. 5718 5719=item * 5720 5721The integer types C<s>, C<S>, C<l>, and C<L> may be 5722followed by a C<!> modifier to specify native shorts or 5723longs. As shown in the example above, a bare C<l> means 5724exactly 32 bits, although the native C<long> as seen by the local C compiler 5725may be larger. This is mainly an issue on 64-bit platforms. You can 5726see whether using C<!> makes any difference this way: 5727 5728 printf "format s is %d, s! is %d\n", 5729 length pack("s"), length pack("s!"); 5730 5731 printf "format l is %d, l! is %d\n", 5732 length pack("l"), length pack("l!"); 5733 5734 5735C<i!> and C<I!> are also allowed, but only for completeness' sake: 5736they are identical to C<i> and C<I>. 5737 5738The actual sizes (in bytes) of native shorts, ints, longs, and long 5739longs on the platform where Perl was built are also available from 5740the command line: 5741 5742 $ perl -V:{short,int,long{,long}}size 5743 shortsize='2'; 5744 intsize='4'; 5745 longsize='4'; 5746 longlongsize='8'; 5747 5748or programmatically via the L<C<Config>|Config> module: 5749 5750 use Config; 5751 print $Config{shortsize}, "\n"; 5752 print $Config{intsize}, "\n"; 5753 print $Config{longsize}, "\n"; 5754 print $Config{longlongsize}, "\n"; 5755 5756C<$Config{longlongsize}> is undefined on systems without 5757long long support. 5758 5759=item * 5760 5761The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are 5762inherently non-portable between processors and operating systems because 5763they obey native byteorder and endianness. For example, a 4-byte integer 57640x12345678 (305419896 decimal) would be ordered natively (arranged in and 5765handled by the CPU registers) into bytes as 5766 5767 0x12 0x34 0x56 0x78 # big-endian 5768 0x78 0x56 0x34 0x12 # little-endian 5769 5770Basically, Intel and VAX CPUs are little-endian, while everybody else, 5771including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are 5772big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) 5773them in little-endian mode, but SGI/Cray uses them in big-endian mode. 5774 5775The names I<big-endian> and I<little-endian> are comic references to the 5776egg-eating habits of the little-endian Lilliputians and the big-endian 5777Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>. 5778This entered computer lingo via the paper "On Holy Wars and a Plea for 5779Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. 5780 5781Some systems may have even weirder byte orders such as 5782 5783 0x56 0x78 0x12 0x34 5784 0x34 0x12 0x78 0x56 5785 5786These are called mid-endian, middle-endian, mixed-endian, or just weird. 5787 5788You can determine your system endianness with this incantation: 5789 5790 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); 5791 5792The byteorder on the platform where Perl was built is also available 5793via L<Config>: 5794 5795 use Config; 5796 print "$Config{byteorder}\n"; 5797 5798or from the command line: 5799 5800 $ perl -V:byteorder 5801 5802Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321"> 5803and C<"87654321"> are big-endian. Systems with multiarchitecture binaries 5804will have C<"ffff">, signifying that static information doesn't work, 5805one must use runtime probing. 5806 5807For portably packed integers, either use the formats C<n>, C<N>, C<v>, 5808and C<V> or else use the C<< > >> and C<< < >> modifiers described 5809immediately below. See also L<perlport>. 5810 5811=item * 5812 5813Also floating point numbers have endianness. Usually (but not always) 5814this agrees with the integer endianness. Even though most platforms 5815these days use the IEEE 754 binary format, there are differences, 5816especially if the long doubles are involved. You can see the 5817C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>, 5818C<longdblsize>): the "kind" values are enums, unlike C<byteorder>. 5819 5820Portability-wise the best option is probably to keep to the IEEE 754 582164-bit doubles, and of agreed-upon endianness. Another possibility 5822is the C<"%a">) format of L<C<printf>|/printf FILEHANDLE FORMAT, LIST>. 5823 5824=item * 5825 5826Starting with Perl 5.10.0, integer and floating-point formats, along with 5827the C<p> and C<P> formats and C<()> groups, may all be followed by the 5828C<< > >> or C<< < >> endianness modifiers to respectively enforce big- 5829or little-endian byte-order. These modifiers are especially useful 5830given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers, 583164-bit integers, or floating-point values. 5832 5833Here are some concerns to keep in mind when using an endianness modifier: 5834 5835=over 5836 5837=item * 5838 5839Exchanging signed integers between different platforms works only 5840when all platforms store them in the same format. Most platforms store 5841signed integers in two's-complement notation, so usually this is not an issue. 5842 5843=item * 5844 5845The C<< > >> or C<< < >> modifiers can only be used on floating-point 5846formats on big- or little-endian machines. Otherwise, attempting to 5847use them raises an exception. 5848 5849=item * 5850 5851Forcing big- or little-endian byte-order on floating-point values for 5852data exchange can work only if all platforms use the same 5853binary representation such as IEEE floating-point. Even if all 5854platforms are using IEEE, there may still be subtle differences. Being able 5855to use C<< > >> or C<< < >> on floating-point values can be useful, 5856but also dangerous if you don't know exactly what you're doing. 5857It is not a general way to portably store floating-point values. 5858 5859=item * 5860 5861When using C<< > >> or C<< < >> on a C<()> group, this affects 5862all types inside the group that accept byte-order modifiers, 5863including all subgroups. It is silently ignored for all other 5864types. You are not allowed to override the byte-order within a group 5865that already has a byte-order modifier suffix. 5866 5867=back 5868 5869=item * 5870 5871Real numbers (floats and doubles) are in native machine format only. 5872Due to the multiplicity of floating-point formats and the lack of a 5873standard "network" representation for them, no facility for interchange has been 5874made. This means that packed floating-point data written on one machine 5875may not be readable on another, even if both use IEEE floating-point 5876arithmetic (because the endianness of the memory representation is not part 5877of the IEEE spec). See also L<perlport>. 5878 5879If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >> 5880modifiers to force big- or little-endian byte-order on floating-point values. 5881 5882Because Perl uses doubles (or long doubles, if configured) internally for 5883all numeric calculation, converting from double into float and thence 5884to double again loses precision, so C<unpack("f", pack("f", $foo)>) 5885will not in general equal $foo. 5886 5887=item * 5888 5889Pack and unpack can operate in two modes: character mode (C<C0> mode) where 5890the packed string is processed per character, and UTF-8 byte mode (C<U0> mode) 5891where the packed string is processed in its UTF-8-encoded Unicode form on 5892a byte-by-byte basis. Character mode is the default 5893unless the format string starts with C<U>. You 5894can always switch mode mid-format with an explicit 5895C<C0> or C<U0> in the format. This mode remains in effect until the next 5896mode change, or until the end of the C<()> group it (directly) applies to. 5897 5898Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode 5899bytes is not necessarily obvious. Probably only the first of these 5900is what you want: 5901 5902 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 5903 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)' 5904 03B1.03C9 5905 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 5906 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' 5907 CE.B1.CF.89 5908 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 5909 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)' 5910 CE.B1.CF.89 5911 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | 5912 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' 5913 C3.8E.C2.B1.C3.8F.C2.89 5914 5915Those examples also illustrate that you should not try to use 5916L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> as a 5917substitute for the L<Encode> module. 5918 5919=item * 5920 5921You must yourself do any alignment or padding by inserting, for example, 5922enough C<"x">es while packing. There is no way for 5923L<C<pack>|/pack TEMPLATE,LIST> and L<C<unpack>|/unpack TEMPLATE,EXPR> 5924to know where characters are going to or coming from, so they 5925handle their output and input as flat sequences of characters. 5926 5927=item * 5928 5929A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may 5930take a repeat count either as postfix, or for 5931L<C<unpack>|/unpack TEMPLATE,EXPR>, also via the C</> 5932template character. Within each repetition of a group, positioning with 5933C<@> starts over at 0. Therefore, the result of 5934 5935 pack("@1A((@2A)@3A)", qw[X Y Z]) 5936 5937is the string C<"\0X\0\0YZ">. 5938 5939=item * 5940 5941C<x> and C<X> accept the C<!> modifier to act as alignment commands: they 5942jump forward or back to the closest position aligned at a multiple of C<count> 5943characters. For example, to L<C<pack>|/pack TEMPLATE,LIST> or 5944L<C<unpack>|/unpack TEMPLATE,EXPR> a C structure like 5945 5946 struct { 5947 char c; /* one signed, 8-bit character */ 5948 double d; 5949 char cc[2]; 5950 } 5951 5952one may need to use the template C<c x![d] d c[2]>. This assumes that 5953doubles must be aligned to the size of double. 5954 5955For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1; 5956both are no-ops. 5957 5958=item * 5959 5960C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to 5961represent signed 16-/32-bit integers in big-/little-endian order. 5962This is portable only when all platforms sharing packed data use the 5963same binary representation for signed integers; for example, when all 5964platforms use two's-complement representation. 5965 5966=item * 5967 5968Comments can be embedded in a TEMPLATE using C<#> through the end of line. 5969White space can separate pack codes from each other, but modifiers and 5970repeat counts must follow immediately. Breaking complex templates into 5971individual line-by-line components, suitably annotated, can do as much to 5972improve legibility and maintainability of pack/unpack formats as C</x> can 5973for complicated pattern matches. 5974 5975=item * 5976 5977If TEMPLATE requires more arguments than L<C<pack>|/pack TEMPLATE,LIST> 5978is given, L<C<pack>|/pack TEMPLATE,LIST> 5979assumes additional C<""> arguments. If TEMPLATE requires fewer arguments 5980than given, extra arguments are ignored. 5981 5982=item * 5983 5984Attempting to pack the special floating point values C<Inf> and C<NaN> 5985(infinity, also in negative, and not-a-number) into packed integer values 5986(like C<"L">) is a fatal error. The reason for this is that there simply 5987isn't any sensible mapping for these special values into integers. 5988 5989=back 5990 5991Examples: 5992 5993 $foo = pack("WWWW",65,66,67,68); 5994 # foo eq "ABCD" 5995 $foo = pack("W4",65,66,67,68); 5996 # same thing 5997 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); 5998 # same thing with Unicode circled letters. 5999 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); 6000 # same thing with Unicode circled letters. You don't get the 6001 # UTF-8 bytes because the U at the start of the format caused 6002 # a switch to U0-mode, so the UTF-8 bytes get joined into 6003 # characters 6004 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); 6005 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" 6006 # This is the UTF-8 encoding of the string in the 6007 # previous example 6008 6009 $foo = pack("ccxxcc",65,66,67,68); 6010 # foo eq "AB\0\0CD" 6011 6012 # NOTE: The examples above featuring "W" and "c" are true 6013 # only on ASCII and ASCII-derived systems such as ISO Latin 1 6014 # and UTF-8. On EBCDIC systems, the first example would be 6015 # $foo = pack("WWWW",193,194,195,196); 6016 6017 $foo = pack("s2",1,2); 6018 # "\001\000\002\000" on little-endian 6019 # "\000\001\000\002" on big-endian 6020 6021 $foo = pack("a4","abcd","x","y","z"); 6022 # "abcd" 6023 6024 $foo = pack("aaaa","abcd","x","y","z"); 6025 # "axyz" 6026 6027 $foo = pack("a14","abcdefg"); 6028 # "abcdefg\0\0\0\0\0\0\0" 6029 6030 $foo = pack("i9pl", gmtime); 6031 # a real struct tm (on my system anyway) 6032 6033 $utmp_template = "Z8 Z8 Z16 L"; 6034 $utmp = pack($utmp_template, @utmp1); 6035 # a struct utmp (BSDish) 6036 6037 @utmp2 = unpack($utmp_template, $utmp); 6038 # "@utmp1" eq "@utmp2" 6039 6040 sub bintodec { 6041 unpack("N", pack("B32", substr("0" x 32 . shift, -32))); 6042 } 6043 6044 $foo = pack('sx2l', 12, 34); 6045 # short 12, two zero bytes padding, long 34 6046 $bar = pack('s@4l', 12, 34); 6047 # short 12, zero fill to position 4, long 34 6048 # $foo eq $bar 6049 $baz = pack('s.l', 12, 4, 34); 6050 # short 12, zero fill to position 4, long 34 6051 6052 $foo = pack('nN', 42, 4711); 6053 # pack big-endian 16- and 32-bit unsigned integers 6054 $foo = pack('S>L>', 42, 4711); 6055 # exactly the same 6056 $foo = pack('s<l<', -42, 4711); 6057 # pack little-endian 16- and 32-bit signed integers 6058 $foo = pack('(sl)<', -42, 4711); 6059 # exactly the same 6060 6061The same template may generally also be used in 6062L<C<unpack>|/unpack TEMPLATE,EXPR>. 6063 6064=item package NAMESPACE 6065 6066=item package NAMESPACE VERSION 6067X<package> X<module> X<namespace> X<version> 6068 6069=item package NAMESPACE BLOCK 6070 6071=item package NAMESPACE VERSION BLOCK 6072X<package> X<module> X<namespace> X<version> 6073 6074=for Pod::Functions declare a separate global namespace 6075 6076Declares the BLOCK or the rest of the compilation unit as being in the 6077given namespace. The scope of the package declaration is either the 6078supplied code BLOCK or, in the absence of a BLOCK, from the declaration 6079itself through the end of current scope (the enclosing block, file, or 6080L<C<eval>|/eval EXPR>). That is, the forms without a BLOCK are 6081operative through the end of the current scope, just like the 6082L<C<my>|/my VARLIST>, L<C<state>|/state VARLIST>, and 6083L<C<our>|/our VARLIST> operators. All unqualified dynamic identifiers 6084in this scope will be in the given namespace, except where overridden by 6085another L<C<package>|/package NAMESPACE> declaration or 6086when they're one of the special identifiers that qualify into C<main::>, 6087like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables. 6088 6089A package statement affects dynamic variables only, including those 6090you've used L<C<local>|/local EXPR> on, but I<not> lexically-scoped 6091variables, which are created with L<C<my>|/my VARLIST>, 6092L<C<state>|/state VARLIST>, or L<C<our>|/our VARLIST>. Typically it 6093would be the first declaration in a file included by 6094L<C<require>|/require VERSION> or L<C<use>|/use Module VERSION LIST>. 6095You can switch into a 6096package in more than one place, since this only determines which default 6097symbol table the compiler uses for the rest of that block. You can refer to 6098identifiers in other packages than the current one by prefixing the identifier 6099with the package name and a double colon, as in C<$SomePack::var> 6100or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main> 6101package is assumed. That is, C<$::sail> is equivalent to 6102C<$main::sail> (as well as to C<$main'sail>, still seen in ancient 6103code, mostly from Perl 4). 6104 6105If VERSION is provided, L<C<package>|/package NAMESPACE> sets the 6106C<$VERSION> variable in the given 6107namespace to a L<version> object with the VERSION provided. VERSION must be a 6108"strict" style version number as defined by the L<version> module: a positive 6109decimal number (integer or decimal-fraction) without exponentiation or else a 6110dotted-decimal v-string with a leading 'v' character and at least three 6111components. You should set C<$VERSION> only once per package. 6112 6113See L<perlmod/"Packages"> for more information about packages, modules, 6114and classes. See L<perlsub> for other scoping issues. 6115 6116=item __PACKAGE__ 6117X<__PACKAGE__> 6118 6119=for Pod::Functions +5.004 the current package 6120 6121A special token that returns the name of the package in which it occurs. 6122 6123=item __CLASS__ 6124X<__CLASS__> 6125 6126=for Pod::Functions the class name of the current instance. 6127 6128Invoked within a L<C<method>|/method NAME BLOCK>, or similar location, such as 6129a field initializer expression, this token returns the name of the class of 6130the invoking instance. Essentially it is equivalent to C<ref($self)> except 6131that it can additionally be used in a field initializer to gain access to 6132class methods, before the instance is fully constructed. 6133 6134 use feature 'class'; 6135 6136 class Example1 { 6137 field $f = __CLASS__->default_f; 6138 6139 sub default_f { 10 } 6140 } 6141 6142In a basic class, this value will be the same as 6143L<C<__PACKAGE__>|/__PACKAGE__>. The distinction can be seen when a subclass 6144is constructed; it will give the class name of the instance being constructed, 6145rather than just the package name that the actual code belongs to. 6146 6147 class Example2 :isa(Example1) { 6148 sub default_f { 20 } 6149 } 6150 6151 my $obj = Example2->new; 6152 # The $f field now has the value 20 6153 6154=item pipe READHANDLE,WRITEHANDLE 6155X<pipe> 6156 6157=for Pod::Functions open a pair of connected filehandles 6158 6159Opens a pair of connected pipes like the corresponding system call. 6160Note that if you set up a loop of piped processes, deadlock can occur 6161unless you are very careful. In addition, note that Perl's pipes use 6162IO buffering, so you may need to set L<C<$E<verbar>>|perlvar/$E<verbar>> 6163to flush your WRITEHANDLE after each command, depending on the 6164application. 6165 6166Returns true on success. 6167 6168See L<IPC::Open2>, L<IPC::Open3>, and 6169L<perlipc/"Bidirectional Communication with Another Process"> 6170for examples of such things. 6171 6172On systems that support a close-on-exec flag on files, that flag is set 6173on all newly opened file descriptors whose 6174L<C<fileno>|/fileno FILEHANDLE>s are I<higher> than the current value of 6175L<C<$^F>|perlvar/$^F> (by default 2 for C<STDERR>). See L<perlvar/$^F>. 6176 6177=item pop ARRAY 6178X<pop> X<stack> 6179 6180=item pop 6181 6182=for Pod::Functions remove the last element from an array and return it 6183 6184Removes and returns the B<last> element of the array, shortening the array by 6185one element. 6186 6187 my @arr = ('cat', 'dog', 'mouse'); 6188 my $item = pop(@arr); # 'mouse' 6189 6190 # @arr is now ('cat', 'dog') 6191 6192Returns C<undef> if the array is empty. 6193 6194B<Note:> C<pop> may also return C<undef> if the last element in the array 6195is C<undef>. 6196 6197 my @arr = ('one', 'two', undef); 6198 my $item = pop(@arr); # undef 6199 6200If ARRAY is omitted, C<pop> operates on the L<C<@ARGV>|perlvar/@ARGV> array 6201in the main program, but the L<C<@_>|perlvar/@_> array in subroutines. C<pop> 6202will operate on the C<@ARGV> array in C<eval STRING>, C<BEGIN {}>, C<INIT {}>, 6203C<CHECK {}> blocks. 6204 6205Starting with Perl 5.14, an experimental feature allowed 6206L<C<pop>|/pop ARRAY> to take a 6207scalar expression. This experiment has been deemed unsuccessful, and was 6208removed as of Perl 5.24. 6209 6210=item pos SCALAR 6211X<pos> X<match, position> 6212 6213=item pos 6214 6215=for Pod::Functions find or set the offset for the last/next m//g search 6216 6217Returns the offset of where the last C<m//g> search left off for the 6218variable in question (L<C<$_>|perlvar/$_> is used when the variable is not 6219specified). This offset is in characters unless the 6220(no-longer-recommended) L<C<use bytes>|bytes> pragma is in effect, in 6221which case the offset is in bytes. Note that 0 is a valid match offset. 6222L<C<undef>|/undef EXPR> indicates 6223that the search position is reset (usually due to match failure, but 6224can also be because no match has yet been run on the scalar). 6225 6226L<C<pos>|/pos SCALAR> directly accesses the location used by the regexp 6227engine to store the offset, so assigning to L<C<pos>|/pos SCALAR> will 6228change that offset, and so will also influence the C<\G> zero-width 6229assertion in regular expressions. Both of these effects take place for 6230the next match, so you can't affect the position with 6231L<C<pos>|/pos SCALAR> during the current match, such as in 6232C<(?{pos() = 5})> or C<s//pos() = 5/e>. 6233 6234Setting L<C<pos>|/pos SCALAR> also resets the I<matched with 6235zero-length> flag, described 6236under L<perlre/"Repeated Patterns Matching a Zero-length Substring">. 6237 6238Because a failed C<m//gc> match doesn't reset the offset, the return 6239from L<C<pos>|/pos SCALAR> won't change either in this case. See 6240L<perlre> and L<perlop>. 6241 6242=item print FILEHANDLE LIST 6243X<print> 6244 6245=item print FILEHANDLE 6246 6247=item print LIST 6248 6249=item print 6250 6251=for Pod::Functions output a list to a filehandle 6252 6253Prints a string or a list of strings. Returns true if successful. 6254FILEHANDLE may be a scalar variable containing the name of or a reference 6255to the filehandle, thus introducing one level of indirection. (NOTE: If 6256FILEHANDLE is a variable and the next token is a term, it may be 6257misinterpreted as an operator unless you interpose a C<+> or put 6258parentheses around the arguments.) If FILEHANDLE is omitted, prints to the 6259last selected (see L<C<select>|/select FILEHANDLE>) output handle. If 6260LIST is omitted, prints L<C<$_>|perlvar/$_> to the currently selected 6261output handle. To use FILEHANDLE alone to print the content of 6262L<C<$_>|perlvar/$_> to it, you must use a bareword filehandle like 6263C<FH>, not an indirect one like C<$fh>. To set the default output handle 6264to something other than STDOUT, use the select operation. 6265 6266The current value of L<C<$,>|perlvar/$,> (if any) is printed between 6267each LIST item. The current value of L<C<$\>|perlvar/$\> (if any) is 6268printed after the entire LIST has been printed. Because print takes a 6269LIST, anything in the LIST is evaluated in list context, including any 6270subroutines whose return lists you pass to 6271L<C<print>|/print FILEHANDLE LIST>. Be careful not to follow the print 6272keyword with a left 6273parenthesis unless you want the corresponding right parenthesis to 6274terminate the arguments to the print; put parentheses around all arguments 6275(or interpose a C<+>, but that doesn't look as good). 6276 6277If you're storing handles in an array or hash, or in general whenever 6278you're using any expression more complex than a bareword handle or a plain, 6279unsubscripted scalar variable to retrieve it, you will have to use a block 6280returning the filehandle value instead, in which case the LIST may not be 6281omitted: 6282 6283 print { $files[$i] } "stuff\n"; 6284 print { $OK ? *STDOUT : *STDERR } "stuff\n"; 6285 6286Printing to a closed pipe or socket will generate a SIGPIPE signal. See 6287L<perlipc> for more on signal handling. 6288 6289=item printf FILEHANDLE FORMAT, LIST 6290X<printf> 6291 6292=item printf FILEHANDLE 6293 6294=item printf FORMAT, LIST 6295 6296=item printf 6297 6298=for Pod::Functions output a formatted list to a filehandle 6299 6300Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that 6301L<C<$\>|perlvar/$\> (the output record separator) is not appended. The 6302FORMAT and the LIST are actually parsed as a single list. The first 6303argument of the list will be interpreted as the 6304L<C<printf>|/printf FILEHANDLE FORMAT, LIST> format. This means that 6305C<printf(@_)> will use C<$_[0]> as the format. See 6306L<sprintf|/sprintf FORMAT, LIST> for an explanation of the format 6307argument. If C<use locale> (including C<use locale ':not_characters'>) 6308is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been 6309called, the character used for the decimal separator in formatted 6310floating-point numbers is affected by the C<LC_NUMERIC> locale setting. 6311See L<perllocale> and L<POSIX>. 6312 6313For historical reasons, if you omit the list, L<C<$_>|perlvar/$_> is 6314used as the format; 6315to use FILEHANDLE without a list, you must use a bareword filehandle like 6316C<FH>, not an indirect one like C<$fh>. However, this will rarely do what 6317you want; if L<C<$_>|perlvar/$_> contains formatting codes, they will be 6318replaced with the empty string and a warning will be emitted if 6319L<warnings> are enabled. Just use L<C<print>|/print FILEHANDLE LIST> if 6320you want to print the contents of L<C<$_>|perlvar/$_>. 6321 6322Don't fall into the trap of using a 6323L<C<printf>|/printf FILEHANDLE FORMAT, LIST> when a simple 6324L<C<print>|/print FILEHANDLE LIST> would do. The 6325L<C<print>|/print FILEHANDLE LIST> is more efficient and less error 6326prone. 6327 6328=item prototype FUNCTION 6329X<prototype> 6330 6331=item prototype 6332 6333=for Pod::Functions +5.002 get the prototype (if any) of a subroutine 6334 6335Returns the prototype of a function as a string (or 6336L<C<undef>|/undef EXPR> if the 6337function has no prototype). FUNCTION is a reference to, or the name of, 6338the function whose prototype you want to retrieve. If FUNCTION is omitted, 6339L<C<$_>|perlvar/$_> is used. 6340 6341If FUNCTION is a string starting with C<CORE::>, the rest is taken as a 6342name for a Perl builtin. If the builtin's arguments 6343cannot be adequately expressed by a prototype 6344(such as L<C<system>|/system LIST>), L<C<prototype>|/prototype FUNCTION> 6345returns L<C<undef>|/undef EXPR>, because the builtin 6346does not really behave like a Perl function. Otherwise, the string 6347describing the equivalent prototype is returned. 6348 6349=item push ARRAY,LIST 6350X<push> X<stack> 6351 6352=for Pod::Functions append one or more elements to an array 6353 6354Adds one or more items to the B<end> of an array. 6355 6356 my @animals = ("cat"); 6357 push(@animals, "mouse"); # ("cat", "mouse") 6358 6359 my @colors = ("red"); 6360 push(@colors, ("blue", "green")); # ("red", "blue", "green") 6361 6362Returns the number of elements in the array following the completed 6363L<C<push>|/push ARRAY,LIST>. 6364 6365 my $color_count = push(@colors, ("yellow", "purple")); 6366 6367 say "There are $color_count colors in the updated array"; 6368 6369Starting with Perl 5.14, an experimental feature allowed 6370L<C<push>|/push ARRAY,LIST> to take a 6371scalar expression. This experiment has been deemed unsuccessful, and was 6372removed as of Perl 5.24. 6373 6374=item q/STRING/ 6375 6376=for Pod::Functions singly quote a string 6377 6378=item qq/STRING/ 6379 6380=for Pod::Functions doubly quote a string 6381 6382=item qw/STRING/ 6383 6384=for Pod::Functions quote a list of words 6385 6386=item qx/STRING/ 6387 6388=for Pod::Functions backquote quote a string 6389 6390Generalized quotes. See L<perlop/"Quote-Like Operators">. 6391 6392=item qr/STRING/ 6393 6394=for Pod::Functions +5.005 compile pattern 6395 6396Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">. 6397 6398=item quotemeta EXPR 6399X<quotemeta> X<metacharacter> 6400 6401=item quotemeta 6402 6403=for Pod::Functions quote regular expression magic characters 6404 6405Returns the value of EXPR with all the ASCII non-"word" 6406characters backslashed. (That is, all ASCII characters not matching 6407C</[A-Za-z_0-9]/> will be preceded by a backslash in the 6408returned string, regardless of any locale settings.) 6409This is the internal function implementing 6410the C<\Q> escape in double-quoted strings. 6411(See below for the behavior on non-ASCII code points.) 6412 6413If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 6414 6415quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into 6416regular expressions, because by default an interpolated variable will be 6417considered a mini-regular expression. For example: 6418 6419 my $sentence = 'The quick brown fox jumped over the lazy dog'; 6420 my $substring = 'quick.*?fox'; 6421 $sentence =~ s{$substring}{big bad wolf}; 6422 6423Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>. 6424 6425On the other hand: 6426 6427 my $sentence = 'The quick brown fox jumped over the lazy dog'; 6428 my $substring = 'quick.*?fox'; 6429 $sentence =~ s{\Q$substring\E}{big bad wolf}; 6430 6431Or: 6432 6433 my $sentence = 'The quick brown fox jumped over the lazy dog'; 6434 my $substring = 'quick.*?fox'; 6435 my $quoted_substring = quotemeta($substring); 6436 $sentence =~ s{$quoted_substring}{big bad wolf}; 6437 6438Will both leave the sentence as is. 6439Normally, when accepting literal string input from the user, 6440L<C<quotemeta>|/quotemeta EXPR> or C<\Q> must be used. 6441 6442Beware that if you put literal backslashes (those not inside 6443interpolated variables) between C<\Q> and C<\E>, double-quotish 6444backslash interpolation may lead to confusing results. If you 6445I<need> to use literal backslashes within C<\Q...\E>, 6446consult L<perlop/"Gory details of parsing quoted constructs">. 6447 6448Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters 6449quoted, there is no way to insert a literal C<$> or C<@> inside a 6450C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become 6451C<"\\\$">; if not, it is interpreted as the start of an interpolated 6452scalar. 6453 6454In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded 6455strings, but not quoted in UTF-8 strings. 6456 6457Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for 6458quoting non-ASCII characters; the quoting of ASCII characters is 6459unchanged. 6460 6461Also unchanged is the quoting of non-UTF-8 strings when outside the 6462scope of a 6463L<C<use feature 'unicode_strings'>|feature/The 'unicode_strings' feature>, 6464which is to quote all 6465characters in the upper Latin1 range. This provides complete backwards 6466compatibility for old programs which do not use Unicode. (Note that 6467C<unicode_strings> is automatically enabled within the scope of a 6468S<C<use v5.12>> or greater.) 6469 6470Within the scope of L<C<use locale>|locale>, all non-ASCII Latin1 code 6471points 6472are quoted whether the string is encoded as UTF-8 or not. As mentioned 6473above, locale does not affect the quoting of ASCII-range characters. 6474This protects against those locales where characters such as C<"|"> are 6475considered to be word characters. 6476 6477Otherwise, Perl quotes non-ASCII characters using an adaptation from 6478Unicode (see L<https://www.unicode.org/reports/tr31/>). 6479The only code points that are quoted are those that have any of the 6480Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, 6481Default_Ignorable_Code_Point, or General_Category=Control. 6482 6483Of these properties, the two important ones are Pattern_Syntax and 6484Pattern_White_Space. They have been set up by Unicode for exactly this 6485purpose of deciding which characters in a regular expression pattern 6486should be quoted. No character that can be in an identifier has these 6487properties. 6488 6489Perl promises, that if we ever add regular expression pattern 6490metacharacters to the dozen already defined 6491(C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the 6492Pattern_Syntax property. Perl also promises, that if we ever add 6493characters that are considered to be white space in regular expressions 6494(currently mostly affected by C</x>), they will all have the 6495Pattern_White_Space property. 6496 6497Unicode promises that the set of code points that have these two 6498properties will never change, so something that is not quoted in v5.16 6499will never need to be quoted in any future Perl release. (Not all the 6500code points that match Pattern_Syntax have actually had characters 6501assigned to them; so there is room to grow, but they are quoted 6502whether assigned or not. Perl, of course, would never use an 6503unassigned code point as an actual metacharacter.) 6504 6505Quoting characters that have the other 3 properties is done to enhance 6506the readability of the regular expression and not because they actually 6507need to be quoted for regular expression purposes (characters with the 6508White_Space property are likely to be indistinguishable on the page or 6509screen from those with the Pattern_White_Space property; and the other 6510two properties contain non-printing characters). 6511 6512=item rand EXPR 6513X<rand> X<random> 6514 6515=item rand 6516 6517=for Pod::Functions retrieve the next pseudorandom number 6518 6519Returns a random fractional number greater than or equal to C<0> and less 6520than the value of EXPR. (EXPR should be positive.) If EXPR is 6521omitted, the value C<1> is used. Currently EXPR with the value C<0> is 6522also special-cased as C<1> (this was undocumented before Perl 5.8.0 6523and is subject to change in future versions of Perl). Automatically calls 6524L<C<srand>|/srand EXPR> unless L<C<srand>|/srand EXPR> has already been 6525called. See also L<C<srand>|/srand EXPR>. 6526 6527Apply L<C<int>|/int EXPR> to the value returned by L<C<rand>|/rand EXPR> 6528if you want random integers instead of random fractional numbers. For 6529example, 6530 6531 int(rand(10)) 6532 6533returns a random integer between C<0> and C<9>, inclusive. 6534 6535(Note: If your rand function consistently returns numbers that are too 6536large or too small, then your version of Perl was probably compiled 6537with the wrong number of RANDBITS.) 6538 6539B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely 6540on it in security-sensitive situations.> As of this writing, a 6541number of third-party CPAN modules offer random number generators 6542intended by their authors to be cryptographically secure, 6543including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, 6544and L<Math::TrulyRandom>. 6545 6546=item read FILEHANDLE,SCALAR,LENGTH,OFFSET 6547X<read> X<file, read> 6548 6549=item read FILEHANDLE,SCALAR,LENGTH 6550 6551=for Pod::Functions fixed-length buffered input from a filehandle 6552 6553Attempts to read LENGTH I<characters> of data into variable SCALAR 6554from the specified FILEHANDLE. Returns the number of characters 6555actually read, C<0> at end of file, or undef if there was an error (in 6556the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will be grown 6557or shrunk 6558so that the last character actually read is the last character of the 6559scalar after the read. 6560 6561An OFFSET may be specified to place the read data at some place in the 6562string other than the beginning. A negative OFFSET specifies 6563placement at that many characters counting backwards from the end of 6564the string. A positive OFFSET greater than the length of SCALAR 6565results in the string being padded to the required size with C<"\0"> 6566bytes before the result of the read is appended. 6567 6568The call is implemented in terms of either Perl's or your system's native 6569L<fread(3)> library function, via the L<PerlIO> layers applied to the 6570handle. To get a true L<read(2)> system call, see 6571L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>. 6572 6573Note the I<characters>: depending on the status of the filehandle, 6574either (8-bit) bytes or characters are read. By default, all 6575filehandles operate on bytes, but for example if the filehandle has 6576been opened with the C<:utf8> I/O layer (see 6577L<C<open>|/open FILEHANDLE,MODE,EXPR>, and the L<open> 6578pragma), the I/O will operate on UTF8-encoded Unicode 6579characters, not bytes. Similarly for the C<:encoding> layer: 6580in that case pretty much any characters can be read. 6581 6582=item readdir DIRHANDLE 6583X<readdir> 6584 6585=for Pod::Functions get a directory from a directory handle 6586 6587Returns the next directory entry for a directory opened by 6588L<C<opendir>|/opendir DIRHANDLE,EXPR>. 6589If used in list context, returns all the rest of the entries in the 6590directory. If there are no more entries, returns the undefined value in 6591scalar context and the empty list in list context. 6592 6593If you're planning to filetest the return values out of a 6594L<C<readdir>|/readdir DIRHANDLE>, you'd better prepend the directory in 6595question. Otherwise, because we didn't L<C<chdir>|/chdir EXPR> there, 6596it would have been testing the wrong file. 6597 6598 opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!"; 6599 my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); 6600 closedir $dh; 6601 6602As of Perl 5.12 you can use a bare L<C<readdir>|/readdir DIRHANDLE> in a 6603C<while> loop, which will set L<C<$_>|perlvar/$_> on every iteration. 6604If either a C<readdir> expression or an explicit assignment of a 6605C<readdir> expression to a scalar is used as a C<while>/C<for> condition, 6606then the condition actually tests for definedness of the expression's 6607value, not for its regular truth value. 6608 6609 opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!"; 6610 while (readdir $dh) { 6611 print "$some_dir/$_\n"; 6612 } 6613 closedir $dh; 6614 6615To avoid confusing would-be users of your code who are running earlier 6616versions of Perl with mysterious failures, put this sort of thing at the 6617top of your file to signal that your code will work I<only> on Perls of a 6618recent vintage: 6619 6620 use v5.12; # so readdir assigns to $_ in a lone while test 6621 6622=item readline EXPR 6623 6624=item readline 6625X<readline> X<gets> X<fgets> 6626 6627=for Pod::Functions fetch a record from a file 6628 6629Reads from the filehandle whose typeglob is contained in EXPR (or from 6630C<*ARGV> if EXPR is not provided). In scalar context, each call reads and 6631returns the next line until end-of-file is reached, whereupon the 6632subsequent call returns L<C<undef>|/undef EXPR>. In list context, reads 6633until end-of-file is reached and returns a list of lines. Note that the 6634notion of "line" used here is whatever you may have defined with 6635L<C<$E<sol>>|perlvar/$E<sol>> (or C<$INPUT_RECORD_SEPARATOR> in 6636L<English>). See L<perlvar/"$/">. 6637 6638When L<C<$E<sol>>|perlvar/$E<sol>> is set to L<C<undef>|/undef EXPR>, 6639when L<C<readline>|/readline EXPR> is in scalar context (i.e., file 6640slurp mode), and when an empty file is read, it returns C<''> the first 6641time, followed by L<C<undef>|/undef EXPR> subsequently. 6642 6643This is the internal function implementing the C<< <EXPR> >> 6644operator, but you can use it directly. The C<< <EXPR> >> 6645operator is discussed in more detail in L<perlop/"I/O Operators">. 6646 6647 my $line = <STDIN>; 6648 my $line = readline(STDIN); # same thing 6649 6650If L<C<readline>|/readline EXPR> encounters an operating system error, 6651L<C<$!>|perlvar/$!> will be set with the corresponding error message. 6652It can be helpful to check L<C<$!>|perlvar/$!> when you are reading from 6653filehandles you don't trust, such as a tty or a socket. The following 6654example uses the operator form of L<C<readline>|/readline EXPR> and dies 6655if the result is not defined. 6656 6657 while ( ! eof($fh) ) { 6658 defined( $_ = readline $fh ) or die "readline failed: $!"; 6659 ... 6660 } 6661 6662Note that you can't handle L<C<readline>|/readline EXPR> errors 6663that way with the C<ARGV> filehandle. In that case, you have to open 6664each element of L<C<@ARGV>|perlvar/@ARGV> yourself since 6665L<C<eof>|/eof FILEHANDLE> handles C<ARGV> differently. 6666 6667 foreach my $arg (@ARGV) { 6668 open(my $fh, $arg) or warn "Can't open $arg: $!"; 6669 6670 while ( ! eof($fh) ) { 6671 defined( $_ = readline $fh ) 6672 or die "readline failed for $arg: $!"; 6673 ... 6674 } 6675 } 6676 6677Like the C<< <EXPR> >> operator, if a C<readline> expression is 6678used as the condition of a C<while> or C<for> loop, then it will be 6679implicitly assigned to C<$_>. If either a C<readline> expression or 6680an explicit assignment of a C<readline> expression to a scalar is used 6681as a C<while>/C<for> condition, then the condition actually tests for 6682definedness of the expression's value, not for its regular truth value. 6683 6684=item readlink EXPR 6685X<readlink> 6686 6687=item readlink 6688 6689=for Pod::Functions determine where a symbolic link is pointing 6690 6691Returns the value of a symbolic link, if symbolic links are 6692implemented. If not, raises an exception. If there is a system 6693error, returns the undefined value and sets L<C<$!>|perlvar/$!> (errno). 6694If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 6695 6696Portability issues: L<perlport/readlink>. 6697 6698=item readpipe EXPR 6699 6700=item readpipe 6701X<readpipe> 6702 6703=for Pod::Functions execute a system command and collect standard output 6704 6705EXPR is executed as a system command. 6706The collected standard output of the command is returned. 6707In scalar context, it comes back as a single (potentially 6708multi-line) string. In list context, returns a list of lines 6709(however you've defined lines with L<C<$E<sol>>|perlvar/$E<sol>> (or 6710C<$INPUT_RECORD_SEPARATOR> in L<English>)). 6711This is the internal function implementing the C<qx/EXPR/> 6712operator, but you can use it directly. The C<qx/EXPR/> 6713operator is discussed in more detail in L<perlop/"C<qx/I<STRING>/>">. 6714If EXPR is omitted, uses L<C<$_>|perlvar/$_>. 6715 6716=item recv SOCKET,SCALAR,LENGTH,FLAGS 6717X<recv> 6718 6719=for Pod::Functions receive a message over a Socket 6720 6721Receives a message on a socket. Attempts to receive LENGTH characters 6722of data into variable SCALAR from the specified SOCKET filehandle. 6723SCALAR will be grown or shrunk to the length actually read. Takes the 6724same flags as the system call of the same name. Returns the address 6725of the sender if SOCKET's protocol supports this; returns an empty 6726string otherwise. If there's an error, returns the undefined value. 6727This call is actually implemented in terms of the L<recvfrom(2)> system call. 6728See L<perlipc/"UDP: Message Passing"> for examples. 6729 6730Note that if the socket has been marked as C<:utf8>, C<recv> will 6731throw an exception. The C<:encoding(...)> layer implicitly introduces 6732the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>. 6733 6734=item redo LABEL 6735X<redo> 6736 6737=item redo EXPR 6738 6739=item redo 6740 6741=for Pod::Functions start this loop iteration over again 6742 6743The L<C<redo>|/redo LABEL> command restarts the loop block without 6744evaluating the conditional again. The L<C<continue>|/continue BLOCK> 6745block, if any, is not executed. If 6746the LABEL is omitted, the command refers to the innermost enclosing 6747loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a 6748label name to be computed at run time, and is otherwise identical to C<redo 6749LABEL>. Programs that want to lie to themselves about what was just input 6750normally use this command: 6751 6752 # a simpleminded Pascal comment stripper 6753 # (warning: assumes no { or } in strings) 6754 LINE: while (<STDIN>) { 6755 while (s|({.*}.*){.*}|$1 |) {} 6756 s|{.*}| |; 6757 if (s|{.*| |) { 6758 my $front = $_; 6759 while (<STDIN>) { 6760 if (/}/) { # end of comment? 6761 s|^|$front\{|; 6762 redo LINE; 6763 } 6764 } 6765 } 6766 print; 6767 } 6768 6769L<C<redo>|/redo LABEL> cannot return a value from a block that typically 6770returns a value, such as C<eval {}>, C<sub {}>, or C<do {}>. It will perform 6771its flow control behavior, which precludes any return value. It should not be 6772used to exit a L<C<grep>|/grep BLOCK LIST> or L<C<map>|/map BLOCK LIST> 6773operation. 6774 6775Note that a block by itself is semantically identical to a loop 6776that executes once. Thus L<C<redo>|/redo LABEL> inside such a block 6777will effectively turn it into a looping construct. 6778 6779See also L<C<continue>|/continue BLOCK> for an illustration of how 6780L<C<last>|/last LABEL>, L<C<next>|/next LABEL>, and 6781L<C<redo>|/redo LABEL> work. 6782 6783Unlike most named operators, this has the same precedence as assignment. 6784It is also exempt from the looks-like-a-function rule, so 6785C<redo ("foo")."bar"> will cause "bar" to be part of the argument to 6786L<C<redo>|/redo LABEL>. 6787 6788=item ref EXPR 6789X<ref> X<reference> 6790 6791=item ref 6792 6793=for Pod::Functions find out the type of thing being referenced 6794 6795Examines the value of EXPR, expecting it to be a reference, and returns 6796a string giving information about the reference and the type of referent. 6797If EXPR is not specified, L<C<$_>|perlvar/$_> will be used. 6798 6799If the operand is not a reference, then the empty string will be returned. 6800An empty string will only be returned in this situation. C<ref> is often 6801useful to just test whether a value is a reference, which can be done 6802by comparing the result to the empty string. It is a common mistake 6803to use the result of C<ref> directly as a truth value: this goes wrong 6804because C<0> (which is false) can be returned for a reference. 6805 6806If the operand is a reference to a blessed object, then the name of 6807the class into which the referent is blessed will be returned. C<ref> 6808doesn't care what the physical type of the referent is; blessing takes 6809precedence over such concerns. Beware that exact comparison of C<ref> 6810results against a class name doesn't perform a class membership test: 6811a class's members also include objects blessed into subclasses, for 6812which C<ref> will return the name of the subclass. Also beware that 6813class names can clash with the built-in type names (described below). 6814 6815If the operand is a reference to an unblessed object, then the return 6816value indicates the type of object. If the unblessed referent is not 6817a scalar, then the return value will be one of the strings C<ARRAY>, 6818C<HASH>, C<CODE>, C<FORMAT>, or C<IO>, indicating only which kind of 6819object it is. If the unblessed referent is a scalar, then the return 6820value will be one of the strings C<SCALAR>, C<VSTRING>, C<REF>, C<GLOB>, 6821C<LVALUE>, or C<REGEXP>, depending on the kind of value the scalar 6822currently has. But note that C<qr//> scalars are created already 6823blessed, so C<ref qr/.../> will likely return C<Regexp>. Beware that 6824these built-in type names can also be used as 6825class names, so C<ref> returning one of these names doesn't unambiguously 6826indicate that the referent is of the kind to which the name refers. 6827 6828The ambiguity between built-in type names and class names significantly 6829limits the utility of C<ref>. For unambiguous information, use 6830L<C<Scalar::Util::blessed()>|Scalar::Util/blessed> for information about 6831blessing, and L<C<Scalar::Util::reftype()>|Scalar::Util/reftype> for 6832information about physical types. Use L<the C<isa> method|UNIVERSAL/C<< 6833$obj->isa( TYPE ) >>> for class membership tests, though one must be 6834sure of blessedness before attempting a method call. Alternatively, the 6835L<C<isa> operator|perlop/"Class Instance Operator"> can test class 6836membership without checking blessedness first. 6837 6838See also L<perlref> and L<perlobj>. 6839 6840=item rename OLDNAME,NEWNAME 6841X<rename> X<move> X<mv> X<ren> 6842 6843=for Pod::Functions change a filename 6844 6845Changes the name of a file; an existing file NEWNAME will be 6846clobbered. Returns true for success; on failure returns false and sets 6847L<C<$!>|perlvar/$!>. 6848 6849Behavior of this function varies wildly depending on your system 6850implementation. For example, it will usually not work across file system 6851boundaries, even though the system I<mv> command sometimes compensates 6852for this. Other restrictions include whether it works on directories, 6853open files, or pre-existing files. Check L<perlport> and either the 6854L<rename(2)> manpage or equivalent system documentation for details. 6855 6856For a platform independent L<C<move>|File::Copy/move> function look at 6857the L<File::Copy> module. 6858 6859Portability issues: L<perlport/rename>. 6860 6861=item require VERSION 6862X<require> 6863 6864=item require EXPR 6865 6866=item require 6867 6868=for Pod::Functions load in external functions from a library at runtime 6869 6870Demands a version of Perl specified by VERSION, or demands some semantics 6871specified by EXPR or by L<C<$_>|perlvar/$_> if EXPR is not supplied. 6872 6873VERSION may be either a literal such as v5.24.1, which will be 6874compared to L<C<$^V>|perlvar/$^V> (or C<$PERL_VERSION> in L<English>), 6875or a numeric argument of the form 5.024001, which will be compared to 6876L<C<$]>|perlvar/$]>. An exception is raised if VERSION is greater than 6877the version of the current Perl interpreter. Compare with 6878L<C<use>|/use Module VERSION LIST>, which can do a similar check at 6879compile time. 6880 6881Specifying VERSION as a numeric argument of the form 5.024001 should 6882generally be avoided as older less readable syntax compared to 6883v5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric 6884form was the only supported syntax, which is why you might see it in 6885older code. 6886 6887 require v5.24.1; # run time version check 6888 require 5.24.1; # ditto 6889 require 5.024_001; # ditto; older syntax compatible 6890 with perl 5.6 6891 6892Otherwise, L<C<require>|/require VERSION> demands that a library file be 6893included if it hasn't already been included. The file is included via 6894the do-FILE mechanism, which is essentially just a variety of 6895L<C<eval>|/eval EXPR> with the 6896caveat that lexical variables in the invoking script will be invisible 6897to the included code. If it were implemented in pure Perl, it 6898would have semantics similar to the following: 6899 6900 use Carp 'croak'; 6901 use version; 6902 6903 sub require { 6904 my ($filename) = @_; 6905 if ( my $version = eval { version->parse($filename) } ) { 6906 if ( $version > $^V ) { 6907 my $vn = $version->normal; 6908 croak "Perl $vn required--this is only $^V, stopped"; 6909 } 6910 return 1; 6911 } 6912 6913 if (exists $INC{$filename}) { 6914 return 1 if $INC{$filename}; 6915 croak "Compilation failed in require"; 6916 } 6917 6918 local $INC; 6919 # this type of loop lets a hook overwrite $INC if they wish 6920 for($INC = 0; $INC < @INC; $INC++) { 6921 my $prefix = $INC[$INC]; 6922 if (!defined $prefix) { 6923 next; 6924 } 6925 if (ref $prefix) { 6926 #... do other stuff - see text below .... 6927 } 6928 # (see text below about possible appending of .pmc 6929 # suffix to $filename) 6930 my $realfilename = "$prefix/$filename"; 6931 next if ! -e $realfilename || -d _ || -b _; 6932 $INC{$filename} = $realfilename; 6933 my $result = do($realfilename); 6934 # but run in caller's namespace 6935 6936 if (!defined $result) { 6937 $INC{$filename} = undef; 6938 croak $@ ? "$@Compilation failed in require" 6939 : "Can't locate $filename: $!\n"; 6940 } 6941 if (!$result) { 6942 delete $INC{$filename}; 6943 croak "$filename did not return true value"; 6944 } 6945 $! = 0; 6946 return $result; 6947 } 6948 croak "Can't locate $filename in \@INC ..."; 6949 } 6950 6951Note that the file will not be included twice under the same specified 6952name. 6953 6954Historically the file must return true as the last statement to indicate 6955successful execution of any initialization code, so it's customary to 6956end such a file with C<1;> unless you're sure it'll return true 6957otherwise. But it's better just to put the C<1;>, in case you add more 6958statements. As of 5.37.6 this requirement may be avoided by enabling 6959the 'module_true' feature, which is enabled by default in modern 6960version bundles. Thus code with C<use v5.37;> no longer needs to concern 6961itself with this issue. See L<feature> for more details. Note that this 6962affects the compilation unit within which the feature is used, and using 6963it before requiring a module will not change the behavior of existing 6964modules that do not themselves also use it. 6965 6966If EXPR is a bareword, L<C<require>|/require VERSION> assumes a F<.pm> 6967extension and replaces C<::> with C</> in the filename for you, 6968to make it easy to load standard modules. This form of loading of 6969modules does not risk altering your namespace, however it will autovivify 6970the stash for the required module. 6971 6972In other words, if you try this: 6973 6974 require Foo::Bar; # a splendid bareword 6975 6976The require function will actually look for the F<Foo/Bar.pm> file in the 6977directories specified in the L<C<@INC>|perlvar/@INC> array, and it will 6978autovivify the C<Foo::Bar::> stash at compile time. 6979 6980But if you try this: 6981 6982 my $class = 'Foo::Bar'; 6983 require $class; # $class is not a bareword 6984 #or 6985 require "Foo::Bar"; # not a bareword because of the "" 6986 6987The require function will look for the F<Foo::Bar> file in the 6988L<C<@INC>|perlvar/@INC> array and 6989will complain about not finding F<Foo::Bar> there. In this case you can do: 6990 6991 eval "require $class"; 6992 6993or you could do 6994 6995 require "Foo/Bar.pm"; 6996 6997Neither of these forms will autovivify any stashes at compile time and 6998only have run time effects. 6999 7000Now that you understand how L<C<require>|/require VERSION> looks for 7001files with a bareword argument, there is a little extra functionality 7002going on behind the scenes. Before L<C<require>|/require VERSION> looks 7003for a F<.pm> extension, it will first look for a similar filename with a 7004F<.pmc> extension. If this file is found, it will be loaded in place of 7005any file ending in a F<.pm> extension. This applies to both the explicit 7006C<require "Foo/Bar.pm";> form and the C<require Foo::Bar;> form. 7007 7008You can also insert hooks into the import facility by putting Perl 7009coderefs or objects directly into the L<C<@INC>|perlvar/@INC> array. 7010There are two types of hooks, INC filters, and INCDIR hooks, and there 7011are three forms of representing a hook: subroutine references, array 7012references, and blessed objects. 7013 7014Subroutine references are the simplest case. When the inclusion system 7015walks through L<C<@INC>|perlvar/@INC> and encounters a subroutine, unless 7016this subroutine is blessed and supports an INCDIR hook this 7017subroutine will be assumed to be an INC hook will be called with two 7018parameters, the first a reference to itself, and the second the name of 7019the file to be included (e.g., F<Foo/Bar.pm>). The subroutine should 7020return either nothing or else a list of up to four values in the 7021following order: 7022 7023=over 7024 7025=item 1 7026 7027A reference to a scalar, containing any initial source code to prepend to 7028the file or generator output. 7029 7030=item 2 7031 7032A filehandle, from which the file will be read. 7033 7034=item 3 7035 7036A reference to a subroutine. If there is no filehandle (previous item), 7037then this subroutine is expected to generate one line of source code per 7038call, writing the line into L<C<$_>|perlvar/$_> and returning 1, then 7039finally at end of file returning 0. If there is a filehandle, then the 7040subroutine will be called to act as a simple source filter, with the 7041line as read in L<C<$_>|perlvar/$_>. 7042Again, return 1 for each valid line, and 0 after all lines have been 7043returned. 7044For historical reasons the subroutine will receive a meaningless argument 7045(in fact always the numeric value zero) as C<$_[0]>. 7046 7047=item 4 7048 7049Optional state for the subroutine. The state is passed in as C<$_[1]>. 7050 7051=back 7052 7053C<AUTOLOAD> cannot be used to resolve the C<INCDIR> method, C<INC> is 7054checked first, and C<AUTOLOAD> would resolve that. 7055 7056If an empty list, L<C<undef>|/undef EXPR>, or nothing that matches the 7057first 3 values above is returned, then L<C<require>|/require VERSION> 7058looks at the remaining elements of L<C<@INC>|perlvar/@INC>. 7059Note that this filehandle must be a real filehandle (strictly a typeglob 7060or reference to a typeglob, whether blessed or unblessed); tied filehandles 7061will be ignored and processing will stop there. 7062 7063If the hook is an object, it should provide an C<INC> or C<INCDIR> 7064method that will be called as above, the first parameter being the 7065object itself. If it does not provide either method, and the object is 7066not CODE ref then an exception will be thrown, otherwise it will simply 7067be executed like an unblessed CODE ref would. Note that you must fully 7068qualify the method name when you declare an C<INC> sub (unlike the 7069C<INCDIR> sub), as the unqualified symbol C<INC> is always forced into 7070package C<main>. Here is a typical code layout for an C<INC> hook: 7071 7072 # In Foo.pm 7073 package Foo; 7074 sub new { ... } 7075 sub Foo::INC { 7076 my ($self, $filename) = @_; 7077 ... 7078 } 7079 7080 # In the main program 7081 push @INC, Foo->new(...); 7082 7083If the hook is an array reference, its first element must be a 7084subroutine reference or an object as described above. When the first 7085element is an object that supports an C<INC> or C<INCDIR> method then 7086the method will be called with the object as the first argument, the 7087filename requested as the second, and the hook array reference as the 7088the third. When the first element is a subroutine then it will be 7089called with the array as the first argument, and the filename as the 7090second, no third parameter will be passed in. In both forms you can 7091modify the contents of the array to provide state between calls, or 7092whatever you like. 7093 7094 7095In other words, you can write: 7096 7097 push @INC, \&my_sub; 7098 sub my_sub { 7099 my ($coderef, $filename) = @_; # $coderef is \&my_sub 7100 ... 7101 } 7102 7103or: 7104 7105 push @INC, [ \&my_sub, $x, $y, ... ]; 7106 sub my_sub { 7107 my ($arrayref, $filename) = @_; 7108 # Retrieve $x, $y, ... 7109 my (undef, @parameters) = @$arrayref; 7110 ... 7111 } 7112 7113or: 7114 7115 push @INC, [ HookObj->new(), $x, $y, ... ]; 7116 sub HookObj::INC { 7117 my ($self, $filename, $arrayref)= @_; 7118 my (undef, @parameters) = @$arrayref; 7119 ... 7120 } 7121 7122These hooks are also permitted to set the L<C<%INC>|perlvar/%INC> entry 7123corresponding to the files they have loaded. See L<perlvar/%INC>. 7124Should an C<INC> hook not do this then perl will set the C<%INC> entry 7125to be the hook reference itself. 7126 7127A hook may also be used to rewrite the C<@INC> array. While this might 7128sound strange, there are situations where it can be very useful to do 7129this. Such hooks usually just return undef and do not mix filtering and 7130C<@INC> modifications. While in older versions of perl having a hook 7131modify C<@INC> was fraught with issues and could even result in 7132segfaults or assert failures, as of 5.37.7 the logic has been made much 7133more robust and the hook now has control over the loop iteration if it 7134wishes to do so. 7135 7136There is a now a facility to control the iterator for the C<@INC> array 7137traversal that is performed during require. The C<$INC> variable will be 7138initialized with the index of the currently executing hook. Once the 7139hook returns the next slot in C<@INC> that will be checked will be the 7140integer successor of value in C<$INC> (or -1 if it is undef). For example 7141the following code 7142 7143 push @INC, sub { 7144 splice @INC, $INC, 1; # remove this hook from @INC 7145 unshift @INC, sub { warn "A" }; 7146 undef $INC; # reset the $INC iterator so we 7147 # execute the newly installed sub 7148 # immediately. 7149 }; 7150 7151would install a sub into C<@INC> that when executed as a hook (by for 7152instance a require of a file that does not exist), the hook will splice 7153itself out of C<@INC>, and add a new sub to the front that will warn 7154whenever someone does a require operation that requires an C<@INC> 7155search, and then immediately execute that hook. 7156 7157Prior to 5.37.7, there was no way to cause perl to use the newly 7158installed hook immediately, or to inspect any changed items in C<@INC> to 7159the left of the iterator, and so the warning would only be generated on 7160the second call to require. In more recent perl the presence of the last 7161statement which undefines C<$INC> will cause perl to restart the 7162traversal of the C<@INC> array at the beginning and execute the newly 7163installed sub immediately. 7164 7165Whatever value C<$INC> held, if any, will be restored at the end of the 7166require. Any changes made to C<$INC> during the lifetime of the hook 7167will be unrolled after the hook exits, and its value only has meaning 7168immediately after execution of the hook, thus setting C<$INC> to some 7169value prior to executing a C<require> will have no effect on how the 7170require executes at all. 7171 7172As of 5.37.7 C<@INC> values of undef will be silently ignored. 7173 7174The function C<require()> is difficult to wrap properly. Many modules 7175consult the stack to find information about their caller, and injecting 7176a new stack frame by wrapping C<require()> often breaks things. 7177Nevertheless it can be very helpful to have the ability to perform 7178actions before and after a C<require>, for instance for trace utilities 7179like C<Devel::TraceUse> or to measure time to load and the memory 7180consumption of the require graph. Because of the difficulties in safely 7181creating a C<require()> wrapper in 5.37.10 we introduced a new mechanism. 7182 7183As of 5.37.10, prior to any other actions it performs, C<require> will 7184check if C<${^HOOK}{require__before}> contains a coderef, and if it does 7185it will be called with the filename form of the item being loaded. The hook 7186may modify C<$_[0]> to load a different filename, or it may throw a fatal 7187exception to cause the require to fail, which will be treated as though the 7188required code itself had thrown an exception. 7189 7190The C<${^HOOK}{require__before}> hook may return a code reference, in 7191which case the code reference will be executed (in an eval with the 7192filname as a parameter) after the require completes. It will be executed 7193regardless of how the compilation completed, and even if the require 7194throws a fatal exception. The function may consult C<%INC> to determine 7195if the require failed or not. For instance the following code will print 7196some diagnostics before and after every C<require> statement. The 7197example also includes logic to chain the signal, so that multiple 7198signals can cooperate. Well behaved C<${^HOOK}{require__before}> 7199handlers should always take this into account. 7200 7201 { 7202 use Scalar::Util qw(reftype); 7203 my $old_hook = ${^HOOK}{require__before}; 7204 local ${^HOOK}{require__before} = sub { 7205 my ($name) = @_; 7206 my $old_hook_ret; 7207 $old_hook_ret = $old_hook->($name) if $old_hook; 7208 warn "Requiring: $name\n"; 7209 return sub { 7210 $old_hook_ret->() if ref($old_hook_ret) 7211 && reftype($old_hook_ret) eq "CODE"; 7212 warn sprintf "Finished requiring %s: %s\n", 7213 $name, $INC{$name} ? "loaded" :"failed"; 7214 }; 7215 }; 7216 require Whatever; 7217 } 7218 7219This hook executes for ALL C<require> statements, unlike C<INC> and 7220C<INCDIR> hooks, which are only executed for relative file names, and it 7221executes first before any other special behaviour inside of require. 7222Note that the initial hook in C<${^HOOK}{require__before}> is *not* 7223executed inside of an eval, and throwing an exception will stop further 7224processing, but the after hook it may return is executed inside of an 7225eval, and any exceptions it throws will be silently ignored. This is 7226because it executes inside of the scope cleanup logic that is triggered 7227after the require completes, and an exception at this time would not 7228stop the module from being loaded, etc. 7229 7230There is a similar hook that fires after require completes, 7231C<${^HOOK}{require__after}>, which will be called after each require statement 7232completes, either via an exception or successfully. It will be called with 7233the filename of the most recently executed require statement. It is executed 7234in an eval, and will not in any way affect execution. 7235 7236For a yet-more-powerful import facility built around C<require>, see 7237L<C<use>|/use Module VERSION LIST> and L<perlmod>. 7238 7239=item reset EXPR 7240X<reset> 7241 7242=item reset 7243 7244=for Pod::Functions clear all variables of a given name 7245 7246Generally used in a L<C<continue>|/continue BLOCK> block at the end of a 7247loop to clear variables and reset C<m?pattern?> searches so that they 7248work again. The 7249expression is interpreted as a list of single characters (hyphens 7250allowed for ranges). All variables (scalars, arrays, and hashes) 7251in the current package beginning with one of 7252those letters are reset to their pristine state. If the expression is 7253omitted, one-match searches (C<m?pattern?>) are reset to match again. 7254Only resets variables or searches in the current package. Always returns 72551. Examples: 7256 7257 reset 'X'; # reset all X variables 7258 reset 'a-z'; # reset lower case variables 7259 reset; # just reset m?one-time? searches 7260 7261Resetting C<"A-Z"> is not recommended because you'll wipe out your 7262L<C<@ARGV>|perlvar/@ARGV> and L<C<@INC>|perlvar/@INC> arrays and your 7263L<C<%ENV>|perlvar/%ENV> hash. 7264 7265Resets only package variables; lexical variables are unaffected, but 7266they clean themselves up on scope exit anyway, so you'll probably want 7267to use them instead. See L<C<my>|/my VARLIST>. 7268 7269=item return EXPR 7270X<return> 7271 7272=item return 7273 7274=for Pod::Functions get out of a function early 7275 7276Returns from a subroutine, L<C<eval>|/eval EXPR>, 7277L<C<do FILE>|/do EXPR>, L<C<sort>|/sort SUBNAME LIST> block or regex 7278eval block (but not a L<C<grep>|/grep BLOCK LIST>, 7279L<C<map>|/map BLOCK LIST>, or L<C<do BLOCK>|/do BLOCK> block) with the value 7280given in EXPR. Evaluation of EXPR may be in list, scalar, or void 7281context, depending on how the return value will be used, and the context 7282may vary from one execution to the next (see 7283L<C<wantarray>|/wantarray>). If no EXPR 7284is given, returns an empty list in list context, the undefined value in 7285scalar context, and (of course) nothing at all in void context. 7286 7287(In the absence of an explicit L<C<return>|/return EXPR>, a subroutine, 7288L<C<eval>|/eval EXPR>, 7289or L<C<do FILE>|/do EXPR> automatically returns the value of the last expression 7290evaluated.) 7291 7292Unlike most named operators, this is also exempt from the 7293looks-like-a-function rule, so C<return ("foo")."bar"> will 7294cause C<"bar"> to be part of the argument to L<C<return>|/return EXPR>. 7295 7296=item reverse LIST 7297X<reverse> X<rev> X<invert> 7298 7299=for Pod::Functions flip a string or a list 7300 7301In list context, returns a list value consisting of the elements 7302of LIST in the opposite order. In scalar context, concatenates the 7303elements of LIST and returns a string value with all characters 7304in the opposite order. 7305 7306 print join(", ", reverse "world", "Hello"); # Hello, world 7307 7308 print scalar reverse "dlrow ,", "olleH"; # Hello, world 7309 7310Used without arguments in scalar context, L<C<reverse>|/reverse LIST> 7311reverses L<C<$_>|perlvar/$_>. 7312 7313 $_ = "dlrow ,olleH"; 7314 print reverse; # No output, list context 7315 print scalar reverse; # Hello, world 7316 7317Note that reversing an array to itself (as in C<@a = reverse @a>) will 7318preserve non-existent elements whenever possible; i.e., for non-magical 7319arrays or for tied arrays with C<EXISTS> and C<DELETE> methods. 7320 7321This operator is also handy for inverting a hash, although there are some 7322caveats. If a value is duplicated in the original hash, only one of those 7323can be represented as a key in the inverted hash. Also, this has to 7324unwind one hash and build a whole new one, which may take some time 7325on a large hash, such as from a DBM file. 7326 7327 my %by_name = reverse %by_address; # Invert the hash 7328 7329=item rewinddir DIRHANDLE 7330X<rewinddir> 7331 7332=for Pod::Functions reset directory handle 7333 7334Sets the current position to the beginning of the directory for the 7335L<C<readdir>|/readdir DIRHANDLE> routine on DIRHANDLE. 7336 7337Portability issues: L<perlport/rewinddir>. 7338 7339=item rindex STR,SUBSTR,POSITION 7340X<rindex> 7341 7342=item rindex STR,SUBSTR 7343 7344=for Pod::Functions right-to-left substring search 7345 7346Works just like L<C<index>|/index STR,SUBSTR,POSITION> except that it 7347returns the position of the I<last> 7348occurrence of SUBSTR in STR. If POSITION is specified, returns the 7349last occurrence beginning at or before that position. 7350 7351=item rmdir FILENAME 7352X<rmdir> X<rd> X<directory, remove> 7353 7354=item rmdir 7355 7356=for Pod::Functions remove a directory 7357 7358Deletes the directory specified by FILENAME if that directory is 7359empty. If it succeeds it returns true; otherwise it returns false and 7360sets L<C<$!>|perlvar/$!> (errno). If FILENAME is omitted, uses 7361L<C<$_>|perlvar/$_>. 7362 7363To remove a directory tree recursively (C<rm -rf> on Unix) look at 7364the L<C<rmtree>|File::Path/rmtree( $dir )> function of the L<File::Path> 7365module. 7366 7367=item s/// 7368 7369=for Pod::Functions replace a pattern with a string 7370 7371The substitution operator. See L<perlop/"Regexp Quote-Like Operators">. 7372 7373=item say FILEHANDLE LIST 7374X<say> 7375 7376=item say FILEHANDLE 7377 7378=item say LIST 7379 7380=item say 7381 7382=for Pod::Functions +say output a list to a filehandle, appending a newline 7383 7384Just like L<C<print>|/print FILEHANDLE LIST>, but implicitly appends a 7385newline at the end of the LIST instead of any value L<C<$\>|perlvar/$\> 7386might have. To use FILEHANDLE without a LIST to 7387print the contents of L<C<$_>|perlvar/$_> to it, you must use a bareword 7388filehandle like C<FH>, not an indirect one like C<$fh>. 7389 7390L<C<say>|/say FILEHANDLE LIST> is available only if the 7391L<C<"say"> feature|feature/The 'say' feature> is enabled or if it is 7392prefixed with C<CORE::>. The 7393L<C<"say"> feature|feature/The 'say' feature> is enabled automatically 7394with a C<use v5.10> (or higher) declaration in the current scope. 7395 7396=item scalar EXPR 7397X<scalar> X<context> 7398 7399=for Pod::Functions force a scalar context 7400 7401Forces EXPR to be interpreted in scalar context and returns the value 7402of EXPR. 7403 7404 my @counts = ( scalar @a, scalar @b, scalar @c ); 7405 7406There is no equivalent operator to force an expression to 7407be interpolated in list context because in practice, this is never 7408needed. If you really wanted to do so, however, you could use 7409the construction C<@{[ (some expression) ]}>, but usually a simple 7410C<(some expression)> suffices. 7411 7412Because L<C<scalar>|/scalar EXPR> is a unary operator, if you 7413accidentally use a 7414parenthesized list for the EXPR, this behaves as a scalar comma expression, 7415evaluating all but the last element in void context and returning the final 7416element evaluated in scalar context. This is seldom what you want. 7417 7418The following single statement: 7419 7420 print uc(scalar(foo(), $bar)), $baz; 7421 7422is the moral equivalent of these two: 7423 7424 foo(); 7425 print(uc($bar), $baz); 7426 7427See L<perlop> for more details on unary operators and the comma operator, 7428and L<perldata> for details on evaluating a hash in scalar context. 7429 7430=item seek FILEHANDLE,POSITION,WHENCE 7431X<seek> X<fseek> X<filehandle, position> 7432 7433=for Pod::Functions reposition file pointer for random-access I/O 7434 7435Sets FILEHANDLE's position, just like the L<fseek(3)> call of C C<stdio>. 7436FILEHANDLE may be an expression whose value gives the name of the 7437filehandle. The values for WHENCE are C<0> to set the new position 7438I<in bytes> to POSITION; C<1> to set it to the current position plus 7439POSITION; and C<2> to set it to EOF plus POSITION, typically 7440negative. For WHENCE you may use the constants C<SEEK_SET>, 7441C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end 7442of the file) from the L<Fcntl> module. Returns C<1> on success, false 7443otherwise. 7444 7445Note the emphasis on bytes: even if the filehandle has been set to operate 7446on characters (for example using the C<:encoding(UTF-8)> I/O layer), the 7447L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 7448L<C<tell>|/tell FILEHANDLE>, and 7449L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> 7450family of functions use byte offsets, not character offsets, 7451because seeking to a character offset would be very slow in a UTF-8 file. 7452 7453If you want to position the file for 7454L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> or 7455L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, don't use 7456L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, because buffering makes its 7457effect on the file's read-write position unpredictable and non-portable. 7458Use L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> instead. 7459 7460Due to the rules and rigors of ANSI C, on some systems you have to do a 7461seek whenever you switch between reading and writing. Amongst other 7462things, this may have the effect of calling stdio's L<clearerr(3)>. 7463A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: 7464 7465 seek($fh, 0, 1); 7466 7467This is also useful for applications emulating C<tail -f>. Once you hit 7468EOF on your read and then sleep for a while, you (probably) have to stick in a 7469dummy L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to reset things. The 7470L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> doesn't change the position, 7471but it I<does> clear the end-of-file condition on the handle, so that the 7472next C<readline FILE> makes Perl try again to read something. (We hope.) 7473 7474If that doesn't work (some I/O implementations are particularly 7475cantankerous), you might need something like this: 7476 7477 for (;;) { 7478 for ($curpos = tell($fh); $_ = readline($fh); 7479 $curpos = tell($fh)) { 7480 # search for some stuff and put it into files 7481 } 7482 sleep($for_a_while); 7483 seek($fh, $curpos, 0); 7484 } 7485 7486=item seekdir DIRHANDLE,POS 7487X<seekdir> 7488 7489=for Pod::Functions reposition directory pointer 7490 7491Sets the current position for the L<C<readdir>|/readdir DIRHANDLE> 7492routine on DIRHANDLE. POS must be a value returned by 7493L<C<telldir>|/telldir DIRHANDLE>. L<C<seekdir>|/seekdir DIRHANDLE,POS> 7494also has the same caveats about possible directory compaction as the 7495corresponding system library routine. 7496 7497=item select FILEHANDLE 7498X<select> X<filehandle, default> 7499 7500=item select 7501 7502=for Pod::Functions reset default output or do I/O multiplexing 7503 7504Returns the currently selected filehandle. If FILEHANDLE is supplied, 7505sets the new current default filehandle for output. This has two 7506effects: first, a L<C<write>|/write FILEHANDLE>, L<C<print>|/print 7507FILEHANDLE LIST>, or L<C<say>|/say FILEHANDLE LIST> without a 7508filehandle will default to this FILEHANDLE. Second, references to variables 7509related to output will refer to this output channel. 7510 7511For example, to set the top-of-form format for more than one 7512output channel, you might do the following: 7513 7514 select(REPORT1); 7515 $^ = 'report1_top'; 7516 select(REPORT2); 7517 $^ = 'report2_top'; 7518 7519FILEHANDLE may be an expression whose value gives the name of the 7520actual filehandle. Thus: 7521 7522 my $oldfh = select(STDERR); $| = 1; select($oldfh); 7523 7524Some programmers may prefer to think of filehandles as objects with 7525methods, preferring to write the last example as: 7526 7527 STDERR->autoflush(1); 7528 7529(Prior to Perl version 5.14, you have to C<use IO::Handle;> explicitly 7530first.) 7531 7532Whilst you can use C<select> to temporarily "capture" the output of 7533C<print> like this: 7534 7535 { 7536 my $old_handle = select $new_handle; 7537 7538 # This goes to $new_handle: 7539 print "ok 1\n"; 7540 ... 7541 7542 select $old_handle; 7543 } 7544 7545you might find it easier to localize the typeglob instead: 7546 7547 { 7548 local *STDOUT = $new_handle; 7549 7550 print "ok 1\n"; 7551 ... 7552 } 7553 7554The two are not exactly equivalent, but the latter might be clearer and will 7555restore STDOUT if the wrapped code dies. The difference is that in the 7556former, the original STDOUT can still be accessed by explicitly using it in a 7557C<print> statement (as C<print STDOUT ...>), whereas in the latter the meaning 7558of the STDOUT handle itself has temporarily been changed. 7559 7560Portability issues: L<perlport/select>. 7561 7562=item select RBITS,WBITS,EBITS,TIMEOUT 7563X<select> 7564 7565This calls the L<select(2)> syscall with the bit masks specified, which 7566can be constructed using L<C<fileno>|/fileno FILEHANDLE> and 7567L<C<vec>|/vec EXPR,OFFSET,BITS>, along these lines: 7568 7569 my $rin = my $win = my $ein = ''; 7570 vec($rin, fileno(STDIN), 1) = 1; 7571 vec($win, fileno(STDOUT), 1) = 1; 7572 $ein = $rin | $win; 7573 7574If you want to select on many filehandles, you may wish to write a 7575subroutine like this: 7576 7577 sub fhbits { 7578 my @fhlist = @_; 7579 my $bits = ""; 7580 for my $fh (@fhlist) { 7581 vec($bits, fileno($fh), 1) = 1; 7582 } 7583 return $bits; 7584 } 7585 my $rin = fhbits(\*STDIN, $tty, $mysock); 7586 7587The usual idiom is: 7588 7589 my ($nfound, $timeleft) = 7590 select(my $rout = $rin, my $wout = $win, my $eout = $ein, 7591 $timeout); 7592 7593or to block until something becomes ready just do this 7594 7595 my $nfound = 7596 select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef); 7597 7598Most systems do not bother to return anything useful in C<$timeleft>, so 7599calling L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> in scalar context 7600just returns C<$nfound>. 7601 7602Any of the bit masks can also be L<C<undef>|/undef EXPR>. The timeout, 7603if specified, is 7604in seconds, which may be fractional. Note: not all implementations are 7605capable of returning the C<$timeleft>. If not, they always return 7606C<$timeleft> equal to the supplied C<$timeout>. 7607 7608You can effect a sleep of 250 milliseconds this way: 7609 7610 select(undef, undef, undef, 0.25); 7611 7612Note that whether L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> gets 7613restarted after signals (say, SIGALRM) is implementation-dependent. See 7614also L<perlport> for notes on the portability of 7615L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>. 7616 7617On error, L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> behaves just 7618like L<select(2)>: it returns C<-1> and sets L<C<$!>|perlvar/$!>. 7619 7620On some Unixes, L<select(2)> may report a socket file descriptor as 7621"ready for reading" even when no data is available, and thus any 7622subsequent L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> would block. 7623This can be avoided if you always use C<O_NONBLOCK> on the socket. See 7624L<select(2)> and L<fcntl(2)> for further details. 7625 7626The standard L<C<IO::Select>|IO::Select> module provides a 7627user-friendlier interface to 7628L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, mostly because it does 7629all the bit-mask work for you. 7630 7631B<WARNING>: One should not attempt to mix buffered I/O (like 7632L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET> or 7633L<C<readline>|/readline EXPR>) with 7634L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT>, except as permitted by 7635POSIX, and even then only on POSIX systems. You have to use 7636L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> instead. 7637 7638Portability issues: L<perlport/select>. 7639 7640=item semctl ID,SEMNUM,CMD,ARG 7641X<semctl> 7642 7643=for Pod::Functions SysV semaphore control operations 7644 7645Calls the System V IPC function L<semctl(2)>. You'll probably have to say 7646 7647 use IPC::SysV; 7648 7649first to get the correct constant definitions. If CMD is IPC_STAT or 7650GETALL, then ARG must be a variable that will hold the returned 7651semid_ds structure or semaphore value array. Returns like 7652L<C<ioctl>|/ioctl FILEHANDLE,FUNCTION,SCALAR>: 7653the undefined value for error, "C<0 but true>" for zero, or the actual 7654return value otherwise. The ARG must consist of a vector of native 7655short integers, which may be created with C<pack("s!",(0)x$nsem)>. 7656See also L<perlipc/"SysV IPC"> and the documentation for 7657L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>. 7658 7659Portability issues: L<perlport/semctl>. 7660 7661=item semget KEY,NSEMS,FLAGS 7662X<semget> 7663 7664=for Pod::Functions get set of SysV semaphores 7665 7666Calls the System V IPC function L<semget(2)>. Returns the semaphore id, or 7667the undefined value on error. See also 7668L<perlipc/"SysV IPC"> and the documentation for 7669L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>. 7670 7671Portability issues: L<perlport/semget>. 7672 7673=item semop KEY,OPSTRING 7674X<semop> 7675 7676=for Pod::Functions SysV semaphore operations 7677 7678Calls the System V IPC function L<semop(2)> for semaphore operations 7679such as signalling and waiting. OPSTRING must be a packed array of 7680semop structures. Each semop structure can be generated with 7681C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING 7682implies the number of semaphore operations. Returns true if 7683successful, false on error. As an example, the 7684following code waits on semaphore $semnum of semaphore id $semid: 7685 7686 my $semop = pack("s!3", $semnum, -1, 0); 7687 die "Semaphore trouble: $!\n" unless semop($semid, $semop); 7688 7689To signal the semaphore, replace C<-1> with C<1>. See also 7690L<perlipc/"SysV IPC"> and the documentation for 7691L<C<IPC::SysV>|IPC::SysV> and L<C<IPC::Semaphore>|IPC::Semaphore>. 7692 7693Portability issues: L<perlport/semop>. 7694 7695=item send SOCKET,MSG,FLAGS,TO 7696X<send> 7697 7698=item send SOCKET,MSG,FLAGS 7699 7700=for Pod::Functions send a message over a socket 7701 7702Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET 7703filehandle. Takes the same flags as the system call of the same name. On 7704unconnected sockets, you must specify a destination to I<send to>, in which 7705case it does a L<sendto(2)> syscall. Returns the number of characters sent, 7706or the undefined value on error. The L<sendmsg(2)> syscall is currently 7707unimplemented. See L<perlipc/"UDP: Message Passing"> for examples. 7708 7709Note that if the socket has been marked as C<:utf8>, C<send> will 7710throw an exception. The C<:encoding(...)> layer implicitly introduces 7711the C<:utf8> layer. See L<C<binmode>|/binmode FILEHANDLE, LAYER>. 7712 7713=item setpgrp PID,PGRP 7714X<setpgrp> X<group> 7715 7716=for Pod::Functions set the process group of a process 7717 7718Sets the current process group for the specified PID, C<0> for the current 7719process. Raises an exception when used on a machine that doesn't 7720implement POSIX L<setpgid(2)> or BSD L<setpgrp(2)>. If the arguments 7721are omitted, it defaults to C<0,0>. Note that the BSD 4.2 version of 7722L<C<setpgrp>|/setpgrp PID,PGRP> does not accept any arguments, so only 7723C<setpgrp(0,0)> is portable. See also 7724L<C<POSIX::setsid()>|POSIX/C<setsid>>. 7725 7726Portability issues: L<perlport/setpgrp>. 7727 7728=item setpriority WHICH,WHO,PRIORITY 7729X<setpriority> X<priority> X<nice> X<renice> 7730 7731=for Pod::Functions set a process's nice value 7732 7733Sets the current priority for a process, a process group, or a user. 7734(See L<setpriority(2)>.) Raises an exception when used on a machine 7735that doesn't implement L<setpriority(2)>. 7736 7737C<WHICH> can be any of C<PRIO_PROCESS>, C<PRIO_PGRP> or C<PRIO_USER> 7738imported from L<POSIX/RESOURCE CONSTANTS>. 7739 7740Portability issues: L<perlport/setpriority>. 7741 7742=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 7743X<setsockopt> 7744 7745=for Pod::Functions set some socket options 7746 7747Sets the socket option requested. Returns L<C<undef>|/undef EXPR> on 7748error. Use integer constants provided by the L<C<Socket>|Socket> module 7749for 7750LEVEL and OPNAME. Values for LEVEL can also be obtained from 7751getprotobyname. OPTVAL might either be a packed string or an integer. 7752An integer OPTVAL is shorthand for pack("i", OPTVAL). 7753 7754An example disabling Nagle's algorithm on a socket: 7755 7756 use Socket qw(IPPROTO_TCP TCP_NODELAY); 7757 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); 7758 7759Portability issues: L<perlport/setsockopt>. 7760 7761=item shift ARRAY 7762X<shift> 7763 7764=item shift 7765 7766=for Pod::Functions remove the first element of an array, and return it 7767 7768Removes and returns the B<first> element of an array. This shortens the 7769array by one and moves everything down. 7770 7771 my @arr = ('cat', 'dog'); 7772 my $item = shift(@arr); # 'cat' 7773 7774 # @arr is now ('dog'); 7775 7776Returns C<undef> if the array is empty. 7777 7778B<Note:> C<shift> may also return C<undef> if the first element in the array 7779is C<undef>. 7780 7781 my @arr = (undef, 'two', 'three'); 7782 my $item = shift(@arr); # undef 7783 7784If ARRAY is omitted, C<shift> operates on the C<@ARGV> array in the main 7785program, and the C<@_> array in subroutines. C<shift> will operate on the 7786C<@ARGV> array in C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}> blocks. 7787 7788Starting with Perl 5.14, an experimental feature allowed 7789L<C<shift>|/shift ARRAY> to take a 7790scalar expression. This experiment has been deemed unsuccessful, and was 7791removed as of Perl 5.24. 7792 7793See also L<C<unshift>|/unshift ARRAY,LIST>, L<C<push>|/push ARRAY,LIST>, 7794and L<C<pop>|/pop ARRAY>. L<C<shift>|/shift ARRAY> and 7795L<C<unshift>|/unshift ARRAY,LIST> do the same thing to the left end of 7796an array that L<C<pop>|/pop ARRAY> and L<C<push>|/push ARRAY,LIST> do to 7797the right end. 7798 7799=item shmctl ID,CMD,ARG 7800X<shmctl> 7801 7802=for Pod::Functions SysV shared memory operations 7803 7804Calls the System V IPC function shmctl. You'll probably have to say 7805 7806 use IPC::SysV; 7807 7808first to get the correct constant definitions. If CMD is C<IPC_STAT>, 7809then ARG must be a variable that will hold the returned C<shmid_ds> 7810structure. Returns like ioctl: L<C<undef>|/undef EXPR> for error; "C<0> 7811but true" for zero; and the actual return value otherwise. 7812See also L<perlipc/"SysV IPC"> and the documentation for 7813L<C<IPC::SysV>|IPC::SysV>. 7814 7815Portability issues: L<perlport/shmctl>. 7816 7817=item shmget KEY,SIZE,FLAGS 7818X<shmget> 7819 7820=for Pod::Functions get SysV shared memory segment identifier 7821 7822Calls the System V IPC function shmget. Returns the shared memory 7823segment id, or L<C<undef>|/undef EXPR> on error. 7824See also L<perlipc/"SysV IPC"> and the documentation for 7825L<C<IPC::SysV>|IPC::SysV>. 7826 7827Portability issues: L<perlport/shmget>. 7828 7829=item shmread ID,VAR,POS,SIZE 7830X<shmread> 7831X<shmwrite> 7832 7833=for Pod::Functions read SysV shared memory 7834 7835=item shmwrite ID,STRING,POS,SIZE 7836 7837=for Pod::Functions write SysV shared memory 7838 7839Reads or writes the System V shared memory segment ID starting at 7840position POS for size SIZE by attaching to it, copying in/out, and 7841detaching from it. When reading, VAR must be a variable that will 7842hold the data read. When writing, if STRING is too long, only SIZE 7843bytes are used; if STRING is too short, nulls are written to fill out 7844SIZE bytes. Return true if successful, false on error. 7845L<C<shmread>|/shmread ID,VAR,POS,SIZE> taints the variable. See also 7846L<perlipc/"SysV IPC"> and the documentation for 7847L<C<IPC::SysV>|IPC::SysV> and the L<C<IPC::Shareable>|IPC::Shareable> 7848module from CPAN. 7849 7850Portability issues: L<perlport/shmread> and L<perlport/shmwrite>. 7851 7852=item shutdown SOCKET,HOW 7853X<shutdown> 7854 7855=for Pod::Functions close down just half of a socket connection 7856 7857Shuts down a socket connection in the manner indicated by HOW, which 7858has the same interpretation as in the syscall of the same name. 7859 7860 shutdown($socket, 0); # I/we have stopped reading data 7861 shutdown($socket, 1); # I/we have stopped writing data 7862 shutdown($socket, 2); # I/we have stopped using this socket 7863 7864This is useful with sockets when you want to tell the other 7865side you're done writing but not done reading, or vice versa. 7866It's also a more insistent form of close because it also 7867disables the file descriptor in any forked copies in other 7868processes. 7869 7870Returns C<1> for success; on error, returns L<C<undef>|/undef EXPR> if 7871the first argument is not a valid filehandle, or returns C<0> and sets 7872L<C<$!>|perlvar/$!> for any other failure. 7873 7874=item sin EXPR 7875X<sin> X<sine> X<asin> X<arcsine> 7876 7877=item sin 7878 7879=for Pod::Functions return the sine of a number 7880 7881Returns the sine of EXPR (expressed in radians). If EXPR is omitted, 7882returns sine of L<C<$_>|perlvar/$_>. 7883 7884For the inverse sine operation, you may use the C<Math::Trig::asin> 7885function, or use this relation: 7886 7887 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } 7888 7889=item sleep EXPR 7890X<sleep> X<pause> 7891 7892=item sleep 7893 7894=for Pod::Functions block for some number of seconds 7895 7896Causes the script to sleep for (integer) EXPR seconds, or forever if no 7897argument is given. Returns the integer number of seconds actually slept. 7898 7899EXPR should be a positive integer. If called with a negative integer, 7900L<C<sleep>|/sleep EXPR> does not sleep but instead emits a warning, sets 7901C<$!> (C<errno>), and returns zero. 7902 7903If called with a non-integer, the fractional part is ignored. 7904 7905 7906C<sleep 0> is permitted, but a function call to the underlying platform 7907implementation still occurs, with any side effects that may have. 7908C<sleep 0> is therefore not exactly identical to not sleeping at all. 7909 7910May be interrupted if the process receives a signal such as C<SIGALRM>. 7911 7912 eval { 7913 local $SIG{ALRM} = sub { die "Alarm!\n" }; 7914 sleep; 7915 }; 7916 die $@ unless $@ eq "Alarm!\n"; 7917 7918You probably cannot mix L<C<alarm>|/alarm SECONDS> and 7919L<C<sleep>|/sleep EXPR> calls, because L<C<sleep>|/sleep EXPR> is often 7920implemented using L<C<alarm>|/alarm SECONDS>. 7921 7922On some older systems, it may sleep up to a full second less than what 7923you requested, depending on how it counts seconds. Most modern systems 7924always sleep the full amount. They may appear to sleep longer than that, 7925however, because your process might not be scheduled right away in a 7926busy multitasking system. 7927 7928For delays of finer granularity than one second, the L<Time::HiRes> 7929module (from CPAN, and starting from Perl 5.8 part of the standard 7930distribution) provides L<C<usleep>|Time::HiRes/usleep ( $useconds )>. 7931You may also use Perl's four-argument 7932version of L<C<select>|/select RBITS,WBITS,EBITS,TIMEOUT> leaving the 7933first three arguments undefined, or you might be able to use the 7934L<C<syscall>|/syscall NUMBER, LIST> interface to access L<setitimer(2)> 7935if your system supports it. See L<perlfaq8> for details. 7936 7937See also the L<POSIX> module's L<C<pause>|POSIX/C<pause>> function. 7938 7939=item socket SOCKET,DOMAIN,TYPE,PROTOCOL 7940X<socket> 7941 7942=for Pod::Functions create a socket 7943 7944Opens a socket of the specified kind and attaches it to filehandle 7945SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for 7946the syscall of the same name. You should C<use Socket> first 7947to get the proper definitions imported. See the examples in 7948L<perlipc/"Sockets: Client/Server Communication">. 7949 7950On systems that support a close-on-exec flag on files, the flag will 7951be set for the newly opened file descriptor, as determined by the 7952value of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>. 7953 7954=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 7955X<socketpair> 7956 7957=for Pod::Functions create a pair of sockets 7958 7959Creates an unnamed pair of sockets in the specified domain, of the 7960specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as 7961for the syscall of the same name. If unimplemented, raises an exception. 7962Returns true if successful. 7963 7964On systems that support a close-on-exec flag on files, the flag will 7965be set for the newly opened file descriptors, as determined by the value 7966of L<C<$^F>|perlvar/$^F>. See L<perlvar/$^F>. 7967 7968Some systems define L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> in terms of 7969L<C<socketpair>|/socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL>, in 7970which a call to C<pipe($rdr, $wtr)> is essentially: 7971 7972 use Socket; 7973 socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); 7974 shutdown($rdr, 1); # no more writing for reader 7975 shutdown($wtr, 0); # no more reading for writer 7976 7977See L<perlipc> for an example of socketpair use. Perl 5.8 and later will 7978emulate socketpair using IP sockets to localhost if your system implements 7979sockets but not socketpair. 7980 7981Portability issues: L<perlport/socketpair>. 7982 7983=item sort SUBNAME LIST 7984X<sort> 7985 7986=item sort BLOCK LIST 7987 7988=item sort LIST 7989 7990=for Pod::Functions sort a list of values 7991 7992In list context, this sorts the LIST and returns the sorted list value. 7993In scalar context, the behaviour of L<C<sort>|/sort SUBNAME LIST> is 7994undefined. 7995 7996If SUBNAME or BLOCK is omitted, L<C<sort>|/sort SUBNAME LIST>s in 7997standard string comparison 7998order. If SUBNAME is specified, it gives the name of a subroutine 7999that returns a numeric value less than, equal to, or greater than C<0>, 8000depending on how the elements of the list are to be ordered. (The 8001C<< <=> >> and C<cmp> operators are extremely useful in such routines.) 8002SUBNAME may be a scalar variable name (unsubscripted), in which case 8003the value provides the name of (or a reference to) the actual 8004subroutine to use. In place of a SUBNAME, you can provide a BLOCK as 8005an anonymous, in-line sort subroutine. 8006 8007If the subroutine's prototype is C<($$)>, the elements to be compared are 8008passed by reference in L<C<@_>|perlvar/@_>, as for a normal subroutine. 8009This is slower than unprototyped subroutines, where the elements to be 8010compared are passed into the subroutine as the package global variables 8011C<$a> and C<$b> (see example below). 8012 8013If the subroutine is an XSUB, the elements to be compared are pushed on 8014to the stack, the way arguments are usually passed to XSUBs. C<$a> and 8015C<$b> are not set. 8016 8017The values to be compared are always passed by reference and should not 8018be modified. 8019 8020You also cannot exit out of the sort block or subroutine using any of the 8021loop control operators described in L<perlsyn> or with 8022L<C<goto>|/goto LABEL>. 8023 8024When L<C<use locale>|locale> (but not C<use locale ':not_characters'>) 8025is in effect, C<sort LIST> sorts LIST according to the 8026current collation locale. See L<perllocale>. 8027 8028L<C<sort>|/sort SUBNAME LIST> returns aliases into the original list, 8029much as a for loop's index variable aliases the list elements. That is, 8030modifying an element of a list returned by L<C<sort>|/sort SUBNAME LIST> 8031(for example, in a C<foreach>, L<C<map>|/map BLOCK LIST> or 8032L<C<grep>|/grep BLOCK LIST>) 8033actually modifies the element in the original list. This is usually 8034something to be avoided when writing clear code. 8035 8036Historically Perl has varied in whether sorting is stable by default. 8037If stability matters, it can be controlled explicitly by using the 8038L<sort> pragma. 8039 8040Examples: 8041 8042 # sort lexically 8043 my @articles = sort @files; 8044 8045 # same thing, but with explicit sort routine 8046 my @articles = sort {$a cmp $b} @files; 8047 8048 # now case-insensitively 8049 my @articles = sort {fc($a) cmp fc($b)} @files; 8050 8051 # same thing in reversed order 8052 my @articles = sort {$b cmp $a} @files; 8053 8054 # sort numerically ascending 8055 my @articles = sort {$a <=> $b} @files; 8056 8057 # sort numerically descending 8058 my @articles = sort {$b <=> $a} @files; 8059 8060 # this sorts the %age hash by value instead of key 8061 # using an in-line function 8062 my @eldest = sort { $age{$b} <=> $age{$a} } keys %age; 8063 8064 # sort using explicit subroutine name 8065 sub byage { 8066 $age{$a} <=> $age{$b}; # presuming numeric 8067 } 8068 my @sortedclass = sort byage @class; 8069 8070 sub backwards { $b cmp $a } 8071 my @harry = qw(dog cat x Cain Abel); 8072 my @george = qw(gone chased yz Punished Axed); 8073 print sort @harry; 8074 # prints AbelCaincatdogx 8075 print sort backwards @harry; 8076 # prints xdogcatCainAbel 8077 print sort @george, 'to', @harry; 8078 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz 8079 8080 # inefficiently sort by descending numeric compare using 8081 # the first integer after the first = sign, or the 8082 # whole record case-insensitively otherwise 8083 8084 my @new = sort { 8085 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] 8086 || 8087 fc($a) cmp fc($b) 8088 } @old; 8089 8090 # same thing, but much more efficiently; 8091 # we'll build auxiliary indices instead 8092 # for speed 8093 my (@nums, @caps); 8094 for (@old) { 8095 push @nums, ( /=(\d+)/ ? $1 : undef ); 8096 push @caps, fc($_); 8097 } 8098 8099 my @new = @old[ sort { 8100 $nums[$b] <=> $nums[$a] 8101 || 8102 $caps[$a] cmp $caps[$b] 8103 } 0..$#old 8104 ]; 8105 8106 # same thing, but without any temps 8107 my @new = map { $_->[0] } 8108 sort { $b->[1] <=> $a->[1] 8109 || 8110 $a->[2] cmp $b->[2] 8111 } map { [$_, /=(\d+)/, fc($_)] } @old; 8112 8113 # using a prototype allows you to use any comparison subroutine 8114 # as a sort subroutine (including other package's subroutines) 8115 package Other; 8116 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are 8117 # not set here 8118 package main; 8119 my @new = sort Other::backwards @old; 8120 8121 ## using a prototype with function signature 8122 use feature 'signatures'; 8123 sub function_with_signature :prototype($$) ($one, $two) { 8124 return $one <=> $two 8125 } 8126 8127 my @new = sort function_with_signature @old; 8128 8129 # guarantee stability 8130 use sort 'stable'; 8131 my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; 8132 8133Warning: syntactical care is required when sorting the list returned from 8134a function. If you want to sort the list returned by the function call 8135C<find_records(@key)>, you can use: 8136 8137 my @contact = sort { $a cmp $b } find_records @key; 8138 my @contact = sort +find_records(@key); 8139 my @contact = sort &find_records(@key); 8140 my @contact = sort(find_records(@key)); 8141 8142If instead you want to sort the array C<@key> with the comparison routine 8143C<find_records()> then you can use: 8144 8145 my @contact = sort { find_records() } @key; 8146 my @contact = sort find_records(@key); 8147 my @contact = sort(find_records @key); 8148 my @contact = sort(find_records (@key)); 8149 8150C<$a> and C<$b> are set as package globals in the package the sort() is 8151called from. That means C<$main::a> and C<$main::b> (or C<$::a> and 8152C<$::b>) in the C<main> package, C<$FooPack::a> and C<$FooPack::b> in the 8153C<FooPack> package, etc. If the sort block is in scope of a C<my> or 8154C<state> declaration of C<$a> and/or C<$b>, you I<must> spell out the full 8155name of the variables in the sort block : 8156 8157 package main; 8158 my $a = "C"; # DANGER, Will Robinson, DANGER !!! 8159 8160 print sort { $a cmp $b } qw(A C E G B D F H); 8161 # WRONG 8162 sub badlexi { $a cmp $b } 8163 print sort badlexi qw(A C E G B D F H); 8164 # WRONG 8165 # the above prints BACFEDGH or some other incorrect ordering 8166 8167 print sort { $::a cmp $::b } qw(A C E G B D F H); 8168 # OK 8169 print sort { our $a cmp our $b } qw(A C E G B D F H); 8170 # also OK 8171 print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H); 8172 # also OK 8173 sub lexi { our $a cmp our $b } 8174 print sort lexi qw(A C E G B D F H); 8175 # also OK 8176 # the above print ABCDEFGH 8177 8178With proper care you may mix package and my (or state) C<$a> and/or C<$b>: 8179 8180 my $a = { 8181 tiny => -2, 8182 small => -1, 8183 normal => 0, 8184 big => 1, 8185 huge => 2 8186 }; 8187 8188 say sort { $a->{our $a} <=> $a->{our $b} } 8189 qw{ huge normal tiny small big}; 8190 8191 # prints tinysmallnormalbighuge 8192 8193C<$a> and C<$b> are implicitly local to the sort() execution and regain their 8194former values upon completing the sort. 8195 8196Sort subroutines written using C<$a> and C<$b> are bound to their calling 8197package. It is possible, but of limited interest, to define them in a 8198different package, since the subroutine must still refer to the calling 8199package's C<$a> and C<$b> : 8200 8201 package Foo; 8202 sub lexi { $Bar::a cmp $Bar::b } 8203 package Bar; 8204 ... sort Foo::lexi ... 8205 8206Use the prototyped versions (see above) for a more generic alternative. 8207 8208The comparison function is required to behave. If it returns 8209inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and 8210sometimes saying the opposite, for example) the results are not 8211well-defined. 8212 8213Because C<< <=> >> returns L<C<undef>|/undef EXPR> when either operand 8214is C<NaN> (not-a-number), be careful when sorting with a 8215comparison function like C<< $a <=> $b >> any lists that might contain a 8216C<NaN>. The following example takes advantage that C<NaN != NaN> to 8217eliminate any C<NaN>s from the input list. 8218 8219 my @result = sort { $a <=> $b } grep { $_ == $_ } @input; 8220 8221In this version of F<perl>, the C<sort> function is implemented via the 8222mergesort algorithm. 8223 8224=item splice ARRAY,OFFSET,LENGTH,LIST 8225X<splice> 8226 8227=item splice ARRAY,OFFSET,LENGTH 8228 8229=item splice ARRAY,OFFSET 8230 8231=item splice ARRAY 8232 8233=for Pod::Functions add or remove elements anywhere in an array 8234 8235Removes the elements designated by OFFSET and LENGTH from an array, and 8236replaces them with the elements of LIST, if any. In list context, 8237returns the elements removed from the array. In scalar context, 8238returns the last element removed, or L<C<undef>|/undef EXPR> if no 8239elements are 8240removed. The array grows or shrinks as necessary. 8241If OFFSET is negative then it starts that far from the end of the array. 8242If LENGTH is omitted, removes everything from OFFSET onward. 8243If LENGTH is negative, removes the elements from OFFSET onward 8244except for -LENGTH elements at the end of the array. 8245If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is 8246past the end of the array and a LENGTH was provided, Perl issues a warning, 8247and splices at the end of the array. 8248 8249The following equivalences hold (assuming C<< $#a >= $i >> ) 8250 8251 push(@a,$x,$y) splice(@a,@a,0,$x,$y) 8252 pop(@a) splice(@a,-1) 8253 shift(@a) splice(@a,0,1) 8254 unshift(@a,$x,$y) splice(@a,0,0,$x,$y) 8255 $a[$i] = $y splice(@a,$i,1,$y) 8256 8257L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> can be used, for example, 8258to implement n-ary queue processing: 8259 8260 sub nary_print { 8261 my $n = shift; 8262 while (my @next_n = splice @_, 0, $n) { 8263 say join q{ -- }, @next_n; 8264 } 8265 } 8266 8267 nary_print(3, qw(a b c d e f g h)); 8268 # prints: 8269 # a -- b -- c 8270 # d -- e -- f 8271 # g -- h 8272 8273Starting with Perl 5.14, an experimental feature allowed 8274L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST> to take a 8275scalar expression. This experiment has been deemed unsuccessful, and was 8276removed as of Perl 5.24. 8277 8278=item split /PATTERN/,EXPR,LIMIT 8279X<split> 8280 8281=item split /PATTERN/,EXPR 8282 8283=item split /PATTERN/ 8284 8285=item split 8286 8287=for Pod::Functions split up a string using a regexp delimiter 8288 8289Splits the string EXPR into a list of strings and returns the 8290list in list context, or the size of the list in scalar context. 8291(Prior to Perl 5.11, it also overwrote C<@_> with the list in 8292void and scalar context. If you target old perls, beware.) 8293 8294If only PATTERN is given, EXPR defaults to L<C<$_>|perlvar/$_>. 8295 8296Anything in EXPR that matches PATTERN is taken to be a separator 8297that separates the EXPR into substrings (called "I<fields>") that 8298do B<not> include the separator. Note that a separator may be 8299longer than one character or even have no characters at all (the 8300empty string, which is a zero-width match). 8301 8302The PATTERN need not be constant; an expression may be used 8303to specify a pattern that varies at runtime. 8304 8305If PATTERN matches the empty string, the EXPR is split at the match 8306position (between characters). As an example, the following: 8307 8308 my @x = split(/b/, "abc"); # ("a", "c") 8309 8310uses the C<b> in C<'abc'> as a separator to produce the list ("a", "c"). 8311However, this: 8312 8313 my @x = split(//, "abc"); # ("a", "b", "c") 8314 8315uses empty string matches as separators; thus, the empty string 8316may be used to split EXPR into a list of its component characters. 8317 8318As a special case for L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT>, 8319the empty pattern given in 8320L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>) 8321specifically matches the empty string, which is contrary to its usual 8322interpretation as the last successful match. 8323 8324If PATTERN is C</^/>, then it is treated as if it used the 8325L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it 8326isn't much use otherwise. 8327 8328C<E<sol>m> and any of the other pattern modifiers valid for C<qr> 8329(summarized in L<perlop/qrE<sol>STRINGE<sol>msixpodualn>) may be 8330specified explicitly. 8331 8332As another special case, 8333L<C<split>|/split E<sol>PATTERNE<sol>,EXPR,LIMIT> emulates the default 8334behavior of the 8335command line tool B<awk> when the PATTERN is either omitted or a 8336string composed of a single space character (such as S<C<' '>> or 8337S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading 8338whitespace in EXPR is removed before splitting occurs, and the PATTERN is 8339instead treated as if it were C</\s+/>; in particular, this means that 8340I<any> contiguous whitespace (not just a single space character) is used as 8341a separator. 8342 8343 my @x = split(" ", " Quick brown fox\n"); 8344 # ("Quick", "brown", "fox") 8345 8346 my @x = split(" ", "RED\tGREEN\tBLUE"); 8347 # ("RED", "GREEN", "BLUE") 8348 8349Using split in this fashion is very similar to how 8350L<C<qwE<sol>E<sol>>|/qwE<sol>STRINGE<sol>> works. 8351 8352However, this special treatment can be avoided by specifying 8353the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing 8354only a single space character to be a separator. In earlier Perls this 8355special case was restricted to the use of a plain S<C<" ">> as the 8356pattern argument to split; in Perl 5.18.0 and later this special case is 8357triggered by any expression which evaluates to the simple string S<C<" ">>. 8358 8359As of Perl 5.28, this special-cased whitespace splitting works as expected in 8360the scope of L<< S<C<"use feature 'unicode_strings'">>|feature/The 8361'unicode_strings' feature >>. In previous versions, and outside the scope of 8362that feature, it exhibits L<perlunicode/The "Unicode Bug">: characters that are 8363whitespace according to Unicode rules but not according to ASCII rules can be 8364treated as part of fields rather than as field separators, depending on the 8365string's internal encoding. 8366 8367As of Perl 5.39.9 the C</x> default modifier does NOT affect 8368C<split STRING> but does affect C<split PATTERN>, this means that 8369C<split " "> will produces the expected I<awk> emulation regardless as 8370to whether it is used in the scope of a C<use re "/x"> statement. If you 8371want to split by spaces under C<use re "/x"> you must do something like 8372C<split /(?-x: )/> or C<split /\x{20}/> instead of C<split / />. 8373 8374If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering 8375the previously described I<awk> emulation. 8376 8377If LIMIT is specified and positive, it represents the maximum number 8378of fields into which the EXPR may be split; in other words, LIMIT is 8379one greater than the maximum number of times EXPR may be split. Thus, 8380the LIMIT value C<1> means that EXPR may be split a maximum of zero 8381times, producing a maximum of one field (namely, the entire value of 8382EXPR). For instance: 8383 8384 my @x = split(/,/, "a,b,c", 1); # ("a,b,c") 8385 my @x = split(/,/, "a,b,c", 2); # ("a", "b,c") 8386 my @x = split(/,/, "a,b,c", 3); # ("a", "b", "c") 8387 my @x = split(/,/, "a,b,c", 4); # ("a", "b", "c") 8388 8389If LIMIT is negative, it is treated as if it were instead arbitrarily 8390large; as many fields as possible are produced. 8391 8392If LIMIT is omitted (or, equivalently, zero), then it is usually 8393treated as if it were instead negative but with the exception that 8394trailing empty fields are stripped (empty leading fields are always 8395preserved); if all fields are empty, then all fields are considered to 8396be trailing (and are thus stripped in this case). Thus, the following: 8397 8398 my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c") 8399 8400produces only a three element list. 8401 8402 my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "") 8403 8404produces a six element list. 8405 8406In time-critical applications, it is worthwhile to avoid splitting 8407into more fields than necessary. Thus, when assigning to a list, 8408if LIMIT is omitted (or zero), then LIMIT is treated as though it 8409were one larger than the number of variables in the list; for the 8410following, LIMIT is implicitly 3: 8411 8412 my ($login, $passwd) = split(/:/); 8413 8414Note that splitting an EXPR that evaluates to the empty string always 8415produces zero fields, regardless of the LIMIT specified. 8416 8417An empty leading field is produced when there is a positive-width 8418match at the beginning of EXPR. For instance: 8419 8420 my @x = split(/ /, " abc"); # ("", "abc") 8421 8422splits into two elements. However, a zero-width match at the 8423beginning of EXPR never produces an empty field, so that: 8424 8425 my @x = split(//, " abc"); # (" ", "a", "b", "c") 8426 8427splits into four elements instead of five. 8428 8429An empty trailing field, on the other hand, is produced when there is a 8430match at the end of EXPR, regardless of the length of the match 8431(of course, unless a non-zero LIMIT is given explicitly, such fields are 8432removed, as in the last example). Thus: 8433 8434 my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "") 8435 8436If the PATTERN contains 8437L<capturing groups|perlretut/Grouping things and hierarchical matching>, 8438then for each separator, an additional field is produced for each substring 8439captured by a group (in the order in which the groups are specified, 8440as per L<backreferences|perlretut/Backreferences>); if any group does not 8441match, then it captures the L<C<undef>|/undef EXPR> value instead of a 8442substring. Also, 8443note that any such additional field is produced whenever there is a 8444separator (that is, whenever a split occurs), and such an additional field 8445does B<not> count towards the LIMIT. Consider the following expressions 8446evaluated in list context (each returned list is provided in the associated 8447comment): 8448 8449 my @x = split(/-|,/ , "1-10,20", 3); 8450 # ("1", "10", "20") 8451 8452 my @x = split(/(-|,)/ , "1-10,20", 3); 8453 # ("1", "-", "10", ",", "20") 8454 8455 my @x = split(/-|(,)/ , "1-10,20", 3); 8456 # ("1", undef, "10", ",", "20") 8457 8458 my @x = split(/(-)|,/ , "1-10,20", 3); 8459 # ("1", "-", "10", undef, "20") 8460 8461 my @x = split(/(-)|(,)/, "1-10,20", 3); 8462 # ("1", "-", undef, "10", undef, ",", "20") 8463 8464=item sprintf FORMAT, LIST 8465X<sprintf> 8466 8467=for Pod::Functions formatted print into a string 8468 8469Returns a string formatted by the usual 8470L<C<printf>|/printf FILEHANDLE FORMAT, LIST> conventions of the C 8471library function L<C<sprintf>|/sprintf FORMAT, LIST>. See below for 8472more details and see L<sprintf(3)> or L<printf(3)> on your system for an 8473explanation of the general principles. 8474 8475For example: 8476 8477 # Format number with up to 8 leading zeroes 8478 my $result = sprintf("%08d", $number); 8479 8480 # Round number to 3 digits after decimal point 8481 my $rounded = sprintf("%.3f", $number); 8482 8483Perl does its own L<C<sprintf>|/sprintf FORMAT, LIST> formatting: it 8484emulates the C 8485function L<sprintf(3)>, but doesn't use it except for floating-point 8486numbers, and even then only standard modifiers are allowed. 8487Non-standard extensions in your local L<sprintf(3)> are 8488therefore unavailable from Perl. 8489 8490Unlike L<C<printf>|/printf FILEHANDLE FORMAT, LIST>, 8491L<C<sprintf>|/sprintf FORMAT, LIST> does not do what you probably mean 8492when you pass it an array as your first argument. 8493The array is given scalar context, 8494and instead of using the 0th element of the array as the format, Perl will 8495use the count of elements in the array as the format, which is almost never 8496useful. 8497 8498Perl's L<C<sprintf>|/sprintf FORMAT, LIST> permits the following 8499universally-known conversions: 8500 8501 %% a percent sign 8502 %c a character with the given number 8503 %s a string 8504 %d a signed integer, in decimal 8505 %u an unsigned integer, in decimal 8506 %o an unsigned integer, in octal 8507 %x an unsigned integer, in hexadecimal 8508 %e a floating-point number, in scientific notation 8509 %f a floating-point number, in fixed decimal notation 8510 %g a floating-point number, in %e or %f notation 8511 8512In addition, Perl permits the following widely-supported conversions: 8513 8514 %X like %x, but using upper-case letters 8515 %E like %e, but using an upper-case "E" 8516 %G like %g, but with an upper-case "E" (if applicable) 8517 %b an unsigned integer, in binary 8518 %B like %b, but using an upper-case "B" with the # flag 8519 %p a pointer (outputs the Perl value's address in hexadecimal) 8520 %n special: *stores* the number of characters output so far 8521 into the next argument in the parameter list 8522 %a hexadecimal floating point 8523 %A like %a, but using upper-case letters 8524 %i a synonym for %d 8525 %D a synonym for %ld 8526 %U a synonym for %lu 8527 %O a synonym for %lo 8528 %F a synonym for %f 8529 8530Note that the number of exponent digits in the scientific notation produced 8531by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the 8532exponent less than 100 is system-dependent: it may be three or less 8533(zero-padded as necessary). In other words, 1.23 times ten to the 853499th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>: 8535the exponent or the hexadecimal digits may float: especially the 8536"long doubles" Perl configuration option may cause surprises. 8537 8538Between the C<%> and the format letter, you may specify several 8539additional attributes controlling the interpretation of the format. 8540In order, these are: 8541 8542=over 4 8543 8544=item format parameter index 8545 8546An explicit format parameter index, such as C<2$>. By default sprintf 8547will format the next unused argument in the list, but this allows you 8548to take the arguments out of order: 8549 8550 printf '%2$d %1$d', 12, 34; # prints "34 12" 8551 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" 8552 8553=item flags 8554 8555one or more of: 8556 8557 space prefix non-negative number with a space 8558 + prefix non-negative number with a plus sign 8559 - left-justify within the field 8560 0 use zeros, not spaces, to right-justify 8561 # ensure the leading "0" for any octal, 8562 prefix non-zero hexadecimal with "0x" or "0X", 8563 prefix non-zero binary with "0b" or "0B" 8564 8565For example: 8566 8567 printf '<% d>', 12; # prints "< 12>" 8568 printf '<% d>', 0; # prints "< 0>" 8569 printf '<% d>', -12; # prints "<-12>" 8570 printf '<%+d>', 12; # prints "<+12>" 8571 printf '<%+d>', 0; # prints "<+0>" 8572 printf '<%+d>', -12; # prints "<-12>" 8573 printf '<%6s>', 12; # prints "< 12>" 8574 printf '<%-6s>', 12; # prints "<12 >" 8575 printf '<%06s>', 12; # prints "<000012>" 8576 printf '<%#o>', 12; # prints "<014>" 8577 printf '<%#x>', 12; # prints "<0xc>" 8578 printf '<%#X>', 12; # prints "<0XC>" 8579 printf '<%#b>', 12; # prints "<0b1100>" 8580 printf '<%#B>', 12; # prints "<0B1100>" 8581 8582When a space and a plus sign are given as the flags at once, 8583the space is ignored. 8584 8585 printf '<%+ d>', 12; # prints "<+12>" 8586 printf '<% +d>', 12; # prints "<+12>" 8587 8588When the # flag and a precision are given in the %o conversion, 8589the precision is incremented if it's necessary for the leading "0". 8590 8591 printf '<%#.5o>', 012; # prints "<00012>" 8592 printf '<%#.5o>', 012345; # prints "<012345>" 8593 printf '<%#.0o>', 0; # prints "<0>" 8594 8595=item vector flag 8596 8597This flag tells Perl to interpret the supplied string as a vector of 8598integers, one for each character in the string. Perl applies the format to 8599each integer in turn, then joins the resulting strings with a separator (a 8600dot C<.> by default). This can be useful for displaying ordinal values of 8601characters in arbitrary strings: 8602 8603 printf "%vd", "AB\x{100}"; # prints "65.66.256" 8604 printf "version is v%vd\n", $^V; # Perl's version 8605 8606Put an asterisk C<*> before the C<v> to override the string to 8607use to separate the numbers: 8608 8609 printf "address is %*vX\n", ":", $addr; # IPv6 address 8610 printf "bits are %0*v8b\n", " ", $bits; # random bitstring 8611 8612You can also explicitly specify the argument number to use for 8613the join string using something like C<*2$v>; for example: 8614 8615 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses 8616 @addr[1..3], ":"; 8617 8618=item (minimum) width 8619 8620Arguments are usually formatted to be only as wide as required to 8621display the given value. You can override the width by putting 8622a number here, or get the width from the next argument (with C<*>) 8623or from a specified argument (e.g., with C<*2$>): 8624 8625 printf "<%s>", "a"; # prints "<a>" 8626 printf "<%6s>", "a"; # prints "< a>" 8627 printf "<%*s>", 6, "a"; # prints "< a>" 8628 printf '<%*2$s>', "a", 6; # prints "< a>" 8629 printf "<%2s>", "long"; # prints "<long>" (does not truncate) 8630 8631If a field width obtained through C<*> is negative, it has the same 8632effect as the C<-> flag: left-justification. 8633 8634=item precision, or maximum width 8635X<precision> 8636 8637You can specify a precision (for numeric conversions) or a maximum 8638width (for string conversions) by specifying a C<.> followed by a number. 8639For floating-point formats except C<g> and C<G>, this specifies 8640how many places right of the decimal point to show (the default being 6). 8641For example: 8642 8643 # these examples are subject to system-specific variation 8644 printf '<%f>', 1; # prints "<1.000000>" 8645 printf '<%.1f>', 1; # prints "<1.0>" 8646 printf '<%.0f>', 1; # prints "<1>" 8647 printf '<%07.2f>', 1.3; # prints "<0001.30>" 8648 printf '<%e>', 10; # prints "<1.000000e+01>" 8649 printf '<%.1e>', 10; # prints "<1.0e+01>" 8650 8651For "g" and "G", this specifies the maximum number of significant digits to 8652show; for example: 8653 8654 # These examples are subject to system-specific variation. 8655 printf '<%g>', 1; # prints "<1>" 8656 printf '<%.10g>', 1; # prints "<1>" 8657 printf '<%g>', 100; # prints "<100>" 8658 printf '<%.1g>', 100; # prints "<1e+02>" 8659 printf '<%.2g>', 100.01; # prints "<1e+02>" 8660 printf '<%.5g>', 100.01; # prints "<100.01>" 8661 printf '<%.4g>', 100.01; # prints "<100>" 8662 printf '<%.1g>', 0.0111; # prints "<0.01>" 8663 printf '<%.2g>', 0.0111; # prints "<0.011>" 8664 printf '<%.3g>', 0.0111; # prints "<0.0111>" 8665 8666For integer conversions, specifying a precision implies that the 8667output of the number itself should be zero-padded to this width, 8668where the 0 flag is ignored: 8669 8670 printf '<%.6d>', 1; # prints "<000001>" 8671 printf '<%+.6d>', 1; # prints "<+000001>" 8672 printf '<%-10.6d>', 1; # prints "<000001 >" 8673 printf '<%10.6d>', 1; # prints "< 000001>" 8674 printf '<%010.6d>', 1; # prints "< 000001>" 8675 printf '<%+10.6d>', 1; # prints "< +000001>" 8676 8677 printf '<%.6x>', 1; # prints "<000001>" 8678 printf '<%#.6x>', 1; # prints "<0x000001>" 8679 printf '<%-10.6x>', 1; # prints "<000001 >" 8680 printf '<%10.6x>', 1; # prints "< 000001>" 8681 printf '<%010.6x>', 1; # prints "< 000001>" 8682 printf '<%#10.6x>', 1; # prints "< 0x000001>" 8683 8684For string conversions, specifying a precision truncates the string 8685to fit the specified width: 8686 8687 printf '<%.5s>', "truncated"; # prints "<trunc>" 8688 printf '<%10.5s>', "truncated"; # prints "< trunc>" 8689 8690You can also get the precision from the next argument using C<.*>, or from a 8691specified argument (e.g., with C<.*2$>): 8692 8693 printf '<%.6x>', 1; # prints "<000001>" 8694 printf '<%.*x>', 6, 1; # prints "<000001>" 8695 8696 printf '<%.*2$x>', 1, 6; # prints "<000001>" 8697 8698 printf '<%6.*2$x>', 1, 4; # prints "< 0001>" 8699 8700If a precision obtained through C<*> is negative, it counts 8701as having no precision at all. 8702 8703 printf '<%.*s>', 7, "string"; # prints "<string>" 8704 printf '<%.*s>', 3, "string"; # prints "<str>" 8705 printf '<%.*s>', 0, "string"; # prints "<>" 8706 printf '<%.*s>', -1, "string"; # prints "<string>" 8707 8708 printf '<%.*d>', 1, 0; # prints "<0>" 8709 printf '<%.*d>', 0, 0; # prints "<>" 8710 printf '<%.*d>', -1, 0; # prints "<0>" 8711 8712=item size 8713 8714For numeric conversions, you can specify the size to interpret the 8715number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer 8716conversions (C<d u o x X b i D U O>), numbers are usually assumed to be 8717whatever the default integer size is on your platform (usually 32 or 64 8718bits), but you can override this to use instead one of the standard C types, 8719as supported by the compiler used to build Perl: 8720 8721 hh interpret integer as C type "char" or "unsigned 8722 char" on Perl 5.14 or later 8723 h interpret integer as C type "short" or 8724 "unsigned short" 8725 j interpret integer as C type "intmax_t" on Perl 8726 5.14 or later. Prior to Perl 5.30 this worked only 8727 with a C99 compiler, hence was unportable before 8728 that release. 8729 l interpret integer as C type "long" or 8730 "unsigned long" 8731 q, L, or ll interpret integer as C type "long long", 8732 "unsigned long long", or "quad" (typically 8733 64-bit integers) 8734 t interpret integer as C type "ptrdiff_t" on Perl 8735 5.14 or later 8736 z interpret integer as C types "size_t" or 8737 "ssize_t" on Perl 5.14 or later 8738 8739Note that, in general, using the C<l> modifier (for example, when writing 8740C<"%ld"> or C<"%lu"> instead of C<"%d"> and C<"%u">) is unnecessary 8741when used from Perl code. Moreover, it may be harmful, for example on 8742Windows 64-bit where a long is 32-bits. 8743 8744As of 5.14, none of these raises an exception if they are not supported on 8745your platform. However, if warnings are enabled, a warning of the 8746L<C<printf>|warnings> warning class is issued on an unsupported 8747conversion flag. Should you instead prefer an exception, do this: 8748 8749 use warnings FATAL => "printf"; 8750 8751If you would like to know about a version dependency before you 8752start running the program, put something like this at its top: 8753 8754 use v5.14; # for hh/j/t/z/ printf modifiers 8755 8756You can find out whether your Perl supports quads via L<Config>: 8757 8758 use Config; 8759 if ($Config{use64bitint} eq "define" 8760 || $Config{longsize} >= 8) { 8761 print "Nice quads!\n"; 8762 } 8763 8764For floating-point conversions (C<e f g E F G>), numbers are usually assumed 8765to be the default floating-point size on your platform (double or long double), 8766but you can force "long double" with C<q>, C<L>, or C<ll> if your 8767platform supports them. You can find out whether your Perl supports long 8768doubles via L<Config>: 8769 8770 use Config; 8771 print "long doubles\n" if $Config{d_longdbl} eq "define"; 8772 8773You can find out whether Perl considers "long double" to be the default 8774floating-point size to use on your platform via L<Config>: 8775 8776 use Config; 8777 if ($Config{uselongdouble} eq "define") { 8778 print "long doubles by default\n"; 8779 } 8780 8781It can also be that long doubles and doubles are the same thing: 8782 8783 use Config; 8784 ($Config{doublesize} == $Config{longdblsize}) && 8785 print "doubles are long doubles\n"; 8786 8787The size specifier C<V> has no effect for Perl code, but is supported for 8788compatibility with XS code. It means "use the standard size for a Perl 8789integer or floating-point number", which is the default. 8790 8791=item order of arguments 8792 8793Normally, L<C<sprintf>|/sprintf FORMAT, LIST> takes the next unused 8794argument as the value to 8795format for each format specification. If the format specification 8796uses C<*> to require additional arguments, these are consumed from 8797the argument list in the order they appear in the format 8798specification I<before> the value to format. Where an argument is 8799specified by an explicit index, this does not affect the normal 8800order for the arguments, even when the explicitly specified index 8801would have been the next argument. 8802 8803So: 8804 8805 printf "<%*.*s>", $x, $y, $z; 8806 8807uses C<$x> for the width, C<$y> for the precision, and C<$z> 8808as the value to format; while: 8809 8810 printf '<%*1$.*s>', $x, $y; 8811 8812would use C<$x> for the width and precision, and C<$y> as the 8813value to format. 8814 8815Here are some more examples; be aware that when using an explicit 8816index, the C<$> may need escaping: 8817 8818 printf "%2\$d %d\n", 12, 34; # will print "34 12\n" 8819 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" 8820 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" 8821 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" 8822 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n" 8823 8824=back 8825 8826If L<C<use locale>|locale> (including C<use locale ':not_characters'>) 8827is in effect and L<C<POSIX::setlocale>|POSIX/C<setlocale>> has been 8828called, 8829the character used for the decimal separator in formatted floating-point 8830numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale> 8831and L<POSIX>. 8832 8833=item sqrt EXPR 8834X<sqrt> X<root> X<square root> 8835 8836=item sqrt 8837 8838=for Pod::Functions square root function 8839 8840Return the positive square root of EXPR. If EXPR is omitted, uses 8841L<C<$_>|perlvar/$_>. Works only for non-negative operands unless you've 8842loaded the L<C<Math::Complex>|Math::Complex> module. 8843 8844 use Math::Complex; 8845 print sqrt(-4); # prints 2i 8846 8847=item srand EXPR 8848X<srand> X<seed> X<randseed> 8849 8850=item srand 8851 8852=for Pod::Functions seed the random number generator 8853 8854Sets and returns the random number seed for the L<C<rand>|/rand EXPR> 8855operator. 8856 8857The point of the function is to "seed" the L<C<rand>|/rand EXPR> 8858function so that L<C<rand>|/rand EXPR> can produce a different sequence 8859each time you run your program. When called with a parameter, 8860L<C<srand>|/srand EXPR> uses that for the seed; otherwise it 8861(semi-)randomly chooses a seed (see below). In either case, starting with Perl 5.14, 8862it returns the seed. To signal that your code will work I<only> on Perls 8863of a recent vintage: 8864 8865 use v5.14; # so srand returns the seed 8866 8867If L<C<srand>|/srand EXPR> is not called explicitly, it is called 8868implicitly without a parameter at the first use of the 8869L<C<rand>|/rand EXPR> operator. However, there are a few situations 8870where programs are likely to want to call L<C<srand>|/srand EXPR>. One 8871is for generating predictable results, generally for testing or 8872debugging. There, you use C<srand($seed)>, with the same C<$seed> each 8873time. Another case is that you may want to call L<C<srand>|/srand EXPR> 8874after a L<C<fork>|/fork> to avoid child processes sharing the same seed 8875value as the parent (and consequently each other). 8876 8877Do B<not> call C<srand()> (i.e., without an argument) more than once per 8878process. The internal state of the random number generator should 8879contain more entropy than can be provided by any seed, so calling 8880L<C<srand>|/srand EXPR> again actually I<loses> randomness. 8881 8882Most implementations of L<C<srand>|/srand EXPR> take an integer and will 8883silently 8884truncate decimal numbers. This means C<srand(42)> will usually 8885produce the same results as C<srand(42.1)>. To be safe, always pass 8886L<C<srand>|/srand EXPR> an integer. 8887 8888A typical use of the returned seed is for a test program which has too many 8889combinations to test comprehensively in the time available to it each run. It 8890can test a random subset each time, and should there be a failure, log the seed 8891used for that run so that it can later be used to reproduce the same results. 8892 8893If the C<PERL_RAND_SEED> environment variable is set to a non-negative 8894integer during process startup then calls to C<srand()> with no 8895arguments will initialize the perl random number generator with a 8896consistent seed each time it is called, whether called explicitly with 8897no arguments or implicitly via use of C<rand()>. The exact seeding that 8898a given C<PERL_RAND_SEED> will produce is deliberately unspecified, but 8899using different values for C<PERL_RAND_SEED> should produce different 8900results. This is intended for debugging and performance analysis and is 8901only guaranteed to produce consistent results between invocations of the 8902same perl executable running the same code when all other factors are 8903equal. The environment variable is read only once during process 8904startup, and changing it during the program flow will not affect the 8905currently running process. See L<perlrun> for more details. 8906 8907B<L<C<rand>|/rand EXPR> is not cryptographically secure. You should not rely 8908on it in security-sensitive situations.> As of this writing, a 8909number of third-party CPAN modules offer random number generators 8910intended by their authors to be cryptographically secure, 8911including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, 8912and L<Math::TrulyRandom>. 8913 8914=item stat FILEHANDLE 8915X<stat> X<file, status> X<ctime> 8916 8917=item stat EXPR 8918 8919=item stat DIRHANDLE 8920 8921=item stat 8922 8923=for Pod::Functions get a file's status information 8924 8925Returns a 13-element list giving the status info for a file, either 8926the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is 8927omitted, it stats L<C<$_>|perlvar/$_> (not C<_>!). Returns the empty 8928list if L<C<stat>|/stat FILEHANDLE> fails. Typically 8929used as follows: 8930 8931 my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, 8932 $atime,$mtime,$ctime,$blksize,$blocks) 8933 = stat($filename); 8934 8935Not all fields are supported on all filesystem types. Here are the 8936meanings of the fields: 8937 8938 0 dev device number of filesystem 8939 1 ino inode number 8940 2 mode file mode (type and permissions) 8941 3 nlink number of (hard) links to the file 8942 4 uid numeric user ID of file's owner 8943 5 gid numeric group ID of file's owner 8944 6 rdev the device identifier (special files only) 8945 7 size total size of file, in bytes 8946 8 atime last access time in seconds since the epoch 8947 9 mtime last modify time in seconds since the epoch 8948 10 ctime inode change time in seconds since the epoch (*) 8949 11 blksize preferred I/O size in bytes for interacting with the 8950 file (may vary from file to file) 8951 12 blocks actual number of system-specific blocks allocated 8952 on disk (often, but not always, 512 bytes each) 8953 8954(The epoch was at 00:00 January 1, 1970 GMT.) 8955 8956(*) Not all fields are supported on all filesystem types. Notably, the 8957ctime field is non-portable. In particular, you cannot expect it to be a 8958"creation time"; see L<perlport/"Files and Filesystems"> for details. 8959 8960If L<C<stat>|/stat FILEHANDLE> is passed the special filehandle 8961consisting of an underline, no stat is done, but the current contents of 8962the stat structure from the last L<C<stat>|/stat FILEHANDLE>, 8963L<C<lstat>|/lstat FILEHANDLE>, or filetest are returned. Example: 8964 8965 if (-x $file && (($d) = stat(_)) && $d < 0) { 8966 print "$file is executable NFS file\n"; 8967 } 8968 8969(This works on machines only for which the device number is negative 8970under NFS.) 8971 8972On some platforms inode numbers are of a type larger than perl knows how 8973to handle as integer numerical values. If necessary, an inode number will 8974be returned as a decimal string in order to preserve the entire value. 8975If used in a numeric context, this will be converted to a floating-point 8976numerical value, with rounding, a fate that is best avoided. Therefore, 8977you should prefer to compare inode numbers using C<eq> rather than C<==>. 8978C<eq> will work fine on inode numbers that are represented numerically, 8979as well as those represented as strings. 8980 8981Because the mode contains both the file type and its permissions, you 8982should mask off the file type portion and (s)printf using a C<"%o"> 8983if you want to see the real permissions. 8984 8985 my $mode = (stat($filename))[2]; 8986 printf "Permissions are %04o\n", $mode & 07777; 8987 8988In scalar context, L<C<stat>|/stat FILEHANDLE> returns a boolean value 8989indicating success 8990or failure, and, if successful, sets the information associated with 8991the special filehandle C<_>. 8992 8993The L<File::stat> module provides a convenient, by-name access mechanism: 8994 8995 use File::stat; 8996 my $sb = stat($filename); 8997 printf "File is %s, size is %s, perm %04o, mtime %s\n", 8998 $filename, $sb->size, $sb->mode & 07777, 8999 scalar localtime $sb->mtime; 9000 9001You can import symbolic mode constants and functions 9002(C<S_I*>) from the L<Fcntl> module: 9003 9004 use Fcntl ':mode'; 9005 9006 my $mode = (stat($filename))[2]; 9007 9008 my $user_rwx = ($mode & S_IRWXU) >> 6; 9009 my $group_read = ($mode & S_IRGRP) >> 3; 9010 my $other_execute = $mode & S_IXOTH; 9011 9012 printf "Permissions are %04o\n", S_IMODE($mode), "\n"; 9013 9014 my $is_setuid = $mode & S_ISUID; 9015 my $is_directory = S_ISDIR($mode); 9016 9017You could write the last two using the C<-u> and C<-d> operators. 9018Commonly available C<S_I*> constants are: 9019 9020 # Permissions: read, write, execute, for user, group, others. 9021 9022 S_IRWXU S_IRUSR S_IWUSR S_IXUSR 9023 S_IRWXG S_IRGRP S_IWGRP S_IXGRP 9024 S_IRWXO S_IROTH S_IWOTH S_IXOTH 9025 9026 # Setuid/Setgid/Stickiness/SaveText/EnforcedLocks. 9027 # Note that the exact meaning of these is system-dependent. 9028 9029 S_ISUID S_ISGID S_ISVTX S_ISTXT S_ENFMT 9030 9031 # File types. Not all are necessarily available on 9032 # your system. 9033 9034 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR 9035 S_IFIFO S_IFSOCK S_IFWHT 9036 9037 # The following are compatibility aliases for S_IRUSR, 9038 # S_IWUSR, and S_IXUSR. 9039 9040 S_IREAD S_IWRITE S_IEXEC 9041 9042and the C<S_I*> functions are 9043 9044 S_IMODE($mode) the part of $mode containing the permission 9045 bits and the setuid/setgid/sticky bits 9046 9047 S_IFMT($mode) the part of $mode containing the file type, 9048 which will match one of the S_IF* constants 9049 (e.g. S_IFMT($mode) == S_IFDIR for directories), 9050 but see the following helper functions 9051 9052 # The operators -f, -d, -l, -b, -c, -p, and -S. 9053 9054 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) 9055 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) 9056 9057 # No direct -X operator counterpart, but for the first one 9058 # the -g operator is often equivalent. The ENFMT stands for 9059 # record flocking enforcement, a platform-dependent feature. 9060 9061 S_ISENFMT($mode) S_ISWHT($mode) 9062 9063See your native L<chmod(2)> and L<stat(2)> documentation for more details 9064about the C<S_I*> constants. To get status info for a symbolic link 9065instead of the target file behind the link, use the 9066L<C<lstat>|/lstat FILEHANDLE> function. 9067 9068Portability issues: L<perlport/stat>. 9069 9070=item state VARLIST 9071X<state> 9072 9073=item state TYPE VARLIST 9074 9075=item state VARLIST : ATTRS 9076 9077=item state TYPE VARLIST : ATTRS 9078 9079=for Pod::Functions +state declare and assign a persistent lexical variable 9080 9081L<C<state>|/state VARLIST> declares a lexically scoped variable, just 9082like L<C<my>|/my VARLIST>. 9083However, those variables will never be reinitialized, contrary to 9084lexical variables that are reinitialized each time their enclosing block 9085is entered. 9086See L<perlsub/"Persistent Private Variables"> for details. 9087 9088If more than one variable is listed, the list must be placed in 9089parentheses. With a parenthesised list, L<C<undef>|/undef EXPR> can be 9090used as a 9091dummy placeholder. However, since initialization of state variables in 9092such lists is currently not possible this would serve no purpose. 9093 9094Like L<C<my>|/my VARLIST>, L<C<local>|/local EXPR>, and 9095L<C<our>|/our VARLIST>, L<C<state>|/state VARLIST> can operate on a variable 9096anywhere it appears in an expression (aside from interpolation in strings). 9097The declaration will not apply to additional uses of the same variable until 9098the next statement. This means additional uses of that variable within the 9099same statement will act as they would have before that declaration occurred, 9100or result in a strict 'vars' error, as appropriate. 9101 9102 package main; 9103 use feature 'state'; 9104 our $x = 2; 9105 foo($x, state $x = $x + 1, $x); # foo() receives (2, 3, 2) 9106 foo($x, $main::x); # foo() receives (3, 2) 9107 9108Redeclaring a variable in the same scope or statement will "shadow" the 9109previous declaration, creating a new instance and preventing access to 9110the previous one. This is usually undesired and, if warnings are enabled, 9111will result in a warning in the C<shadow> category. 9112 9113L<C<state>|/state VARLIST> is available only if the 9114L<C<"state"> feature|feature/The 'state' feature> is enabled or if it is 9115prefixed with C<CORE::>. The 9116L<C<"state"> feature|feature/The 'state' feature> is enabled 9117automatically with a C<use v5.10> (or higher) declaration in the current 9118scope. 9119 9120 9121=item study SCALAR 9122X<study> 9123 9124=item study 9125 9126=for Pod::Functions no-op, formerly optimized input data for repeated searches 9127 9128At this time, C<study> does nothing. This may change in the future. 9129 9130Prior to Perl version 5.16, it would create an inverted index of all characters 9131that occurred in the given SCALAR (or L<C<$_>|perlvar/$_> if unspecified). When 9132matching a pattern, the rarest character from the pattern would be looked up in 9133this index. Rarity was based on some static frequency tables constructed from 9134some C programs and English text. 9135 9136 9137=item sub NAME BLOCK 9138X<sub> 9139 9140=item sub NAME (PROTO) BLOCK 9141 9142=item sub NAME : ATTRS BLOCK 9143 9144=item sub NAME (PROTO) : ATTRS BLOCK 9145 9146=for Pod::Functions declare a subroutine, possibly anonymously 9147 9148This is subroutine definition, not a real function I<per se>. Without a 9149BLOCK it's just a forward declaration. Without a NAME, it's an anonymous 9150function declaration, so does return a value: the CODE ref of the closure 9151just created. 9152 9153See L<perlsub> and L<perlref> for details about subroutines and 9154references; see L<attributes> and L<Attribute::Handlers> for more 9155information about attributes. 9156 9157=item __SUB__ 9158X<__SUB__> 9159 9160=for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine 9161 9162A special token that returns a reference to the current subroutine, or 9163L<C<undef>|/undef EXPR> outside of a subroutine. 9164 9165The behaviour of L<C<__SUB__>|/__SUB__> within a regex code block (such 9166as C</(?{...})/>) is subject to change. 9167 9168This token is only available under C<use v5.16> or the 9169L<C<"current_sub"> feature|feature/The 'current_sub' feature>. 9170See L<feature>. 9171 9172=item substr EXPR,OFFSET,LENGTH,REPLACEMENT 9173X<substr> X<substring> X<mid> X<left> X<right> 9174 9175=item substr EXPR,OFFSET,LENGTH 9176 9177=item substr EXPR,OFFSET 9178 9179=for Pod::Functions get or alter a portion of a string 9180 9181Extracts a substring out of EXPR and returns it. First character is at 9182offset zero. If OFFSET is negative, starts 9183that far back from the end of the string. If LENGTH is omitted, returns 9184everything through the end of the string. If LENGTH is negative, leaves that 9185many characters off the end of the string. 9186 9187 my $s = "The black cat climbed the green tree"; 9188 my $color = substr $s, 4, 5; # black 9189 my $middle = substr $s, 4, -11; # black cat climbed the 9190 my $end = substr $s, 14; # climbed the green tree 9191 my $tail = substr $s, -4; # tree 9192 my $z = substr $s, -4, 2; # tr 9193 9194You can use the L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> 9195function as an lvalue, in which case EXPR 9196must itself be an lvalue. If you assign something shorter than LENGTH, 9197the string will shrink, and if you assign something longer than LENGTH, 9198the string will grow to accommodate it. To keep the string the same 9199length, you may need to pad or chop your value using 9200L<C<sprintf>|/sprintf FORMAT, LIST>. 9201 9202If OFFSET and LENGTH specify a substring that is partly outside the 9203string, only the part within the string is returned. If the substring 9204is beyond either end of the string, 9205L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> returns the undefined 9206value and produces a warning. When used as an lvalue, specifying a 9207substring that is entirely outside the string raises an exception. 9208Here's an example showing the behavior for boundary cases: 9209 9210 my $name = 'fred'; 9211 substr($name, 4) = 'dy'; # $name is now 'freddy' 9212 my $null = substr $name, 6, 2; # returns "" (no warning) 9213 my $oops = substr $name, 7; # returns undef, with warning 9214 substr($name, 7) = 'gap'; # raises an exception 9215 9216An alternative to using 9217L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> as an lvalue is to 9218specify the 9219REPLACEMENT string as the 4th argument. This allows you to replace 9220parts of the EXPR and return what was there before in one operation, 9221just as you can with 9222L<C<splice>|/splice ARRAY,OFFSET,LENGTH,LIST>. 9223 9224 my $s = "The black cat climbed the green tree"; 9225 my $z = substr $s, 14, 7, "jumped from"; # climbed 9226 # $s is now "The black cat jumped from the green tree" 9227 9228Note that the lvalue returned by the three-argument version of 9229L<C<substr>|/substr EXPR,OFFSET,LENGTH,REPLACEMENT> acts as 9230a 'magic bullet'; each time it is assigned to, it remembers which part 9231of the original string is being modified; for example: 9232 9233 my $x = '1234'; 9234 for (substr($x,1,2)) { 9235 $_ = 'a'; print $x,"\n"; # prints 1a4 9236 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 9237 $x = '56789'; 9238 $_ = 'pq'; print $x,"\n"; # prints 5pq9 9239 } 9240 9241With negative offsets, it remembers its position from the end of the string 9242when the target string is modified: 9243 9244 my $x = '1234'; 9245 for (substr($x, -3, 2)) { 9246 $_ = 'a'; print $x,"\n"; # prints 1a4, as above 9247 $x = 'abcdefg'; 9248 print $_,"\n"; # prints f 9249 } 9250 9251Prior to Perl version 5.10, the result of using an lvalue multiple times was 9252unspecified. Prior to 5.16, the result with negative offsets was 9253unspecified. 9254 9255=item symlink OLDFILE,NEWFILE 9256X<symlink> X<link> X<symbolic link> X<link, symbolic> 9257 9258=for Pod::Functions create a symbolic link to a file 9259 9260Creates a new filename symbolically linked to the old filename. 9261Returns C<1> for success, C<0> otherwise. On systems that don't support 9262symbolic links, raises an exception. To check for that, 9263use eval: 9264 9265 my $symlink_exists = eval { symlink("",""); 1 }; 9266 9267Portability issues: L<perlport/symlink>. 9268 9269=item syscall NUMBER, LIST 9270X<syscall> X<system call> 9271 9272=for Pod::Functions execute an arbitrary system call 9273 9274Calls the system call specified as the first element of the list, 9275passing the remaining elements as arguments to the system call. If 9276unimplemented, raises an exception. The arguments are interpreted 9277as follows: if a given argument is numeric, the argument is passed as 9278an int. If not, the pointer to the string value is passed. You are 9279responsible to make sure a string is pre-extended long enough to 9280receive any result that might be written into a string. You can't use a 9281string literal (or other read-only string) as an argument to 9282L<C<syscall>|/syscall NUMBER, LIST> because Perl has to assume that any 9283string pointer might be written through. If your 9284integer arguments are not literals and have never been interpreted in a 9285numeric context, you may need to add C<0> to them to force them to look 9286like numbers. This emulates the 9287L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET> function (or 9288vice versa): 9289 9290 require 'syscall.ph'; # may need to run h2ph 9291 my $s = "hi there\n"; 9292 syscall(SYS_write(), fileno(STDOUT), $s, length $s); 9293 9294Note that Perl supports passing of up to only 14 arguments to your syscall, 9295which in practice should (usually) suffice. 9296 9297Syscall returns whatever value returned by the system call it calls. 9298If the system call fails, L<C<syscall>|/syscall NUMBER, LIST> returns 9299C<-1> and sets L<C<$!>|perlvar/$!> (errno). 9300Note that some system calls I<can> legitimately return C<-1>. The proper 9301way to handle such calls is to assign C<$! = 0> before the call, then 9302check the value of L<C<$!>|perlvar/$!> if 9303L<C<syscall>|/syscall NUMBER, LIST> returns C<-1>. 9304 9305There's a problem with C<syscall(SYS_pipe())>: it returns the file 9306number of the read end of the pipe it creates, but there is no way 9307to retrieve the file number of the other end. You can avoid this 9308problem by using L<C<pipe>|/pipe READHANDLE,WRITEHANDLE> instead. 9309 9310Portability issues: L<perlport/syscall>. 9311 9312=item sysopen FILEHANDLE,FILENAME,MODE 9313X<sysopen> 9314 9315=item sysopen FILEHANDLE,FILENAME,MODE,PERMS 9316 9317=for Pod::Functions +5.002 open a file, pipe, or descriptor 9318 9319Opens the file whose filename is given by FILENAME, and associates it with 9320FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real 9321filehandle wanted; an undefined scalar will be suitably autovivified. This 9322function calls the underlying operating system's L<open(2)> function with the 9323parameters FILENAME, MODE, and PERMS. 9324 9325Returns true on success and L<C<undef>|/undef EXPR> otherwise. 9326 9327L<PerlIO> layers will be applied to the handle the same way they would in an 9328L<C<open>|/open FILEHANDLE,MODE,EXPR> call that does not specify layers. That is, 9329the current value of L<C<${^OPEN}>|perlvar/${^OPEN}> as set by the L<open> 9330pragma in a lexical scope, or the C<-C> command-line option or C<PERL_UNICODE> 9331environment variable in the main program scope, falling back to the platform 9332defaults as described in L<PerlIO/Defaults and how to override them>. If you 9333want to remove any layers that may transform the byte stream, use 9334L<C<binmode>|/binmode FILEHANDLE, LAYER> after opening it. 9335 9336The possible values and flag bits of the MODE parameter are 9337system-dependent; they are available via the standard module 9338L<C<Fcntl>|Fcntl>. See the documentation of your operating system's 9339L<open(2)> syscall to see 9340which values and flag bits are available. You may combine several flags 9341using the C<|>-operator. 9342 9343Some of the most common values are C<O_RDONLY> for opening the file in 9344read-only mode, C<O_WRONLY> for opening the file in write-only mode, 9345and C<O_RDWR> for opening the file in read-write mode. 9346X<O_RDONLY> X<O_RDWR> X<O_WRONLY> 9347 9348For historical reasons, some values work on almost every system 9349supported by Perl: 0 means read-only, 1 means write-only, and 2 9350means read/write. We know that these values do I<not> work under 9351OS/390; you probably don't want to use them in new code. 9352 9353If the file named by FILENAME does not exist and the 9354L<C<open>|/open FILEHANDLE,MODE,EXPR> call creates 9355it (typically because MODE includes the C<O_CREAT> flag), then the value of 9356PERMS specifies the permissions of the newly created file. If you omit 9357the PERMS argument to L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, 9358Perl uses the octal value C<0666>. 9359These permission values need to be in octal, and are modified by your 9360process's current L<C<umask>|/umask EXPR>. 9361X<O_CREAT> 9362 9363In many systems the C<O_EXCL> flag is available for opening files in 9364exclusive mode. This is B<not> locking: exclusiveness means here that 9365if the file already exists, 9366L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> fails. C<O_EXCL> may 9367not work 9368on network filesystems, and has no effect unless the C<O_CREAT> flag 9369is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from 9370being opened if it is a symbolic link. It does not protect against 9371symbolic links in the file's path. 9372X<O_EXCL> 9373 9374Sometimes you may want to truncate an already-existing file. This 9375can be done using the C<O_TRUNC> flag. The behavior of 9376C<O_TRUNC> with C<O_RDONLY> is undefined. 9377X<O_TRUNC> 9378 9379You should seldom if ever use C<0644> as argument to 9380L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>, because 9381that takes away the user's option to have a more permissive umask. 9382Better to omit it. See L<C<umask>|/umask EXPR> for more on this. 9383 9384This function has no direct relation to the usage of 9385L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>, 9386L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, 9387or L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. A handle opened with 9388this function can be used with buffered IO just as one opened with 9389L<C<open>|/open FILEHANDLE,MODE,EXPR> can be used with unbuffered IO. 9390 9391Note that under Perls older than 5.8.0, 9392L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> depends on the 9393L<fdopen(3)> C library function. On many Unix systems, L<fdopen(3)> is known 9394to fail when file descriptors exceed a certain value, typically 255. If 9395you need more file descriptors than that, consider using the 9396L<C<POSIX::open>|POSIX/C<open>> function. For Perls 5.8.0 and later, 9397PerlIO is (most often) the default. 9398 9399See L<perlopentut> for a kinder, gentler explanation of opening files. 9400 9401Portability issues: L<perlport/sysopen>. 9402 9403=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET 9404X<sysread> 9405 9406=item sysread FILEHANDLE,SCALAR,LENGTH 9407 9408=for Pod::Functions fixed-length unbuffered input from a filehandle 9409 9410Attempts to read LENGTH bytes of data into variable SCALAR from the 9411specified FILEHANDLE, using L<read(2)>. It bypasses any L<PerlIO> layers 9412including buffered IO (but is affected by the presence of the C<:utf8> 9413layer as described later), so mixing this with other kinds of reads, 9414L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>, 9415L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 9416L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> can cause 9417confusion because the 9418C<:perlio> or C<:crlf> layers usually buffer data. Returns the number of 9419bytes actually read, C<0> at end of file, or undef if there was an 9420error (in the latter case L<C<$!>|perlvar/$!> is also set). SCALAR will 9421be grown or 9422shrunk so that the last byte actually read is the last byte of the 9423scalar after the read. 9424 9425An OFFSET may be specified to place the read data at some place in the 9426string other than the beginning. A negative OFFSET specifies 9427placement at that many characters counting backwards from the end of 9428the string. A positive OFFSET greater than the length of SCALAR 9429results in the string being padded to the required size with C<"\0"> 9430bytes before the result of the read is appended. 9431 9432There is no syseof() function, which is ok, since 9433L<C<eof>|/eof FILEHANDLE> doesn't work well on device files (like ttys) 9434anyway. Use L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> and 9435check for a return value of 0 to decide whether you're done. 9436 9437Note that if the filehandle has been marked as C<:utf8>, C<sysread> will 9438throw an exception. The C<:encoding(...)> layer implicitly 9439introduces the C<:utf8> layer. See 9440L<C<binmode>|/binmode FILEHANDLE, LAYER>, 9441L<C<open>|/open FILEHANDLE,MODE,EXPR>, and the L<open> pragma. 9442 9443=item sysseek FILEHANDLE,POSITION,WHENCE 9444X<sysseek> X<lseek> 9445 9446=for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite 9447 9448Sets FILEHANDLE's system position I<in bytes> using L<lseek(2)>. FILEHANDLE may 9449be an expression whose value gives the name of the filehandle. The values 9450for WHENCE are C<0> to set the new position to POSITION; C<1> to set it 9451to the current position plus POSITION; and C<2> to set it to EOF plus 9452POSITION, typically negative. 9453 9454Note the emphasis on bytes: even if the filehandle has been set to operate 9455on characters (for example using the C<:encoding(UTF-8)> I/O layer), the 9456L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 9457L<C<tell>|/tell FILEHANDLE>, and 9458L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> 9459family of functions use byte offsets, not character offsets, 9460because seeking to a character offset would be very slow in a UTF-8 file. 9461 9462L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> bypasses normal 9463buffered IO, so mixing it with reads other than 9464L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET> (for example 9465L<C<readline>|/readline EXPR> or 9466L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>), 9467L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>, 9468L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 9469L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause 9470confusion. 9471 9472For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, 9473and C<SEEK_END> (start of the file, current position, end of the file) 9474from the L<Fcntl> module. Use of the constants is also more portable 9475than relying on 0, 1, and 2. For example to define a "systell" function: 9476 9477 use Fcntl 'SEEK_CUR'; 9478 sub systell { sysseek($_[0], 0, SEEK_CUR) } 9479 9480Returns the new position, or the undefined value on failure. A position 9481of zero is returned as the string C<"0 but true">; thus 9482L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> returns 9483true on success and false on failure, yet you can still easily determine 9484the new position. 9485 9486=item system LIST 9487X<system> X<shell> 9488 9489=item system PROGRAM LIST 9490 9491=for Pod::Functions run a separate program 9492 9493Does exactly the same thing as L<C<exec>|/exec LIST>, except that a fork is 9494done first and the parent process waits for the child process to 9495exit. Note that argument processing varies depending on the 9496number of arguments. If there is more than one argument in LIST, 9497or if LIST is an array with more than one value, starts the program 9498given by the first element of the list with arguments given by the 9499rest of the list. If there is only one scalar argument, the argument 9500is checked for shell metacharacters, and if there are any, the 9501entire argument is passed to the system's command shell for parsing 9502(this is C</bin/sh -c> on Unix platforms, but varies on other 9503platforms). If there are no shell metacharacters in the argument, 9504it is split into words and passed directly to C<execvp>, which is 9505more efficient. On Windows, only the C<system PROGRAM LIST> syntax will 9506reliably avoid using the shell; C<system LIST>, even with more than one 9507element, will fall back to the shell if the first spawn fails. 9508 9509Perl will attempt to flush all files opened for 9510output before any operation that may do a fork, but this may not be 9511supported on some platforms (see L<perlport>). To be safe, you may need 9512to set L<C<$E<verbar>>|perlvar/$E<verbar>> (C<$AUTOFLUSH> in L<English>) 9513or call the C<autoflush> method of L<C<IO::Handle>|IO::Handle/METHODS> 9514on any open handles. 9515 9516The return value is the exit status of the program as returned by the 9517L<C<wait>|/wait> call. To get the actual exit value, shift right by 9518eight (see below). See also L<C<exec>|/exec LIST>. This is I<not> what 9519you want to use to capture the output from a command; for that you 9520should use merely backticks or 9521L<C<qxE<sol>E<sol>>|/qxE<sol>STRINGE<sol>>, as described in 9522L<perlop/"`STRING`">. Return value of -1 indicates a failure to start 9523the program or an error of the L<wait(2)> system call (inspect 9524L<C<$!>|perlvar/$!> for the reason). 9525 9526If you'd like to make L<C<system>|/system LIST> (and many other bits of 9527Perl) die on error, have a look at the L<autodie> pragma. 9528 9529Like L<C<exec>|/exec LIST>, L<C<system>|/system LIST> allows you to lie 9530to a program about its name if you use the C<system PROGRAM LIST> 9531syntax. Again, see L<C<exec>|/exec LIST>. 9532 9533Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of 9534L<C<system>|/system LIST>, if you expect your program to terminate on 9535receipt of these signals you will need to arrange to do so yourself 9536based on the return value. 9537 9538 my @args = ("command", "arg1", "arg2"); 9539 system(@args) == 0 9540 or die "system @args failed: $?"; 9541 9542If you'd like to manually inspect L<C<system>|/system LIST>'s failure, 9543you can check all possible failure modes by inspecting 9544L<C<$?>|perlvar/$?> like this: 9545 9546 if ($? == -1) { 9547 print "failed to execute: $!\n"; 9548 } 9549 elsif ($? & 127) { 9550 printf "child died with signal %d, %s coredump\n", 9551 ($? & 127), ($? & 128) ? 'with' : 'without'; 9552 } 9553 else { 9554 printf "child exited with value %d\n", $? >> 8; 9555 } 9556 9557Alternatively, you may inspect the value of 9558L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}> with the 9559L<C<W*()>|POSIX/C<WIFEXITED>> calls from the L<POSIX> module. 9560 9561When L<C<system>|/system LIST>'s arguments are executed indirectly by 9562the shell, results and return codes are subject to its quirks. 9563See L<perlop/"`STRING`"> and L<C<exec>|/exec LIST> for details. 9564 9565Since L<C<system>|/system LIST> does a L<C<fork>|/fork> and 9566L<C<wait>|/wait> it may affect a C<SIGCHLD> handler. See L<perlipc> for 9567details. 9568 9569Portability issues: L<perlport/system>. 9570 9571=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET 9572X<syswrite> 9573 9574=item syswrite FILEHANDLE,SCALAR,LENGTH 9575 9576=item syswrite FILEHANDLE,SCALAR 9577 9578=for Pod::Functions fixed-length unbuffered output to a filehandle 9579 9580Attempts to write LENGTH bytes of data from variable SCALAR to the 9581specified FILEHANDLE, using L<write(2)>. If LENGTH is 9582not specified, writes whole SCALAR. It bypasses any L<PerlIO> layers 9583including buffered IO (but is affected by the presence of the C<:utf8> 9584layer as described later), so 9585mixing this with reads (other than C<sysread)>), 9586L<C<print>|/print FILEHANDLE LIST>, L<C<write>|/write FILEHANDLE>, 9587L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 9588L<C<tell>|/tell FILEHANDLE>, or L<C<eof>|/eof FILEHANDLE> may cause 9589confusion because the C<:perlio> and C<:crlf> layers usually buffer data. 9590Returns the number of bytes actually written, or L<C<undef>|/undef EXPR> 9591if there was an error (in this case the errno variable 9592L<C<$!>|perlvar/$!> is also set). If the LENGTH is greater than the 9593data available in the SCALAR after the OFFSET, only as much data as is 9594available will be written. 9595 9596An OFFSET may be specified to write the data from some part of the 9597string other than the beginning. A negative OFFSET specifies writing 9598that many characters counting backwards from the end of the string. 9599If SCALAR is of length zero, you can only use an OFFSET of 0. 9600 9601B<WARNING>: If the filehandle is marked C<:utf8>, C<syswrite> will raise an exception. 9602The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. 9603Alternately, if the handle is not marked with an encoding but you 9604attempt to write characters with code points over 255, raises an exception. 9605See L<C<binmode>|/binmode FILEHANDLE, LAYER>, 9606L<C<open>|/open FILEHANDLE,MODE,EXPR>, and the L<open> pragma. 9607 9608=item tell FILEHANDLE 9609X<tell> 9610 9611=item tell 9612 9613=for Pod::Functions get current seekpointer on a filehandle 9614 9615Returns the current position I<in bytes> for FILEHANDLE, or -1 on 9616error. FILEHANDLE may be an expression whose value gives the name of 9617the actual filehandle. If FILEHANDLE is omitted, assumes the file 9618last read. 9619 9620Note the emphasis on bytes: even if the filehandle has been set to operate 9621on characters (for example using the C<:encoding(UTF-8)> I/O layer), the 9622L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE>, 9623L<C<tell>|/tell FILEHANDLE>, and 9624L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE> 9625family of functions use byte offsets, not character offsets, 9626because seeking to a character offset would be very slow in a UTF-8 file. 9627 9628The return value of L<C<tell>|/tell FILEHANDLE> for the standard streams 9629like the STDIN depends on the operating system: it may return -1 or 9630something else. L<C<tell>|/tell FILEHANDLE> on pipes, fifos, and 9631sockets usually returns -1. 9632 9633There is no C<systell> function. Use 9634L<C<sysseek($fh, 0, 1)>|/sysseek FILEHANDLE,POSITION,WHENCE> for that. 9635 9636Do not use L<C<tell>|/tell FILEHANDLE> (or other buffered I/O 9637operations) on a filehandle that has been manipulated by 9638L<C<sysread>|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>, 9639L<C<syswrite>|/syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET>, or 9640L<C<sysseek>|/sysseek FILEHANDLE,POSITION,WHENCE>. Those functions 9641ignore the buffering, while L<C<tell>|/tell FILEHANDLE> does not. 9642 9643=item telldir DIRHANDLE 9644X<telldir> 9645 9646=for Pod::Functions get current seekpointer on a directory handle 9647 9648Returns the current position of the L<C<readdir>|/readdir DIRHANDLE> 9649routines on DIRHANDLE. Value may be given to 9650L<C<seekdir>|/seekdir DIRHANDLE,POS> to access a particular location in 9651a directory. L<C<telldir>|/telldir DIRHANDLE> has the same caveats 9652about possible directory compaction as the corresponding system library 9653routine. 9654 9655=item tie VARIABLE,CLASSNAME,LIST 9656X<tie> 9657 9658=for Pod::Functions +5.002 bind a variable to an object class 9659 9660This function binds a variable to a package class that will provide the 9661implementation for the variable. VARIABLE is the name of the variable 9662to be enchanted. CLASSNAME is the name of a class implementing objects 9663of correct type. Any additional arguments are passed to the 9664appropriate constructor 9665method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, 9666or C<TIEHASH>). Typically these are arguments such as might be passed 9667to the L<dbm_open(3)> function of C. The object returned by the 9668constructor is also returned by the 9669L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function, which would be useful 9670if you want to access other methods in CLASSNAME. 9671 9672Note that functions such as L<C<keys>|/keys HASH> and 9673L<C<values>|/values HASH> may return huge lists when used on large 9674objects, like DBM files. You may prefer to use the L<C<each>|/each 9675HASH> function to iterate over such. Example: 9676 9677 # print out history file offsets 9678 use NDBM_File; 9679 tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); 9680 while (my ($key,$val) = each %HIST) { 9681 print $key, ' = ', unpack('L', $val), "\n"; 9682 } 9683 9684A class implementing a hash should have the following methods: 9685 9686 TIEHASH classname, LIST 9687 FETCH this, key 9688 STORE this, key, value 9689 DELETE this, key 9690 CLEAR this 9691 EXISTS this, key 9692 FIRSTKEY this 9693 NEXTKEY this, lastkey 9694 SCALAR this 9695 DESTROY this 9696 UNTIE this 9697 9698A class implementing an ordinary array should have the following methods: 9699 9700 TIEARRAY classname, LIST 9701 FETCH this, key 9702 STORE this, key, value 9703 FETCHSIZE this 9704 STORESIZE this, count 9705 CLEAR this 9706 PUSH this, LIST 9707 POP this 9708 SHIFT this 9709 UNSHIFT this, LIST 9710 SPLICE this, offset, length, LIST 9711 EXTEND this, count 9712 DELETE this, key 9713 EXISTS this, key 9714 DESTROY this 9715 UNTIE this 9716 9717A class implementing a filehandle should have the following methods: 9718 9719 TIEHANDLE classname, LIST 9720 READ this, scalar, length, offset 9721 READLINE this 9722 GETC this 9723 WRITE this, scalar, length, offset 9724 PRINT this, LIST 9725 PRINTF this, format, LIST 9726 BINMODE this 9727 EOF this 9728 FILENO this 9729 SEEK this, position, whence 9730 TELL this 9731 OPEN this, mode, LIST 9732 CLOSE this 9733 DESTROY this 9734 UNTIE this 9735 9736A class implementing a scalar should have the following methods: 9737 9738 TIESCALAR classname, LIST 9739 FETCH this, 9740 STORE this, value 9741 DESTROY this 9742 UNTIE this 9743 9744Not all methods indicated above need be implemented. See L<perltie>, 9745L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. 9746 9747Unlike L<C<dbmopen>|/dbmopen HASH,DBNAME,MASK>, the 9748L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> function will not 9749L<C<use>|/use Module VERSION LIST> or L<C<require>|/require VERSION> a 9750module for you; you need to do that explicitly yourself. See L<DB_File> 9751or the L<Config> module for interesting 9752L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> implementations. 9753 9754For further details see L<perltie>, L<C<tied>|/tied VARIABLE>. 9755 9756=item tied VARIABLE 9757X<tied> 9758 9759=for Pod::Functions get a reference to the object underlying a tied variable 9760 9761Returns a reference to the object underlying VARIABLE (the same value 9762that was originally returned by the 9763L<C<tie>|/tie VARIABLE,CLASSNAME,LIST> call that bound the variable 9764to a package.) Returns the undefined value if VARIABLE isn't tied to a 9765package. 9766 9767=item time 9768X<time> X<epoch> 9769 9770=for Pod::Functions return number of seconds since 1970 9771 9772Returns the number of non-leap seconds since whatever time the system 9773considers to be the epoch, suitable for feeding to 9774L<C<gmtime>|/gmtime EXPR> and L<C<localtime>|/localtime EXPR>. On most 9775systems the epoch is 00:00:00 UTC, January 1, 1970; 9776a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 97771904 in the current local time zone for its epoch. 9778 9779For measuring time in better granularity than one second, use the 9780L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or, 9781if you have L<gettimeofday(2)>, you may be able to use the 9782L<C<syscall>|/syscall NUMBER, LIST> interface of Perl. See L<perlfaq8> 9783for details. 9784 9785For date and time processing look at the many related modules on CPAN. 9786For a comprehensive date and time representation look at the 9787L<DateTime> module. 9788 9789=item times 9790X<times> 9791 9792=for Pod::Functions return elapsed time for self and child processes 9793 9794Returns a four-element list giving the user and system times in 9795seconds for this process and any exited children of this process. 9796 9797 my ($user,$system,$cuser,$csystem) = times; 9798 9799In scalar context, L<C<times>|/times> returns C<$user>. 9800 9801Children's times are only included for terminated children. 9802 9803Portability issues: L<perlport/times>. 9804 9805=item tr/// 9806 9807=for Pod::Functions transliterate a string 9808 9809The transliteration operator. Same as 9810L<C<yE<sol>E<sol>E<sol>>|/yE<sol>E<sol>E<sol>>. See 9811L<perlop/"Quote-Like Operators">. 9812 9813=item truncate FILEHANDLE,LENGTH 9814X<truncate> 9815 9816=item truncate EXPR,LENGTH 9817 9818=for Pod::Functions shorten a file 9819 9820Truncates the file opened on FILEHANDLE, or named by EXPR, to the 9821specified length. Raises an exception if truncate isn't implemented 9822on your system. Returns true if successful, L<C<undef>|/undef EXPR> on 9823error. 9824 9825The behavior is undefined if LENGTH is greater than the length of the 9826file. 9827 9828The position in the file of FILEHANDLE is left unchanged. You may want to 9829call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the 9830file. 9831 9832Portability issues: L<perlport/truncate>. 9833 9834=item uc EXPR 9835X<uc> X<uppercase> X<toupper> 9836 9837=item uc 9838 9839=for Pod::Functions return upper-case version of a string 9840 9841Returns an uppercased version of EXPR. If EXPR is omitted, uses 9842L<C<$_>|perlvar/$_>. 9843 9844 my $str = uc("Perl is GREAT"); # "PERL IS GREAT" 9845 9846This function behaves the same way under various pragmas, such as in a locale, 9847as L<C<lc>|/lc EXPR> does. 9848 9849If you want titlecase mapping on initial letters see 9850L<C<ucfirst>|/ucfirst EXPR> instead. 9851 9852B<Note:> This is the internal function implementing the 9853L<C<\U>|perlop/"Quote and Quote-like Operators"> escape in double-quoted 9854strings. 9855 9856 my $str = "Perl is \Ugreat\E"; # "Perl is GREAT" 9857 9858=item ucfirst EXPR 9859X<ucfirst> X<uppercase> 9860 9861=item ucfirst 9862 9863=for Pod::Functions return a string with the first letter in upper case 9864 9865Returns the value of EXPR with the B<first> character in uppercase 9866(Unicode calls this titlecase). If EXPR is omitted, C<ucfirst> uses L<C<$_>|perlvar/$_>. 9867 9868 my $str = ucfirst("hello world!"); # "Hello world!" 9869 9870This function behaves the same way under various pragmas, such as in a locale, 9871as L<C<lc>|/lc EXPR> does. 9872 9873B<Note:> This is the internal function implementing the C<\u> escape in 9874double-quoted strings. 9875 9876 my $str = "\uperl\E is great"; # "Perl is great" 9877 9878=item umask EXPR 9879X<umask> 9880 9881=item umask 9882 9883=for Pod::Functions set file creation mode mask 9884 9885Sets the umask for the process to EXPR and returns the previous value. 9886If EXPR is omitted, merely returns the current umask. 9887 9888The Unix permission C<rwxr-x---> is represented as three sets of three 9889bits, or three octal digits: C<0750> (the leading 0 indicates octal 9890and isn't one of the digits). The L<C<umask>|/umask EXPR> value is such 9891a number representing disabled permissions bits. The permission (or 9892"mode") values you pass L<C<mkdir>|/mkdir FILENAME,MODE> or 9893L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> are modified by your 9894umask, so even if you tell 9895L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> to create a file with 9896permissions C<0777>, if your umask is C<0022>, then the file will 9897actually be created with permissions C<0755>. If your 9898L<C<umask>|/umask EXPR> were C<0027> (group can't write; others can't 9899read, write, or execute), then passing 9900L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE> C<0666> would create a 9901file with mode C<0640> (because C<0666 &~ 027> is C<0640>). 9902 9903Here's some advice: supply a creation mode of C<0666> for regular 9904files (in L<C<sysopen>|/sysopen FILEHANDLE,FILENAME,MODE>) and one of 9905C<0777> for directories (in L<C<mkdir>|/mkdir FILENAME,MODE>) and 9906executable files. This gives users the freedom of 9907choice: if they want protected files, they might choose process umasks 9908of C<022>, C<027>, or even the particularly antisocial mask of C<077>. 9909Programs should rarely if ever make policy decisions better left to 9910the user. The exception to this is when writing files that should be 9911kept private: mail files, web browser cookies, F<.rhosts> files, and 9912so on. 9913 9914If L<umask(2)> is not implemented on your system and you are trying to 9915restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>), 9916raises an exception. If L<umask(2)> is not implemented and you are 9917not trying to restrict access for yourself, returns 9918L<C<undef>|/undef EXPR>. 9919 9920Remember that a umask is a number, usually given in octal; it is I<not> a 9921string of octal digits. See also L<C<oct>|/oct EXPR>, if all you have 9922is a string. 9923 9924Portability issues: L<perlport/umask>. 9925 9926=item undef EXPR 9927X<undef> X<undefine> 9928 9929=item undef 9930 9931=for Pod::Functions remove a variable or function definition 9932 9933Undefines the value of EXPR, which must be an lvalue. Use only on a 9934scalar value, an array (using C<@>), a hash (using C<%>), a subroutine 9935(using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}> 9936will probably not do what you expect on most predefined variables or 9937DBM list values, so don't do that; see L<C<delete>|/delete EXPR>. 9938Always returns the undefined value. 9939You can omit the EXPR, in which case nothing is 9940undefined, but you still get an undefined value that you could, for 9941instance, return from a subroutine, assign to a variable, or pass as a 9942parameter. Examples: 9943 9944 undef $foo; 9945 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; 9946 undef @ary; 9947 undef %hash; 9948 undef &mysub; 9949 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. 9950 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; 9951 select undef, undef, undef, 0.25; 9952 my ($x, $y, undef, $z) = foo(); # Ignore third value returned 9953 9954Note that this is a unary operator, not a list operator. 9955 9956=item unlink LIST 9957X<unlink> X<delete> X<remove> X<rm> X<del> 9958 9959=item unlink 9960 9961=for Pod::Functions remove one link to a file 9962 9963Deletes a list of files. On success, it returns the number of files 9964it successfully deleted. On failure, it returns false and sets 9965L<C<$!>|perlvar/$!> (errno): 9966 9967 my $unlinked = unlink 'a', 'b', 'c'; 9968 unlink @goners; 9969 unlink glob "*.bak"; 9970 9971On error, L<C<unlink>|/unlink LIST> will not tell you which files it 9972could not remove. 9973If you want to know which files you could not remove, try them one 9974at a time: 9975 9976 foreach my $file ( @goners ) { 9977 unlink $file or warn "Could not unlink $file: $!"; 9978 } 9979 9980Note: L<C<unlink>|/unlink LIST> will not attempt to delete directories 9981unless you are 9982superuser and the B<-U> flag is supplied to Perl. Even if these 9983conditions are met, be warned that unlinking a directory can inflict 9984damage on your filesystem. Finally, using L<C<unlink>|/unlink LIST> on 9985directories is not supported on many operating systems. Use 9986L<C<rmdir>|/rmdir FILENAME> instead. 9987 9988If LIST is omitted, L<C<unlink>|/unlink LIST> uses L<C<$_>|perlvar/$_>. 9989 9990=item unpack TEMPLATE,EXPR 9991X<unpack> 9992 9993=item unpack TEMPLATE 9994 9995=for Pod::Functions convert binary structure into normal perl variables 9996 9997L<C<unpack>|/unpack TEMPLATE,EXPR> does the reverse of 9998L<C<pack>|/pack TEMPLATE,LIST>: it takes a string 9999and expands it out into a list of values. 10000(In scalar context, it returns merely the first value produced.) 10001 10002If EXPR is omitted, unpacks the L<C<$_>|perlvar/$_> string. 10003See L<perlpacktut> for an introduction to this function. 10004 10005The string is broken into chunks described by the TEMPLATE. Each chunk 10006is converted separately to a value. Typically, either the string is a result 10007of L<C<pack>|/pack TEMPLATE,LIST>, or the characters of the string 10008represent a C structure of some kind. 10009 10010The TEMPLATE has the same format as in the 10011L<C<pack>|/pack TEMPLATE,LIST> function. 10012Here's a subroutine that does substring: 10013 10014 sub substr { 10015 my ($what, $where, $howmuch) = @_; 10016 unpack("x$where a$howmuch", $what); 10017 } 10018 10019and then there's 10020 10021 sub ordinal { unpack("W",$_[0]); } # same as ord() 10022 10023In addition to fields allowed in L<C<pack>|/pack TEMPLATE,LIST>, you may 10024prefix a field with a %<number> to indicate that 10025you want a <number>-bit checksum of the items instead of the items 10026themselves. Default is a 16-bit checksum. The checksum is calculated by 10027summing numeric values of expanded values (for string fields the sum of 10028C<ord($char)> is taken; for bit fields the sum of zeroes and ones). 10029 10030For example, the following 10031computes the same number as the System V sum program: 10032 10033 my $checksum = do { 10034 local $/; # slurp! 10035 unpack("%32W*", readline) % 65535; 10036 }; 10037 10038The following efficiently counts the number of set bits in a bit vector: 10039 10040 my $setbits = unpack("%32b*", $selectmask); 10041 10042The C<p> and C<P> formats should be used with care. Since Perl 10043has no way of checking whether the value passed to 10044L<C<unpack>|/unpack TEMPLATE,EXPR> 10045corresponds to a valid memory location, passing a pointer value that's 10046not known to be valid is likely to have disastrous consequences. 10047 10048If there are more pack codes or if the repeat count of a field or a group 10049is larger than what the remainder of the input string allows, the result 10050is not well defined: the repeat count may be decreased, or 10051L<C<unpack>|/unpack TEMPLATE,EXPR> may produce empty strings or zeros, 10052or it may raise an exception. 10053If the input string is longer than one described by the TEMPLATE, 10054the remainder of that input string is ignored. 10055 10056See L<C<pack>|/pack TEMPLATE,LIST> for more examples and notes. 10057 10058=item unshift ARRAY,LIST 10059X<unshift> 10060 10061=for Pod::Functions prepend more elements to the beginning of a list 10062 10063Add one or more elements to the B<beginning> of an array. This is the 10064opposite of a L<C<shift>|/shift ARRAY>. 10065 10066 my @animals = ("cat"); 10067 unshift(@animals, "mouse"); # ("mouse", "cat") 10068 10069 my @colors = ("red"); 10070 unshift(@colors, ("blue", "green")); # ("blue", "green", "red") 10071 10072Returns the new number of elements in the updated array. 10073 10074 # Return value is the number of items in the updated array 10075 my $color_count = unshift(@colors, ("yellow", "purple")); 10076 10077 say "There are $color_count colors in the updated array"; 10078 10079Note the LIST is prepended whole, not one element at a time, so the 10080prepended elements stay in the same order. Use 10081L<C<reverse>|/reverse LIST> to do the reverse. 10082 10083Starting with Perl 5.14, an experimental feature allowed 10084L<C<unshift>|/unshift ARRAY,LIST> to take 10085a scalar expression. This experiment has been deemed unsuccessful, and was 10086removed as of Perl 5.24. 10087 10088=item untie VARIABLE 10089X<untie> 10090 10091=for Pod::Functions break a tie binding to a variable 10092 10093Breaks the binding between a variable and a package. 10094(See L<tie|/tie VARIABLE,CLASSNAME,LIST>.) 10095Has no effect if the variable is not tied. 10096 10097=item use Module VERSION LIST 10098X<use> X<module> X<import> 10099 10100=item use Module VERSION 10101 10102=item use Module LIST 10103 10104=item use Module 10105 10106=for Pod::Functions load in a module at compile time and import its namespace 10107 10108Imports some semantics into the current package from the named module, 10109generally by aliasing certain subroutine or variable names into your 10110package. It is exactly equivalent to 10111 10112 BEGIN { require Module; Module->import( LIST ); } 10113 10114except that Module I<must> be a bareword. 10115The importation can be made conditional by using the L<if> module. 10116 10117The C<BEGIN> forces the L<C<require>|/require VERSION> and 10118L<C<import>|/import LIST> to happen at compile time. The 10119L<C<require>|/require VERSION> makes sure the module is loaded into 10120memory if it hasn't been yet. The L<C<import>|/import LIST> is not a 10121builtin; it's just an ordinary static method 10122call into the C<Module> package to tell the module to import the list of 10123features back into the current package. The module can implement its 10124L<C<import>|/import LIST> method any way it likes, though most modules 10125just choose to derive their L<C<import>|/import LIST> method via 10126inheritance from the C<Exporter> class that is defined in the 10127L<C<Exporter>|Exporter> module. See L<Exporter>. If no 10128L<C<import>|/import LIST> method can be found, then the call is skipped, 10129even if there is an AUTOLOAD method. 10130 10131If you do not want to call the package's L<C<import>|/import LIST> 10132method (for instance, 10133to stop your namespace from being altered), explicitly supply the empty list: 10134 10135 use Module (); 10136 10137That is exactly equivalent to 10138 10139 BEGIN { require Module } 10140 10141If the VERSION argument is present between Module and LIST, then the 10142L<C<use>|/use Module VERSION LIST> will call the C<VERSION> method in 10143class Module with the given version as an argument: 10144 10145 use Module 12.34; 10146 10147is equivalent to: 10148 10149 BEGIN { require Module; Module->VERSION(12.34) } 10150 10151The L<default C<VERSION> method|UNIVERSAL/C<VERSION ( [ REQUIRE ] )>>, 10152inherited from the L<C<UNIVERSAL>|UNIVERSAL> class, croaks if the given 10153version is larger than the value of the variable C<$Module::VERSION>. 10154 10155The VERSION argument cannot be an arbitrary expression. It only counts 10156as a VERSION argument if it is a version number literal, starting with 10157either a digit or C<v> followed by a digit. Anything that doesn't 10158look like a version literal will be parsed as the start of the LIST. 10159Nevertheless, many attempts to use an arbitrary expression as a VERSION 10160argument will appear to work, because L<Exporter>'s C<import> method 10161handles numeric arguments specially, performing version checks rather 10162than treating them as things to export. 10163 10164Again, there is a distinction between omitting LIST (L<C<import>|/import 10165LIST> called with no arguments) and an explicit empty LIST C<()> 10166(L<C<import>|/import LIST> not called). Note that there is no comma 10167after VERSION! 10168 10169Because this is a wide-open interface, pragmas (compiler directives) 10170are also implemented this way. Some of the currently implemented 10171pragmas are: 10172 10173 use constant; 10174 use diagnostics; 10175 use integer; 10176 use sigtrap qw(SEGV BUS); 10177 use strict qw(subs vars refs); 10178 use subs qw(afunc blurfl); 10179 use warnings qw(all); 10180 use sort qw(stable); 10181 10182Some of these pseudo-modules import semantics into the current 10183block scope (like L<C<strict>|strict> or L<C<integer>|integer>, unlike 10184ordinary modules, which import symbols into the current package (which 10185are effective through the end of the file). 10186 10187Because L<C<use>|/use Module VERSION LIST> takes effect at compile time, 10188it doesn't respect the ordinary flow control of the code being compiled. 10189In particular, putting a L<C<use>|/use Module VERSION LIST> inside the 10190false branch of a conditional doesn't prevent it 10191from being processed. If a module or pragma only needs to be loaded 10192conditionally, this can be done using the L<if> pragma: 10193 10194 use if $] < 5.008, "utf8"; 10195 use if WANT_WARNINGS, warnings => qw(all); 10196 10197There's a corresponding L<C<no>|/no MODULE VERSION LIST> declaration 10198that unimports meanings imported by L<C<use>|/use Module VERSION LIST>, 10199i.e., it calls C<< Module->unimport(LIST) >> instead of 10200L<C<import>|/import LIST>. It behaves just as L<C<import>|/import LIST> 10201does with VERSION, an omitted or empty LIST, 10202or no unimport method being found. 10203 10204 no integer; 10205 no strict 'refs'; 10206 no warnings; 10207 10208See L<perlmodlib> for a list of standard modules and pragmas. See 10209L<perlrun|perlrun/-m[-]module> for the C<-M> and C<-m> command-line 10210options to Perl that give L<C<use>|/use Module VERSION LIST> 10211functionality from the command-line. 10212 10213=item use VERSION 10214 10215=for Pod::Functions enable Perl language features and declare required version 10216 10217Lexically enables all features available in the requested version as 10218defined by the L<feature> pragma, disabling any features not in the 10219requested version's feature bundle. See L<feature>. 10220 10221VERSION may be either a v-string such as v5.24.1, which will be compared 10222to L<C<$^V>|perlvar/$^V> (aka $PERL_VERSION), or a numeric argument of the 10223form 5.024001, which will be compared to L<C<$]>|perlvar/$]>. An 10224exception is raised if VERSION is greater than the version of the current 10225Perl interpreter; Perl will not attempt to parse the rest of the file. 10226Compare with L<C<require>|/require VERSION>, which can do a similar check 10227at run time. 10228 10229If the specified Perl version is 5.12 or higher, strictures are enabled 10230lexically as with L<C<use strict>|strict>. 10231 10232If the specified Perl version is 5.35.0 or higher, L<warnings> are enabled. 10233 10234If the specified Perl version is 5.39.0 or higher, builtin functions are 10235imported lexically as with L<C<use builtin>|builtin> with a corresponding 10236version bundle. 10237 10238Use of C<use VERSION> while another is in effect is not allowed with a 10239C<use v5.39;> or greater version. For lower versions, C<use VERSION> will 10240override most behavior of a previous C<use VERSION>, possibly removing 10241C<warnings> and C<feature> effects added by it. This behavior is deprecated, 10242and a future release of perl will disallow changing the version once one has 10243been declared. Additionally, a C<use VERSION> with a version less than 5.11 10244is not allowed after a C<use VERSION> with a version greater than 5.11. 10245 10246C<use VERSION> does not load the F<feature.pm>, F<strict.pm>, F<warnings.pm> 10247or F<builtin.pm> files, but instead implements the equivalent functionality 10248directly. 10249 10250In the current implementation, any explicit use of C<no strict> overrides 10251C<use VERSION>, even if it comes before it. However, this may be subject to 10252change in a future release of Perl, so new code should not rely on this fact. 10253It is recommended that a C<use VERSION> declaration be the first significant 10254statement within a file (possibly after a C<package> statement or any amount 10255of whitespace or comment), so that its effects happen first, and other pragmata 10256are applied after it. 10257 10258Specifying VERSION as a numeric argument of the form 5.024001 should 10259generally be avoided as older less readable syntax compared to 10260v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric 10261form was the only supported syntax, which is why you might see it in 10262older code. 10263 10264 use v5.24.1; # compile time version check 10265 use 5.24.1; # ditto 10266 use 5.024_001; # ditto; older syntax compatible with perl 5.6 10267 10268This is often useful if you need to check the current Perl version before 10269L<C<use>|/use Module VERSION LIST>ing library modules that won't work 10270with older versions of Perl. 10271(We try not to do this more than we have to.) 10272 10273Symmetrically, C<no VERSION> allows you to specify that you want a version 10274of Perl older than the specified one. Historically this was added during 10275early designs of the Raku language (formerly "Perl 6"), so that a Perl 5 10276program could begin 10277 10278 no 6; 10279 10280to declare that it is not a Perl 6 program. As the two languages have 10281different implementations, file naming conventions, and other 10282infrastructure, this feature is now little used in practice and should be 10283avoided in newly-written code. 10284 10285Care should be taken when using the C<no VERSION> form, as it is I<only> 10286meant to be used to assert that the running Perl is of a earlier version 10287than its argument and I<not> to undo the feature-enabling side effects 10288of C<use VERSION>. 10289 10290=item utime LIST 10291X<utime> 10292 10293=for Pod::Functions set a file's last access and modify times 10294 10295Changes the access and modification times on each file of a list of 10296files. The first two elements of the list must be the NUMERIC access 10297and modification times, in that order. Returns the number of files 10298successfully changed. The inode change time of each file is set 10299to the current time. For example, this code has the same effect as the 10300Unix L<touch(1)> command when the files I<already exist> and belong to 10301the user running the program: 10302 10303 #!/usr/bin/perl 10304 my $atime = my $mtime = time; 10305 utime $atime, $mtime, @ARGV; 10306 10307Since Perl 5.8.0, if the first two elements of the list are 10308L<C<undef>|/undef EXPR>, 10309the L<utime(2)> syscall from your C library is called with a null second 10310argument. On most systems, this will set the file's access and 10311modification times to the current time (i.e., equivalent to the example 10312above) and will work even on files you don't own provided you have write 10313permission: 10314 10315 for my $file (@ARGV) { 10316 utime(undef, undef, $file) 10317 || warn "Couldn't touch $file: $!"; 10318 } 10319 10320Under NFS this will use the time of the NFS server, not the time of 10321the local machine. If there is a time synchronization problem, the 10322NFS server and local machine will have different times. The Unix 10323L<touch(1)> command will in fact normally use this form instead of the 10324one shown in the first example. 10325 10326Passing only one of the first two elements as L<C<undef>|/undef EXPR> is 10327equivalent to passing a 0 and will not have the effect described when 10328both are L<C<undef>|/undef EXPR>. This also triggers an 10329uninitialized warning. 10330 10331On systems that support L<futimes(2)>, you may pass filehandles among the 10332files. On systems that don't support L<futimes(2)>, passing filehandles raises 10333an exception. Filehandles must be passed as globs or glob references to be 10334recognized; barewords are considered filenames. 10335 10336Portability issues: L<perlport/utime>. 10337 10338=item values HASH 10339X<values> 10340 10341=item values ARRAY 10342 10343=for Pod::Functions return a list of the values in a hash 10344 10345In list context, returns a list consisting of all the values of the named 10346hash. In Perl 5.12 or later only, will also return a list of the values of 10347an array; prior to that release, attempting to use an array argument will 10348produce a syntax error. In scalar context, returns the number of values. 10349 10350Hash entries are returned in an apparently random order. The actual random 10351order is specific to a given hash; the exact same series of operations 10352on two hashes may result in a different order for each hash. Any insertion 10353into the hash may change the order, as will any deletion, with the exception 10354that the most recent key returned by L<C<each>|/each HASH> or 10355L<C<keys>|/keys HASH> may be deleted without changing the order. So 10356long as a given hash is unmodified you may rely on 10357L<C<keys>|/keys HASH>, L<C<values>|/values HASH> and 10358L<C<each>|/each HASH> to repeatedly return the same order 10359as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for 10360details on why hash order is randomized. Aside from the guarantees 10361provided here the exact details of Perl's hash algorithm and the hash 10362traversal order are subject to change in any release of Perl. Tied hashes 10363may behave differently to Perl's hashes with respect to changes in order on 10364insertion and deletion of items. 10365 10366As a side effect, calling L<C<values>|/values HASH> resets the HASH or 10367ARRAY's internal iterator (see L<C<each>|/each HASH>) before yielding the 10368values. In particular, 10369calling L<C<values>|/values HASH> in void context resets the iterator 10370with no other overhead. 10371 10372Apart from resetting the iterator, 10373C<values @array> in list context is the same as plain C<@array>. 10374(We recommend that you use void context C<keys @array> for this, but 10375reasoned that taking C<values @array> out would require more 10376documentation than leaving it in.) 10377 10378Note that the values are not copied, which means modifying them will 10379modify the contents of the hash: 10380 10381 for (values %hash) { s/foo/bar/g } # modifies %hash values 10382 for (@hash{keys %hash}) { s/foo/bar/g } # same 10383 10384Starting with Perl 5.14, an experimental feature allowed 10385L<C<values>|/values HASH> to take a 10386scalar expression. This experiment has been deemed unsuccessful, and was 10387removed as of Perl 5.24. 10388 10389To avoid confusing would-be users of your code who are running earlier 10390versions of Perl with mysterious syntax errors, put this sort of thing at 10391the top of your file to signal that your code will work I<only> on Perls of 10392a recent vintage: 10393 10394 use v5.12; # so keys/values/each work on arrays 10395 10396See also L<C<keys>|/keys HASH>, L<C<each>|/each HASH>, and 10397L<C<sort>|/sort SUBNAME LIST>. 10398 10399=item vec EXPR,OFFSET,BITS 10400X<vec> X<bit> X<bit vector> 10401 10402=for Pod::Functions test or set particular bits in a string 10403 10404Treats the string in EXPR as a bit vector made up of elements of 10405width BITS and returns the value of the element specified by OFFSET 10406as an unsigned integer. BITS therefore specifies the number of bits 10407that are reserved for each element in the bit vector. This must 10408be a power of two from 1 to 32 (or 64, if your platform supports 10409that). 10410 10411If BITS is 8, "elements" coincide with bytes of the input string. 10412 10413If BITS is 16 or more, bytes of the input string are grouped into chunks 10414of size BITS/8, and each group is converted to a number as with 10415L<C<pack>|/pack TEMPLATE,LIST>/L<C<unpack>|/unpack TEMPLATE,EXPR> with 10416big-endian formats C<n>/C<N> (and analogously for BITS==64). See 10417L<C<pack>|/pack TEMPLATE,LIST> for details. 10418 10419If bits is 4 or less, the string is broken into bytes, then the bits 10420of each byte are broken into 8/BITS groups. Bits of a byte are 10421numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, 10422C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, 10423breaking the single input byte C<chr(0x36)> into two groups gives a list 10424C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. 10425 10426L<C<vec>|/vec EXPR,OFFSET,BITS> may also be assigned to, in which case 10427parentheses are needed 10428to give the expression the correct precedence as in 10429 10430 vec($image, $max_x * $x + $y, 8) = 3; 10431 10432If the selected element is outside the string, the value 0 is returned. 10433If an element off the end of the string is written to, Perl will first 10434extend the string with sufficiently many zero bytes. It is an error 10435to try to write off the beginning of the string (i.e., negative OFFSET). 10436 10437If the string happens to be encoded as UTF-8 internally (and thus has 10438the UTF8 flag set), L<C<vec>|/vec EXPR,OFFSET,BITS> tries to convert it 10439to use a one-byte-per-character internal representation. However, if the 10440string contains characters with values of 256 or higher, a fatal error 10441will occur. 10442 10443Strings created with L<C<vec>|/vec EXPR,OFFSET,BITS> can also be 10444manipulated with the logical 10445operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit 10446vector operation is desired when both operands are strings. 10447See L<perlop/"Bitwise String Operators">. 10448 10449The following code will build up an ASCII string saying C<'PerlPerlPerl'>. 10450The comments show the string after each step. Note that this code works 10451in the same way on big-endian or little-endian machines. 10452 10453 my $foo = ''; 10454 vec($foo, 0, 32) = 0x5065726C; # 'Perl' 10455 10456 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits 10457 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') 10458 10459 vec($foo, 2, 16) = 0x5065; # 'PerlPe' 10460 vec($foo, 3, 16) = 0x726C; # 'PerlPerl' 10461 vec($foo, 8, 8) = 0x50; # 'PerlPerlP' 10462 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' 10463 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" 10464 vec($foo, 21, 4) = 7; # 'PerlPerlPer' 10465 # 'r' is "\x72" 10466 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" 10467 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" 10468 vec($foo, 94, 1) = 1; # 'PerlPerlPerl' 10469 # 'l' is "\x6c" 10470 10471To transform a bit vector into a string or list of 0's and 1's, use these: 10472 10473 my $bits = unpack("b*", $vector); 10474 my @bits = split(//, unpack("b*", $vector)); 10475 10476If you know the exact length in bits, it can be used in place of the C<*>. 10477 10478Here is an example to illustrate how the bits actually fall in place: 10479 10480 #!/usr/bin/perl -wl 10481 10482 print <<'EOT'; 10483 0 1 2 3 10484 unpack("V",$_) 01234567890123456789012345678901 10485 ------------------------------------------------------------------ 10486 EOT 10487 10488 for $w (0..3) { 10489 $width = 2**$w; 10490 for ($shift=0; $shift < $width; ++$shift) { 10491 for ($off=0; $off < 32/$width; ++$off) { 10492 $str = pack("B*", "0"x32); 10493 $bits = (1<<$shift); 10494 vec($str, $off, $width) = $bits; 10495 $res = unpack("b*",$str); 10496 $val = unpack("V", $str); 10497 write; 10498 } 10499 } 10500 } 10501 10502 format STDOUT = 10503 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 10504 $off, $width, $bits, $val, $res 10505 . 10506 __END__ 10507 10508Regardless of the machine architecture on which it runs, the 10509example above should print the following table: 10510 10511 0 1 2 3 10512 unpack("V",$_) 01234567890123456789012345678901 10513 ------------------------------------------------------------------ 10514 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 10515 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 10516 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 10517 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 10518 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 10519 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 10520 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 10521 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 10522 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 10523 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 10524 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 10525 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 10526 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 10527 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 10528 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 10529 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 10530 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 10531 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 10532 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 10533 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 10534 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 10535 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 10536 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 10537 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 10538 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 10539 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 10540 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 10541 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 10542 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 10543 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 10544 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 10545 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 10546 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 10547 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 10548 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 10549 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 10550 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 10551 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 10552 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 10553 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 10554 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 10555 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 10556 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 10557 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 10558 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 10559 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 10560 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 10561 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 10562 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 10563 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 10564 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 10565 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 10566 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 10567 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 10568 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 10569 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 10570 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 10571 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 10572 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 10573 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 10574 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 10575 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 10576 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 10577 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 10578 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 10579 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 10580 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 10581 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 10582 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 10583 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 10584 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 10585 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 10586 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 10587 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 10588 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 10589 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 10590 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 10591 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 10592 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 10593 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 10594 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 10595 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 10596 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 10597 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 10598 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 10599 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 10600 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 10601 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 10602 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 10603 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 10604 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 10605 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 10606 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 10607 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 10608 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 10609 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 10610 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 10611 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 10612 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 10613 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 10614 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 10615 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 10616 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 10617 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 10618 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 10619 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 10620 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 10621 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 10622 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 10623 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 10624 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 10625 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 10626 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 10627 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 10628 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 10629 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 10630 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 10631 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 10632 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 10633 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 10634 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 10635 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 10636 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 10637 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 10638 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 10639 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 10640 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 10641 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 10642 10643=item wait 10644X<wait> 10645 10646=for Pod::Functions wait for any child process to die 10647 10648Behaves like L<wait(2)> on your system: it waits for a child 10649process to terminate and returns the pid of the deceased process, or 10650C<-1> if there are no child processes. The status is returned in 10651L<C<$?>|perlvar/$?> and 10652L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>. 10653Note that a return value of C<-1> could mean that child processes are 10654being automatically reaped, as described in L<perlipc>. 10655 10656If you use L<C<wait>|/wait> in your handler for 10657L<C<$SIG{CHLD}>|perlvar/%SIG>, it may accidentally wait for the child 10658created by L<C<qx>|/qxE<sol>STRINGE<sol>> or L<C<system>|/system LIST>. 10659See L<perlipc> for details. 10660 10661Equivalent to L<C<waitpid(-1, 0)>|/waitpid PID,FLAGS>. 10662 10663Portability issues: L<perlport/wait>. 10664 10665=item waitpid PID,FLAGS 10666X<waitpid> 10667 10668=for Pod::Functions wait for a particular child process to die 10669 10670Waits for a particular child process to terminate and returns the pid of 10671the deceased process, or C<-1> if there is no such child process. A 10672non-blocking wait (with L<WNOHANG|POSIX/C<WNOHANG>> in FLAGS) can return 0 if 10673there are child processes matching PID but none have terminated yet. 10674The status is returned in L<C<$?>|perlvar/$?> and 10675L<C<${^CHILD_ERROR_NATIVE}>|perlvar/${^CHILD_ERROR_NATIVE}>. 10676 10677A PID of C<0> indicates to wait for any child process whose process group ID is 10678equal to that of the current process. A PID of less than C<-1> indicates to 10679wait for any child process whose process group ID is equal to -PID. A PID of 10680C<-1> indicates to wait for any child process. 10681 10682If you say 10683 10684 use POSIX ":sys_wait_h"; 10685 10686 my $kid; 10687 do { 10688 $kid = waitpid(-1, WNOHANG); 10689 } while $kid > 0; 10690 10691or 10692 10693 1 while waitpid(-1, WNOHANG) > 0; 10694 10695then you can do a non-blocking wait for all pending zombie processes (see 10696L<POSIX/WAIT>). 10697Non-blocking wait is available on machines supporting either the 10698L<waitpid(2)> or L<wait4(2)> syscalls. However, waiting for a particular 10699pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the 10700system call by remembering the status values of processes that have 10701exited but have not been harvested by the Perl script yet.) 10702 10703Note that on some systems, a return value of C<-1> could mean that child 10704processes are being automatically reaped. See L<perlipc> for details, 10705and for other examples. 10706 10707Portability issues: L<perlport/waitpid>. 10708 10709=item wantarray 10710X<wantarray> X<context> 10711 10712=for Pod::Functions get void vs scalar vs list context of current subroutine call 10713 10714Returns true if the context of the currently executing subroutine or 10715L<C<eval>|/eval EXPR> is looking for a list value. Returns false if the 10716context is 10717looking for a scalar. Returns the undefined value if the context is 10718looking for no value (void context). 10719 10720 return unless defined wantarray; # don't bother doing more 10721 my @a = complex_calculation(); 10722 return wantarray ? @a : "@a"; 10723 10724L<C<wantarray>|/wantarray>'s result is unspecified in the top level of a file, 10725in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or 10726in a C<DESTROY> method. 10727 10728This function should have been named wantlist() instead. 10729 10730=item warn LIST 10731X<warn> X<warning> X<STDERR> 10732 10733=for Pod::Functions print debugging info 10734 10735Emits a warning, usually by printing it to C<STDERR>. C<warn> interprets 10736its operand LIST in the same way as C<die>, but is slightly different 10737in what it defaults to when LIST is empty or makes an empty string. 10738If it is empty and L<C<$@>|perlvar/$@> already contains an exception 10739value then that value is used after appending C<"\t...caught">. If it 10740is empty and C<$@> is also empty then the string C<"Warning: Something's 10741wrong"> is used. 10742 10743By default, the exception derived from the operand LIST is stringified 10744and printed to C<STDERR>. This behaviour can be altered by installing 10745a L<C<$SIG{__WARN__}>|perlvar/%SIG> handler. If there is such a 10746handler then no message is automatically printed; it is the handler's 10747responsibility to deal with the exception 10748as it sees fit (like, for instance, converting it into a 10749L<C<die>|/die LIST>). Most 10750handlers must therefore arrange to actually display the 10751warnings that they are not prepared to deal with, by calling 10752L<C<warn>|/warn LIST> 10753again in the handler. Note that this is quite safe and will not 10754produce an endless loop, since C<__WARN__> hooks are not called from 10755inside one. 10756 10757You will find this behavior is slightly different from that of 10758L<C<$SIG{__DIE__}>|perlvar/%SIG> handlers (which don't suppress the 10759error text, but can instead call L<C<die>|/die LIST> again to change 10760it). 10761 10762Using a C<__WARN__> handler provides a powerful way to silence all 10763warnings (even the so-called mandatory ones). An example: 10764 10765 # wipe out *all* compile-time warnings 10766 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } 10767 my $foo = 10; 10768 my $foo = 20; # no warning about duplicate my $foo, 10769 # but hey, you asked for it! 10770 # no compile-time or run-time warnings before here 10771 $DOWARN = 1; 10772 10773 # run-time warnings enabled after here 10774 warn "\$foo is alive and $foo!"; # does show up 10775 10776See L<perlvar> for details on setting L<C<%SIG>|perlvar/%SIG> entries 10777and for more 10778examples. See the L<Carp> module for other kinds of warnings using its 10779C<carp> and C<cluck> functions. 10780 10781=item write FILEHANDLE 10782X<write> 10783 10784=item write EXPR 10785 10786=item write 10787 10788=for Pod::Functions print a picture record 10789 10790Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, 10791using the format associated with that file. By default the format for 10792a file is the one having the same name as the filehandle, but the 10793format for the current output channel (see the 10794L<C<select>|/select FILEHANDLE> function) may be set explicitly by 10795assigning the name of the format to the L<C<$~>|perlvar/$~> variable. 10796 10797Top of form processing is handled automatically: if there is insufficient 10798room on the current page for the formatted record, the page is advanced by 10799writing a form feed and a special top-of-page 10800format is used to format the new 10801page header before the record is written. By default, the top-of-page 10802format is the name of the filehandle with C<_TOP> appended, or C<top> 10803in the current package if the former does not exist. This would be a 10804problem with autovivified filehandles, but it may be dynamically set to the 10805format of your choice by assigning the name to the L<C<$^>|perlvar/$^> 10806variable while that filehandle is selected. The number of lines 10807remaining on the current page is in variable L<C<$->|perlvar/$->, which 10808can be set to C<0> to force a new page. 10809 10810If FILEHANDLE is unspecified, output goes to the current default output 10811channel, which starts out as STDOUT but may be changed by the 10812L<C<select>|/select FILEHANDLE> operator. If the FILEHANDLE is an EXPR, 10813then the expression 10814is evaluated and the resulting string is used to look up the name of 10815the FILEHANDLE at run time. For more on formats, see L<perlform>. 10816 10817Note that write is I<not> the opposite of 10818L<C<read>|/read FILEHANDLE,SCALAR,LENGTH,OFFSET>. Unfortunately. 10819 10820=item y/// 10821 10822=for Pod::Functions transliterate a string 10823 10824The transliteration operator. Same as 10825L<C<trE<sol>E<sol>E<sol>>|/trE<sol>E<sol>E<sol>>. See 10826L<perlop/"Quote-Like Operators">. 10827 10828=back 10829 10830=head2 Non-function Keywords by Cross-reference 10831 10832=head3 perldata 10833 10834=over 10835 10836=item __DATA__ 10837 10838=item __END__ 10839 10840These keywords are documented in L<perldata/"Special Literals">. 10841 10842=back 10843 10844=head3 perlmod 10845 10846=over 10847 10848=item BEGIN 10849 10850=item CHECK 10851 10852=item END 10853 10854=item INIT 10855 10856=item UNITCHECK 10857 10858These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">. 10859 10860=back 10861 10862=head3 perlobj 10863 10864=over 10865 10866=item DESTROY 10867 10868This method keyword is documented in L<perlobj/"Destructors">. 10869 10870=back 10871 10872=head3 perlop 10873 10874=over 10875 10876=item and 10877 10878=item cmp 10879 10880=item eq 10881 10882=item ge 10883 10884=item gt 10885 10886=item isa 10887 10888=item le 10889 10890=item lt 10891 10892=item ne 10893 10894=item not 10895 10896=item or 10897 10898=item x 10899 10900=item xor 10901 10902These operators are documented in L<perlop>. 10903 10904=back 10905 10906=head3 perlsub 10907 10908=over 10909 10910=item AUTOLOAD 10911 10912This keyword is documented in L<perlsub/"Autoloading">. 10913 10914=back 10915 10916=head3 perlsyn 10917 10918=over 10919 10920=item else 10921 10922=item elsif 10923 10924=item for 10925 10926=item foreach 10927 10928=item if 10929 10930=item unless 10931 10932=item until 10933 10934=item while 10935 10936These flow-control keywords are documented in L<perlsyn/"Compound Statements">. 10937 10938=item elseif 10939 10940The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif> 10941or C<else if> either. It does parse C<elseif>, but only to warn you 10942about not using it. 10943 10944See the documentation for flow-control keywords in L<perlsyn/"Compound 10945Statements">. 10946 10947=back 10948 10949=over 10950 10951=item default 10952 10953=item given 10954 10955=item when 10956 10957These flow-control keywords related to the experimental switch feature are 10958documented in L<perlsyn/"Switch Statements">. 10959 10960=back 10961 10962=over 10963 10964=item try 10965 10966=item catch 10967 10968=item finally 10969 10970These flow-control keywords related to the experimental C<try> feature are 10971documented in L<perlsyn/"Try Catch Exception Handling">. 10972 10973=back 10974 10975=over 10976 10977=item defer 10978 10979This flow-control keyword related to the experimental C<defer> feature is 10980documented in L<perlsyn/"defer blocks">. 10981 10982=back 10983 10984=over 10985 10986=item ADJUST 10987 10988This class-related phaser block is documented in L<perlclass>. 10989 10990=back 10991 10992=cut 10993