1.\" $OpenBSD: signal.3,v 1.34 2007/05/31 19:19:28 jmc Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.Dd $Mdocdate: May 31 2007 $ 31.Dt SIGNAL 3 32.Os 33.Sh NAME 34.Nm signal 35.Nd simplified software signal facilities 36.Sh SYNOPSIS 37.Fd #include <signal.h> 38.\" The following is Quite Ugly, but syntactically correct. 39.\" Don't try to fix it. 40.Ft void 41.Fn \*(lp*signal "int sigcatch" "void \*(lp*func\*(rp\*(lpint sigraised\*(rp\*(rp\*(rp\*(lpint" 42.Ft void 43.Fn \*(lp*bsd_signal "int sigcatch" "void \*(lp*func\*(rp\*(lpint sigraised\*(rp\*(rp\*(rp\*(lpint" 44.Sh DESCRIPTION 45The 46.Fn signal 47and 48.Fn bsd_signal 49facilities are simplified interfaces to the more general 50.Xr sigaction 2 51facility. 52The 53.Fn bsd_signal 54interface is provided for source compatibility only. 55It is mainly used on systems where the standard 56.Fn signal 57does not have 58.Bx 59semantics. 60On 61.Ox 62the two interfaces are identical. 63.Pp 64Signals allow the manipulation of a process from outside its 65domain as well as allowing the process to manipulate itself or 66copies of itself (children). 67There are two general types of signals: 68those that cause termination of a process and those that do not. 69Signals which cause termination of a program might result from 70an irrecoverable error or might be the result of a user at a terminal 71typing the 72.Dq interrupt 73character. 74.Pp 75Signals are used when a process is stopped because it wishes to access 76its control terminal while in the background (see 77.Xr tty 4 ) . 78Signals are optionally generated 79when a process resumes after being stopped, 80when the status of child processes changes, 81or when input is ready at the control terminal. 82Most signals result in the termination of the process receiving them 83if no action 84is taken; some signals instead cause the process receiving them 85to be stopped, or are simply discarded if the process has not 86requested otherwise. 87.Pp 88Except for the 89.Dv SIGKILL 90and 91.Dv SIGSTOP 92signals, the 93.Fn signal 94function allows for any signal to be caught, to be ignored, or to generate 95an interrupt. 96These signals are defined in the file 97.Aq Pa signal.h : 98.Bl -column SIGVTALA "create core imag" 99.It Sy "Name Default Action Description" 100.It Dv SIGHUP No " terminate process" " terminal line hangup" 101.It Dv SIGINT No " terminate process" " interrupt program" 102.It Dv SIGQUIT No " create core image" " quit program" 103.It Dv SIGILL No " create core image" " illegal instruction" 104.It Dv SIGTRAP No " create core image" " trace trap" 105.It Dv SIGABRT No " create core image" Xr abort 3 106call (formerly 107.Dv SIGIOT ) 108.It Dv SIGEMT No " create core image" " emulate instruction executed" 109.It Dv SIGFPE No " create core image" " floating-point exception" 110.It Dv SIGKILL No " terminate process" " kill program" 111.It Dv SIGBUS No " create core image" " bus error" 112.It Dv SIGSEGV No " create core image" " segmentation violation" 113.It Dv SIGSYS No " create core image" " system call given invalid argument" 114.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader" 115.It Dv SIGALRM No " terminate process" " real-time timer expired" 116.It Dv SIGTERM No " terminate process" " software termination signal" 117.It Dv SIGURG No " discard signal" " urgent condition present on socket" 118.It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)" 119.It Dv SIGTSTP No " stop process" " stop signal generated from keyboard" 120.It Dv SIGCONT No " discard signal" " continue after stop" 121.It Dv SIGCHLD No " discard signal" " child status has changed" 122.It Dv SIGTTIN No " stop process" " background read attempted from control terminal" 123.It Dv SIGTTOU No " stop process" " background write attempted to control terminal" 124.It Dv SIGIO No " discard signal" Tn " I/O" 125is possible on a descriptor (see 126.Xr fcntl 2 ) 127.It Dv SIGXCPU No " terminate process" " CPU time limit exceeded (see" 128.Xr setrlimit 2 ) 129.It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see" 130.Xr setrlimit 2 ) 131.It Dv SIGVTALRM No " terminate process" " virtual time alarm (see" 132.Xr setitimer 2 ) 133.It Dv SIGPROF No " terminate process" " profiling timer alarm (see" 134.Xr setitimer 2 ) 135.It Dv SIGWINCH No " discard signal" " window size change" 136.It Dv SIGINFO No " discard signal" " status request from keyboard" 137.It Dv SIGUSR1 No " terminate process" " user-defined signal 1" 138.It Dv SIGUSR2 No " terminate process" " user-defined signal 2" 139.El 140.Pp 141The 142.Fa func 143argument is a function to be called as the action upon receipt of the signal 144.Fa sigcatch . 145The function will be called with one argument, 146.Fa sigraised , 147which is the signal raised (thus the same function, 148.Fa func , 149can be used by more than one signal). 150To set the default action of the signal to occur as listed above, 151.Fa func 152should be 153.Dv SIG_DFL . 154A 155.Dv SIG_DFL 156resets the default action. 157To ignore the signal, 158.Fa func 159should be 160.Dv SIG_IGN . 161This will cause subsequent instances of the signal to be ignored 162and pending instances to be discarded. 163If 164.Dv SIG_IGN 165is not used, 166further occurrences of the signal are 167automatically blocked and 168.Fa func 169is called. 170.Pp 171If the 172.Fa func 173is set to 174.Dv SIG_IGN 175for the 176.Dv SIGCHLD 177signal, the system will not create zombie processes when children of 178the calling process exit. 179If the calling process subsequently issues a 180.Xr wait 2 181(or equivalent), it blocks until all of the calling process's child 182processes terminate, and then returns a value of \-1 with 183.Va errno 184set to 185.Dv ECHILD . 186.Bf -symbolic 187This differs from historical 188.Bx 189behavior but is consistent with 190.At V 191as well as the 192.St -xpg4.2 . 193.Ef 194.Pp 195The handled signal is unblocked when 196.Fa func 197returns and 198the process continues from where it left off when the signal occurred. 199.Bf -symbolic 200Unlike previous signal facilities, the handler 201func() remains installed after a signal has been delivered. 202.Ef 203.Pp 204For some system calls, if a signal is caught while the call is 205executing and the call is prematurely terminated, 206the call is automatically restarted. 207(The handler is installed using the 208.Dv SA_RESTART 209flag with 210.Xr sigaction 2 . ) 211The affected system calls include 212.Xr read 2 , 213.Xr write 2 , 214.Xr sendto 2 , 215.Xr recvfrom 2 , 216.Xr sendmsg 2 , 217and 218.Xr recvmsg 2 219on a communications channel or a low-speed device 220and during a 221.Xr ioctl 2 222or 223.Xr wait 2 . 224However, calls that have already committed are not restarted, 225but instead return a partial success (for example, a short read count). 226The 227.Xr siginterrupt 3 228function can be used to change the system call restart behavior for 229a specific signal. 230.Pp 231When a process which has installed signal handlers forks, 232the child process inherits the signals. 233All caught signals may be reset to their default action by a call 234to the 235.Xr execve 2 236function; 237ignored signals remain ignored. 238.Pp 239The following functions are either reentrant or not interruptible 240by signals and are asynchronous-signal safe. 241Therefore applications may 242invoke them, without restriction, from signal-catching functions: 243.Pp 244Base Interfaces: 245.Pp 246.Fn _exit , 247.Fn access , 248.Fn alarm , 249.Fn cfgetispeed , 250.Fn cfgetospeed , 251.Fn cfsetispeed , 252.Fn cfsetospeed , 253.Fn chdir , 254.Fn chmod , 255.Fn chown , 256.Fn close , 257.Fn creat , 258.Fn dup , 259.Fn dup2 , 260.Fn execle , 261.Fn execve , 262.Fn fcntl , 263.Fn fork , 264.Fn fpathconf , 265.Fn fstat , 266.Fn fsync , 267.Fn getegid , 268.Fn geteuid , 269.Fn getgid , 270.Fn getgroups , 271.Fn getpgrp , 272.Fn getpid , 273.Fn getppid , 274.Fn getuid , 275.Fn kill , 276.Fn link , 277.Fn lseek , 278.Fn mkdir , 279.Fn mkfifo , 280.Fn open , 281.Fn pathconf , 282.Fn pause , 283.Fn pipe , 284.Fn raise , 285.Fn read , 286.Fn rename , 287.Fn rmdir , 288.Fn setgid , 289.Fn setpgid , 290.Fn setsid , 291.Fn setuid , 292.Fn sigaction , 293.Fn sigaddset , 294.Fn sigdelset , 295.Fn sigemptyset , 296.Fn sigfillset , 297.Fn sigismember , 298.Fn signal , 299.Fn sigpending , 300.Fn sigprocmask , 301.Fn sigsuspend , 302.Fn sleep , 303.Fn stat , 304.Fn sysconf , 305.Fn tcdrain , 306.Fn tcflow , 307.Fn tcflush , 308.Fn tcgetattr , 309.Fn tcgetpgrp , 310.Fn tcsendbreak , 311.Fn tcsetattr , 312.Fn tcsetpgrp , 313.Fn time , 314.Fn times , 315.Fn umask , 316.Fn uname , 317.Fn unlink , 318.Fn utime , 319.Fn wait , 320.Fn waitpid , 321.Fn write . 322.Pp 323Realtime Interfaces: 324.Pp 325.Fn aio_error , 326.Fn clock_gettime , 327.Fn sigpause , 328.Fn timer_getoverrun , 329.Fn aio_return , 330.Fn fdatasync , 331.Fn sigqueue , 332.Fn timer_gettime , 333.Fn aio_suspend , 334.Fn sem_post , 335.Fn sigset , 336.Fn timer_settime . 337.Pp 338ANSI C Interfaces: 339.Pp 340.Fn strcpy , 341.Fn strcat , 342.Fn strncpy , 343.Fn strncat , 344and perhaps some others. 345.Pp 346Extension Interfaces: 347.Pp 348.Fn strlcpy , 349.Fn strlcat . 350.Pp 351Most functions not in the above lists are considered to be unsafe 352with respect to signals. 353That is to say, the behaviour of such functions when called from a 354signal handler is undefined. 355In general though, signal handlers should do little more than set a 356flag; most other actions are not safe. 357.Pp 358Additionally, inside the signal handler it is also considered more safe to 359make a copy of the global variable 360.Va errno 361and restore it before returning from the signal handler. 362.Pp 363A few other functions are signal race safe in 364.Ox 365but probably not on other systems: 366.Pp 367.Bl -tag -offset indent -compact -width foofoofoofoo 368.It Fn snprintf 369Safe. 370.It Fn vsnprintf 371Safe. 372.It Fn syslog_r 373Safe if the 374.Va syslog_data 375struct is initialized as a local variable. 376.El 377.Sh RETURN VALUES 378The previous action is returned on a successful call. 379Otherwise, 380.Dv SIG_ERR 381is returned and the global variable 382.Va errno 383is set to indicate the error. 384.Sh ERRORS 385.Fn signal 386will fail and no action will take place if one of the 387following occurs: 388.Bl -tag -width Er 389.It Bq Er EINVAL 390A specified signal 391is not a valid signal number. 392.It Bq Er EINVAL 393An attempt is made to ignore or supply a handler for 394.Dv SIGKILL 395or 396.Dv SIGSTOP . 397.El 398.Sh SEE ALSO 399.Xr kill 1 , 400.Xr kill 2 , 401.Xr ptrace 2 , 402.Xr sigaction 2 , 403.Xr sigaltstack 2 , 404.Xr sigprocmask 2 , 405.Xr sigsuspend 2 , 406.Xr setjmp 3 , 407.Xr siginterrupt 3 , 408.Xr tty 4 409.Sh HISTORY 410This 411.Fn signal 412facility appeared in 413.Bx 4.0 . 414