.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond .\" Modified 1 Jan 2002, Martin Schulze .\" Modified 4 Jan 2002, Michael Kerrisk .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" .TH SEMGET 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual" .SH NAME semget \- get a semaphore set identifier .SH SYNOPSIS .nf .B #include .B #include .B #include .fi .sp .BI "int semget(key_t " key , .BI "int " nsems , .BI "int " semflg ); .SH DESCRIPTION This function returns the semaphore set identifier associated with the argument .IR key . A new set of .I nsems semaphores is created if .I key has the value .B IPC_PRIVATE or if no existing semaphore set is associated to .I key and .B IPC_CREAT is asserted in .I semflg (i.e. .IR semflg " &" .B IPC_CREAT isn't zero). .PP The presence in .I semflg of the fields .B IPC_CREAT and .B IPC_EXCL plays the same role, with respect to the existence of the semaphore set, as the presence of .B O_CREAT and .B O_EXCL in the mode argument of the .BR open (2) system call: i.e. the .B semget function fails if .I semflg asserts both .B IPC_CREAT and .B IPC_EXCL and a semaphore set already exists for .IR key . .PP Upon creation, the low-order 9 bits of the argument .I semflg define the access permissions (for owner, group and others) for the semaphore set. These bits have the same format, and the same meaning, as the mode argument in the .BR open (2) or .BR creat (2) system calls (though the execute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values). .PP When creating a new semaphore set, .B semget initializes the semaphore set's associated data structure .B semid_ds as follows: .IP .B sem_perm.cuid and .B sem_perm.uid are set to the effective user\-ID of the calling process. .IP .B sem_perm.cgid and .B sem_perm.gid are set to the effective group\-ID of the calling process. .IP The low-order 9 bits of .B sem_perm.mode are set to the low-order 9 bits of .IR semflg . .IP .B sem_nsems is set to the value of .IR nsems . .IP .B sem_otime is set to 0. .IP .B sem_ctime is set to the current time. .PP The argument .I nsems can be .B 0 (a don't care) when a semaphore set is not being created. Otherwise .I nsems must be greater than .B 0 and less than or equal to the maximum number of semaphores per semaphore set .RB ( SEMMSL ). .PP If the semaphore set already exists, the access permissions are verified. .\" and a check is made to see if it is marked for destruction. .SH "RETURN VALUE" If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise .B \-1 is returned, with .I errno indicating the error. .SH ERRORS On failure .I errno will be set to one of the following: .TP 11 .B EACCES A semaphore set exists for .IR key , but the calling process does not have permission to access the set, and does not have the .BR CAP_IPC_OWNER capability. .TP .B EEXIST A semaphore set exists for .B key and .I semflg was asserting both .B IPC_CREAT and .BR IPC_EXCL . .\" .TP .\" .B EIDRM .\" The semaphore set is marked to be deleted. .TP .B EINVAL .IR nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set .RB ( SEMMSL ), or a semaphore set corresponding to .I key already exists, and .I nsems is larger than the number of semaphores in that set. .TP .B ENOENT No semaphore set exists for .I key and .I semflg wasn't asserting .BR IPC_CREAT . .TP .B ENOMEM A semaphore set has to be created but the system has not enough memory for the new data structure. .TP .B ENOSPC A semaphore set has to be created but the system limit for the maximum number of semaphore sets .RB ( SEMMNI ), or the system wide maximum number of semaphores .RB ( SEMMNS ), would be exceeded. .SH NOTES .B IPC_PRIVATE isn't a flag field but a .B key_t type. If this special value is used for .IR key , the system call ignores everything but the low-order 9 bits of .I semflg and creates a new semaphore set (on success). .PP The followings are limits on semaphore set resources affecting a .B semget call: .TP 11 .B SEMMNI System wide maximum number of semaphore sets: policy dependent. .TP .B SEMMSL Maximum number of semaphores per semid: implementation dependent (500 currently). .TP .B SEMMNS System wide maximum number of semaphores: policy dependent. Values greater than .B SEMMSL * SEMMNI makes it irrelevant. .SH BUGS The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function. .LP The data structure associated with each semaphore in the set isn't initialized by the system call. In order to initialize those data structures, one has to execute a subsequent call to .BR semctl (2) to perform a .B SETVAL or a .B SETALL command on the semaphore set. .SH "CONFORMING TO" SVr4, SVID. SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT. .SH "SEE ALSO" .BR semctl (2), .BR semop (2), .BR ftok (3), .BR ipc (5), .BR capabilities (7)