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Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)intro.2 8.5 (Berkeley) 2/27/95 35.\" 36.Dd June 7, 1999 37.Dt INTRO 2 38.Os 39.Sh NAME 40.Nm intro , 41.Nm errno 42.Nd introduction to system calls and error numbers 43.Sh SYNOPSIS 44.Fd #include <errno.h> 45.Sh DESCRIPTION 46This section provides an overview of the system calls, 47their error returns, and other common definitions and concepts. 48.\".Pp 49.\".Sy System call restart 50.\".Pp 51.\"<more later...> 52.Sh DIAGNOSTICS 53Nearly all of the system calls provide an error number in the external 54variable 55.Va errno , 56which is defined as: 57.Pp 58.Dl extern int errno; 59.Pp 60When a system call detects an error, 61it returns an integer value 62indicating failure (usually -1) 63and sets the variable 64.Va errno 65accordingly. 66<This allows interpretation of the failure on receiving 67a -1 and to take action accordingly.> 68Successful calls never set 69.Va errno ; 70once set, it remains until another error occurs. 71It should only be examined after an error. 72Note that a number of system calls overload the meanings of these 73error numbers, and that the meanings must be interpreted according 74to the type and circumstances of the call. 75.Pp 76The following is a complete list of the errors and their 77names as given in 78.Aq Pa errno.h . 79.Bl -hang -width Ds 80.It Er 0 Em "Error 0" . 81Not used. 82.It Er 1 EPERM Em "Operation not permitted" . 83An attempt was made to perform an operation limited to processes 84with appropriate privileges or to the owner of a file or other 85resources. 86.It Er 2 ENOENT Em "No such file or directory" . 87A component of a specified pathname did not exist, or the 88pathname was an empty string. 89.It Er 3 ESRCH Em "No such process" . 90No process could be found corresponding to that specified by the given 91process ID. 92.It Er 4 EINTR Em "Interrupted function call" . 93An asynchronous signal (such as 94.Dv SIGINT 95or 96.Dv SIGQUIT ) 97was caught by the process during the execution of an interruptible 98function. If the signal handler performs a normal return, the 99interrupted function call will seem to have returned the error condition. 100.It Er 5 EIO Em "Input/output error" . 101Some physical input or output error occurred. 102This error will not be reported until a subsequent operation on the same file 103descriptor and may be lost (over written) by any subsequent errors. 104.It Er 6 ENXIO Em "\&No such device or address" . 105Input or output on a special file referred to a device that did not 106exist, or 107made a request beyond the limits of the device. 108This error may also occur when, for example, 109a tape drive is not online or no disk pack is 110loaded on a drive. 111.It Er 7 E2BIG Em "Arg list too long" . 112The number of bytes used for the argument and environment 113list of the new process exceeded the current limit of 114.if t 2\u\s-218\s+2\d 115.if n 2**18 116bytes 117.Pf ( Dv ARG_MAX 118in 119.Aq Pa sys/syslimits.h ) . 120.It Er 8 ENOEXEC Em "Exec format error" . 121A request was made to execute a file 122that, although it has the appropriate permissions, 123was not in the format required for an 124executable file. 125.It Er 9 EBADF Em "Bad file descriptor" . 126A file descriptor argument was out of range, referred to no open file, 127had been revoked by 128.Xr revoke 2 , 129or a 130.Xr read 2 131(or 132.Xr write 2 ) 133request was made to a file that was 134only open for writing (or reading). 135.It Er 10 ECHILD Em "\&No child processes" . 136A 137.Xr wait 2 138or 139.Xr waitpid 2 140function was executed by a process that had no existing or unwaited-for 141child processes. 142.It Er 11 EDEADLK Em "Resource deadlock avoided" . 143An attempt was made to lock a system resource that 144would have resulted in a deadlock situation. 145.It Er 12 ENOMEM Em "Cannot allocate memory" . 146The new process image required more memory than was allowed by the hardware 147or by system-imposed memory management constraints. 148A lack of swap space is normally temporary; however, 149a lack of core is not. 150Soft limits may be increased to their corresponding hard limits. 151.It Er 13 EACCES Em "Permission denied" . 152An attempt was made to access a file in a way forbidden 153by its file access permissions. 154.It Er 14 EFAULT Em "Bad address" . 155The system detected an invalid address in attempting to 156use an argument of a call. 157The reliable detection of this error cannot be guaranteed and when not detected 158may result in the generation of a signal, indicating an address violation, 159which is sent to the process. 160.It Er 15 ENOTBLK Em "Not a block device" . 161A block device operation was attempted on a non-block device or file. 162.It Er 16 EBUSY Em "Resource busy" . 163An attempt to use a system resource which was in use at the time 164in a manner which would have conflicted with the request. 165.It Er 17 EEXIST Em "File exists" . 166An existing file was mentioned in an inappropriate context, 167for instance, as the new link name in a 168.Xr link 2 169function. 170.It Er 18 EXDEV Em "Improper link" . 171A hard link to a file on another file system 172was attempted. 173.It Er 19 ENODEV Em "Operation not supported by device" . 174An attempt was made to apply an inappropriate 175function to a device, 176for example, 177trying to read a write-only device such as a printer. 178.It Er 20 ENOTDIR Em "Not a directory" . 179A component of the specified pathname existed, but it was 180not a directory, when a directory was expected. 181.It Er 21 EISDIR Em "Is a directory" . 182An attempt was made to open a directory with write mode specified. 183.It Er 22 EINVAL Em "Invalid argument" . 184Some invalid argument was supplied. (For example, 185specifying an undefined signal to a 186.Xr signal 3 187or 188.Xr kill 2 189function). 190.It Er 23 ENFILE Em "Too many open files in system" . 191Maximum number of file descriptors allowable on the system 192has been reached and a requests for an open cannot be satisfied 193until at least one has been closed. 194.It Er 24 EMFILE Em "Too many open files" . 195<As released, the limit on the number of 196open files per process is 64.> 197The 198.Xr getrlimit 3 199call with the 200.Ar RLIMIT_NOFILE 201resource will obtain the current limit. 202.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 203A control function (see 204.Xr ioctl 2 ) 205was attempted for a file or 206special device for which the operation was inappropriate. 207.It Er 26 ETXTBSY Em "Text file busy" . 208The new process was a pure procedure (shared text) file 209which was open for writing by another process, or 210while the pure procedure file was being executed an 211.Xr open 2 212call requested write access. 213.It Er 27 EFBIG Em "File too large" . 214The size of a file exceeded the maximum. (The system-wide maximum file size is 215.if t 2\u\s-263\s+2\d 216.if n 2**63 217bytes. Each file system may impose a lower limit for files contained within 218it.) 219.It Er 28 ENOSPC Em "Device out of space" . 220A 221.Xr write 2 222to an ordinary file, the creation of a 223directory or symbolic link, or the creation of a directory 224entry failed because no more disk blocks were available 225on the file system, or the allocation of an inode for a newly 226created file failed because no more inodes were available 227on the file system. 228.It Er 29 ESPIPE Em "Illegal seek" . 229An 230.Xr lseek 2 231function was issued on a socket, pipe or 232.Tn FIFO . 233.It Er 30 EROFS Em "Read-only file system" . 234An attempt was made to modify a file or directory 235was made 236on a file system that was read-only at the time. 237.It Er 31 EMLINK Em "Too many links" . 238The number of hard links to a single file has exceeded the maximum. (The 239system-wide maximum number of hard links is 32767. Each file system may 240impose a lower limit for files contained within it.) 241.It Er 32 EPIPE Em "Broken pipe" . 242A write on a pipe, socket or 243.Tn FIFO 244for which there is no process 245to read the data. 246.It Er 33 EDOM Em "Numerical argument out of domain" . 247A numerical input argument was outside the defined domain of the mathematical 248function. 249.It Er 34 ERANGE Em "Numerical result out of range" . 250A numerical result of the function was too large to fit in the 251available space (perhaps exceeded precision). 252.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 253This is a temporary condition and later calls to the 254same routine may complete normally. 255.It Er 36 EINPROGRESS Em "Operation now in progress" . 256An operation that takes a long time to complete (such as 257a 258.Xr connect 2 ) 259was attempted on a non-blocking object (see 260.Xr fcntl 2 ) . 261.It Er 37 EALREADY Em "Operation already in progress" . 262An operation was attempted on a non-blocking object that already 263had an operation in progress. 264.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 265Self-explanatory. 266.It Er 39 EDESTADDRREQ Em "Destination address required" . 267A required address was omitted from an operation on a socket. 268.It Er 40 EMSGSIZE Em "Message too long" . 269A message sent on a socket was larger than the internal message buffer 270or some other network limit. 271.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 272A protocol was specified that does not support the semantics of the 273socket type requested. For example, you cannot use the 274.Tn ARPA 275Internet 276.Tn UDP 277protocol with type 278.Dv SOCK_STREAM . 279.It Er 42 ENOPROTOOPT Em "Protocol not available" . 280A bad option or level was specified in a 281.Xr getsockopt 2 282or 283.Xr setsockopt 2 284call. 285.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 286The protocol has not been configured into the 287system or no implementation for it exists. 288.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 289The support for the socket type has not been configured into the 290system or no implementation for it exists. 291.It Er 45 EOPNOTSUPP Em "Operation not supported" . 292The attempted operation is not supported for the type of object referenced. 293Usually this occurs when a file descriptor refers to a file or socket 294that cannot support this operation, 295for example, trying to 296.Em accept 297a connection on a datagram socket. 298.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 299The protocol family has not been configured into the 300system or no implementation for it exists. 301.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 302An address incompatible with the requested protocol was used. 303For example, you shouldn't necessarily expect to be able to use 304.Tn NS 305addresses with 306.Tn ARPA 307Internet protocols. 308.It Er 48 EADDRINUSE Em "Address already in use" . 309Only one usage of each address is normally permitted. 310.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 311Normally results from an attempt to create a socket with an 312address not on this machine. 313.It Er 50 ENETDOWN Em "Network is down" . 314A socket operation encountered a dead network. 315.It Er 51 ENETUNREACH Em "Network is unreachable" . 316A socket operation was attempted to an unreachable network. 317.It Er 52 ENETRESET Em "Network dropped connection on reset" . 318The host you were connected to crashed and rebooted. 319.It Er 53 ECONNABORTED Em "Software caused connection abort" . 320A connection abort was caused internal to your host machine. 321.It Er 54 ECONNRESET Em "Connection reset by peer" . 322A connection was forcibly closed by a peer. This normally 323results from a loss of the connection on the remote socket 324due to a timeout or a reboot. 325.It Er 55 ENOBUFS Em "\&No buffer space available" . 326An operation on a socket or pipe was not performed because 327the system lacked sufficient buffer space or because a queue was full. 328.It Er 56 EISCONN Em "Socket is already connected" . 329A 330.Xr connect 2 331request was made on an already connected socket; or, 332a 333.Xr sendto 2 334or 335.Xr sendmsg 2 336request on a connected socket specified a destination 337when already connected. 338.It Er 57 ENOTCONN Em "Socket is not connected" . 339An request to send or receive data was disallowed because 340the socket was not connected and (when sending on a datagram socket) 341no address was supplied. 342.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 343A request to send data was disallowed because the socket 344had already been shut down with a previous 345.Xr shutdown 2 346call. 347.It Er 60 ETIMEDOUT Em "Operation timed out" . 348A 349.Xr connect 2 350or 351.Xr send 2 352request failed because the connected party did not 353properly respond after a period of time. (The timeout 354period is dependent on the communication protocol.) 355.It Er 61 ECONNREFUSED Em "Connection refused" . 356No connection could be made because the target machine actively 357refused it. This usually results from trying to connect 358to a service that is inactive on the foreign host. 359.It Er 62 ELOOP Em "Too many levels of symbolic links" . 360A path name lookup involved more than 8 symbolic links. 361.It Er 63 ENAMETOOLONG Em "File name too long" . 362A component of a path name exceeded 255 363.Pq Dv MAXNAMELEN 364characters, or an entire 365path name exceeded 1023 366.Pq Dv MAXPATHLEN Ns -1 367characters. 368.It Er 64 EHOSTDOWN Em "Host is down" . 369A socket operation failed because the destination host was down. 370.It Er 65 EHOSTUNREACH Em "No route to host" . 371A socket operation was attempted to an unreachable host. 372.It Er 66 ENOTEMPTY Em "Directory not empty" . 373A directory with entries other than 374.Ql \&. 375and 376.Ql \&.. 377was supplied to a remove directory or rename call. 378.It Er 67 EPROCLIM Em "Too many processes" . 379.It Er 68 EUSERS Em "Too many users" . 380The quota system ran out of table entries. 381.It Er 69 EDQUOT Em "Disc quota exceeded" . 382A 383.Xr write 2 384to an ordinary file, the creation of a 385directory or symbolic link, or the creation of a directory 386entry failed because the user's quota of disk blocks was 387exhausted, or the allocation of an inode for a newly 388created file failed because the user's quota of inodes 389was exhausted. 390.It Er 70 ESTALE Em "Stale NFS file handle" . 391An attempt was made to access an open file (on an 392.Tn NFS 393filesystem) 394which is now unavailable as referenced by the file descriptor. 395This may indicate the file was deleted on the 396.Tn NFS 397server or some 398other catastrophic event occurred. 399.It Er 72 EBADRPC Em "RPC struct is bad" . 400Exchange of 401.Tn RPC 402information was unsuccessful. 403.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 404The version of 405.Tn RPC 406on the remote peer is not compatible with 407the local version. 408.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 409The requested program is not registered on the remote host. 410.It Er 75 EPROGMISMATCH Em "Program version wrong" . 411The requested version of the program is not available 412on the remote host 413.Pq Tn RPC . 414.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 415An 416.Tn RPC 417call was attempted for a procedure which doesn't exist 418in the remote program. 419.It Er 77 ENOLCK Em "No locks available" . 420A system-imposed limit on the number of simultaneous file 421locks was reached. 422.It Er 78 ENOSYS Em "Function not implemented" . 423Attempted a system call that is not available on this 424system. 425.It Er 79 EFTYPE Em "Inappropriate file type or format" . 426Attempted a file operation on a file of a type for which it was invalid. 427.It Er 80 EAUTH Em "Authentication error" . 428Attempted to use an invalid authentication ticket to mount an 429.Tn NFS 430filesystem. 431.It Er 81 ENEEDAUTH Em "Need authenticator" . 432An authentication ticket must be obtained before the given 433.Tn NFS 434filesystem may be mounted. 435.It Er 82 EIDRM Em "Identifier removed" . 436An IPC identifier was removed while the current process was waiting on it. 437.It Er 83 ENOMSG Em "No message of the desired type" . 438An IPC message queue does not contain a message of the desired type, 439or a message catalog does not contain the requested message. 440.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 441A numerical result of the function was too large to be stored in the 442caller-provided space. 443.El 444.Sh DEFINITIONS 445.Bl -tag -width Ds 446.It Process ID . 447Each active process in the system is uniquely identified by a non-negative 448integer called a process ID. The range of this ID is from 0 to 30000. 449.It Parent process ID 450A new process is created by a currently active process; (see 451.Xr fork 2 ) . 452The parent process ID of a process is initially the process ID of its creator. 453If the creating process exits, 454the parent process ID of each child is set to the ID of a system process, 455.Xr init 8 . 456.It Process Group 457Each active process is a member of a process group that is identified by 458a non-negative integer called the process group ID. This is the process 459ID of the group leader. This grouping permits the signaling of related 460processes (see 461.Xr termios 4 ) 462and the job control mechanisms of 463.Xr csh 1 . 464.It Session 465A session is a set of one or more process groups. 466A session is created by a successful call to 467.Xr setsid 2 , 468which causes the caller to become the only member of the only process 469group in the new session. 470.It Session leader 471A process that has created a new session by a successful call to 472.Xr setsid 2 , 473is known as a session leader. 474Only a session leader may acquire a terminal as its controlling terminal (see 475.Xr termios 4 ) . 476.It Controlling process 477A session leader with a controlling terminal is a controlling process. 478.It Controlling terminal 479A terminal that is associated with a session is known as the controlling 480terminal for that session and its members. 481.It "Terminal Process Group ID" 482A terminal may be acquired by a session leader as its controlling terminal. 483Once a terminal is associated with a session, any of the process groups 484within the session may be placed into the foreground by setting 485the terminal process group ID to the ID of the process group. 486This facility is used 487to arbitrate between multiple jobs contending for the same terminal. 488(See 489.Xr csh 1 490and 491.Xr tty 4 492for more information on job control.) 493.It "Orphaned Process Group" 494A process group is considered to be 495.Em orphaned 496if it is not under the control of a job control shell. 497More precisely, a process group is orphaned 498when none of its members has a parent process that is in the same session 499as the group, 500but is in a different process group. 501Note that when a process exits, the parent process for its children 502is changed to be 503.Xr init 8 , 504which is in a separate session. 505Not all members of an orphaned process group are necessarily orphaned 506processes (those whose creating process has exited). 507The process group of a session leader is orphaned by definition. 508.It "Real User ID and Real Group ID" 509Each user on the system is identified by a positive integer 510termed the real user ID. 511.Pp 512Each user is also a member of one or more groups. 513One of these groups is distinguished from others and 514used in implementing accounting facilities. The positive 515integer corresponding to this distinguished group is termed 516the real group ID. 517.Pp 518All processes have a real user ID and real group ID. 519These are initialized from the equivalent attributes 520of the process that created it. 521.It "Effective User Id, Effective Group Id, and Group Access List" 522Access to system resources is governed by two values: 523the effective user ID, and the group access list. 524The first member of the group access list is also known as the 525effective group ID. 526(In POSIX.1, the group access list is known as the set of supplementary 527group IDs, and it is unspecified whether the effective group ID is 528a member of the list.) 529.Pp 530The effective user ID and effective group ID are initially the 531process's real user ID and real group ID respectively. Either 532may be modified through execution of a set-user-ID or set-group-ID 533file (possibly by one its ancestors) (see 534.Xr execve 2 ) . 535By convention, the effective group ID (the first member of the group access 536list) is duplicated, so that the execution of a set-group-ID program 537does not result in the loss of the original (real) group ID. 538.Pp 539The group access list is a set of group IDs 540used only in determining resource accessibility. Access checks 541are performed as described below in ``File Access Permissions''. 542.It "Saved Set User ID and Saved Set Group ID" 543When a process executes a new file, the effective user ID is set 544to the owner of the file if the file is set-user-ID, and the effective 545group ID (first element of the group access list) is set to the group 546of the file if the file is set-group-ID. 547The effective user ID of the process is then recorded as the saved set-user-ID, 548and the effective group ID of the process is recorded as the saved set-group-ID. 549These values may be used to regain those values as the effective user 550or group ID after reverting to the real ID (see 551.Xr setuid 2 ) . 552(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 553and are used in setuid and setgid, but this does not work as desired 554for the super-user.) 555.It Super-user 556A process is recognized as a 557.Em super-user 558process and is granted special privileges if its effective user ID is 0. 559.It Special Processes 560The processes with process IDs of 0, 1, 2 and 3 are special. 561Process 0 is the scheduler. Process 1 is the initialization process 562.Xr init 8 , 563and is the ancestor of every other process in the system. 564It is used to control the process structure. 565Process 2 is the paging daemon. Process 3 is the reaper, which handles 566deallocating resources for exiting processes. 567.It Descriptor 568An integer assigned by the system when a file is referenced 569by 570.Xr open 2 571or 572.Xr dup 2 , 573or when a socket is created by 574.Xr pipe 2 , 575.Xr socket 2 , 576or 577.Xr socketpair 2 , 578which uniquely identifies an access path to that file or socket from 579a given process or any of its children. 580.It File Name 581Names consisting of up to 255 582.Pq Dv MAXNAMELEN 583characters may be used to name 584an ordinary file, special file, or directory. 585.Pp 586These characters may be selected from the set of all 587.Tn ASCII 588character 589excluding 0 (NUL) and the 590.Tn ASCII 591code for 592.Ql \&/ 593(slash). (The parity bit, 594bit 7, must be 0.) 595.Pp 596Note that it is generally unwise to use 597.Ql \&* , 598.Ql \&? , 599.Ql \&[ 600or 601.Ql \&] 602as part of 603file names because of the special meaning attached to these characters 604by the shell. 605.It Path Name 606A path name is a 607.Tn NUL Ns -terminated 608character string starting with an 609optional slash 610.Ql \&/ , 611followed by zero or more directory names separated 612by slashes, optionally followed by a file name. 613The total length of a path name must be less than 1024 614.Pq Dv MAXPATHLEN 615characters. 616.Pp 617If a path name begins with a slash, the path search begins at the 618.Em root 619directory. 620Otherwise, the search begins from the current working directory. 621A slash by itself names the root directory. An empty 622pathname refers to the current directory. 623.It Directory 624A directory is a special type of file that contains entries 625that are references to other files. 626Directory entries are called links. By convention, a directory 627contains at least two links, 628.Ql \&. 629and 630.Ql \&.. , 631referred to as 632.Em dot 633and 634.Em dot-dot 635respectively. Dot refers to the directory itself and 636dot-dot refers to its parent directory. 637.It "Root Directory and Current Working Directory" 638Each process has associated with it a concept of a root directory 639and a current working directory for the purpose of resolving path 640name searches. A process's root directory need not be the root 641directory of the root file system. 642.It File Access Permissions 643Every file in the file system has a set of access permissions. 644These permissions are used in determining whether a process 645may perform a requested operation on the file (such as opening 646a file for writing). Access permissions are established at the 647time a file is created. They may be changed at some later time 648through the 649.Xr chmod 2 650call. 651.Pp 652File access is broken down according to whether a file may be: read, 653written, or executed. Directory files use the execute 654permission to control if the directory may be searched. 655.Pp 656File access permissions are interpreted by the system as 657they apply to three different classes of users: the owner 658of the file, those users in the file's group, anyone else. 659Every file has an independent set of access permissions for 660each of these classes. When an access check is made, the system 661decides if permission should be granted by checking the access 662information applicable to the caller. 663.Pp 664Read, write, and execute/search permissions on 665a file are granted to a process if: 666.Pp 667The process's effective user ID is that of the super-user. (Note: 668even the super-user cannot execute a non-executable file.) 669.Pp 670The process's effective user ID matches the user ID of the owner 671of the file and the owner permissions allow the access. 672.Pp 673The process's effective user ID does not match the user ID of the 674owner of the file, and either the process's effective 675group ID matches the group ID 676of the file, or the group ID of the file is in 677the process's group access list, 678and the group permissions allow the access. 679.Pp 680Neither the effective user ID nor effective group ID 681and group access list of the process 682match the corresponding user ID and group ID of the file, 683but the permissions for ``other users'' allow access. 684.Pp 685Otherwise, permission is denied. 686.It Sockets and Address Families 687A socket is an endpoint for communication between processes. 688Each socket has queues for sending and receiving data. 689.Pp 690Sockets are typed according to their communications properties. 691These properties include whether messages sent and received 692at a socket require the name of the partner, whether communication 693is reliable, the format used in naming message recipients, etc. 694.Pp 695Each instance of the system supports some 696collection of socket types; consult 697.Xr socket 2 698for more information about the types available and 699their properties. 700.Pp 701Each instance of the system supports some number of sets of 702communications protocols. Each protocol set supports addresses 703of a certain format. An Address Family is the set of addresses 704for a specific group of protocols. Each socket has an address 705chosen from the address family in which the socket was created. 706.El 707.Sh SEE ALSO 708.Xr intro 3 , 709.Xr perror 3 710.Sh HISTORY 711An 712.Nm intro 713manual page appeared in 714.At v6 . 715