.\" 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this one .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 .TH SEMGET 2 "November 1, 1993" "Linux 0.99.13" "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 The function returns the semaphore set identifier associated to the value of the argument .IR key . A new set of .I nsems semaphores is created if .I key has value .B IPC_PRIVATE or .I key isn't .BR IPC_PRIVATE , no existing message queue is associated to .IR key , and .B IPC_CREAT is asserted in .I semflg (i.e. .IB semflg &IPC_CREAT isn't zero). 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 msgget 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 lower 9 bits of the argument .I semflg define the access permissions (for owner, group and others) to the semaphore set in the same format, and with the same meaning, as for the access permissions parameter in the .BR open (2) or .BR creat (2) system calls (though the execute permissions are not used by the system, and write permissions, for a semaphore set, effectively means alter permissions). .PP Furthermore, while creating, the system call initializes the system semaphore set 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 lowest order 9 bits of .B sem_perm.mode are set to the lowest order 9 bit 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 the system call isn't a create one. Otherwise .I nsems must be greater than .B 0 and less or equal to the maximum number of semaphores per semid, .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 positive integer), otherwise .B \-1 with .B errno indicating the error. .SH ERRORS For a failing return, .B errno will be set to one among the following values: .TP 11 .B EACCES A semaphore set exists for .IR key , but the calling process has no access permissions to the set. .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 as to be deleted. .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 lowest 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 Use of .B IPC_PRIVATE doesn't inhibit to other processes the access to the allocated semaphore set. .PP As for the files, there is currently no intrinsic way for a process to ensure exclusive access to a semaphore set. Asserting both .B IPC_CREAT and .B IPC_EXCL in .I semflg only ensures (on success) that a new semaphore set will be created, it doesn't imply exclusive access to the semaphore set. .PP 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 EINVAL, EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT. .SH "SEE ALSO" .BR ftok (3), .BR ipc (5), .BR semctl (2), .BR semop (2).