NAME
semget - get a System V semaphore set identifier SYNOPSIS
#include
#include
#include
int semget(keyt key, int nsems, int semflg); DESCRIPTION The semget() system call returns the System V semaphore set identifier associated with the argument key. A new set of nsems semaphores is created if key has the value IPCPRIVATE or if no existing semaphore set is associated with key and IPCCREAT is specified in semflg. If semflg specifies both IPCCREAT and IPCEXCL and a semaphore set already exists for key, then semget() fails with errno set to EEXIST. (This is analogous to the effect of the combination OCREAT | OEXCL for open(2).) Upon creation, the least significant 9 bits of the argument semflg define the permissions (for owner, group and others) for the semaphore set. These bits have the same format, and the same meaning, as the mode argument of open(2) (though the execute permissions are not mean‐ ingful for semaphores, and write permissions mean permission to alter semaphore values). The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 is explicit on this point.) Although Linux, like many other implementations, initializes the semaphore values to 0, a porta‐ ble application cannot rely on this: it should explicitly initialize the semaphores to the desired values. When creating a new semaphore set, semget() initializes the set's asso‐ ciated data structure, semidds (see semctl(2)), as follows: semperm.cuid and semperm.uid are set to the effective user ID of the calling process. semperm.cgid and semperm.gid are set to the effective group ID of the calling process. The least significant 9 bits of semperm.mode are set to the least significant 9 bits of semflg. semnsems is set to the value of nsems. semotime is set to 0. semctime is set to the current time. The argument nsems can be 0 (a don't care) when a semaphore set is not being created. Otherwise nsems must be greater than 0 and less than or equal to the maximum number of semaphores per semaphore set (SEMMSL). If the semaphore set already exists, the permissions are verified. RETURN VALUE If successful, the return value will be the semaphore set identifier (a
nonnegative integer), otherwise -1 is returned, with errno indicating the error. ERRORS On failure errno will be set to one of the following: EACCES A semaphore set exists for key, but the calling process does not have permission to access the set, and does not have the CAPIPCOWNER capability. EEXIST A semaphore set exists for key and semflg specified both IPCCREAT and IPCEXCL. EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corre‐ sponding to key already exists, and nsems is larger than the number of semaphores in that set. ENOENT No semaphore set exists for key and semflg did not specify IPCCREAT. ENOMEM A semaphore set has to be created but the system does not have enough memory for the new data structure. ENOSPC A semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI), or the system wide maximum number of semaphores (SEMMNS), would be exceeded. CONFORMING TO
SVr4, POSIX.1-2001. NOTES The inclusion of
and isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. IPCPRIVATE isn't a flag field but a keyt type. If this special value is used for key, the system call ignores everything but the least sig‐ nificant 9 bits of semflg and creates a new semaphore set (on success). The following limits on semaphore set resources affect the semget() call: SEMMNI System wide maximum number of semaphore sets: policy dependent (on Linux, this limit can be read and modified via the fourth field of /proc/sys/kernel/sem). SEMMSL Maximum number of semaphores per semid: implementation dependent (on Linux, this limit can be read and modified via the first field of /proc/sys/kernel/sem). SEMMNS System wide maximum number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of /proc/sys/kernel/sem). Values greater than SEMMSL * SEMMNI makes it irrelevant. BUGS The name choice IPCPRIVATE was perhaps unfortunate, IPCNEW would more clearly show its function. The semaphores in a set are not initialized by semget(). In order to initialize the semaphores, semctl(2) must be used to perform a SETVAL or a SETALL operation on the semaphore set. (Where multiple peers do not know who will be the first to initialize the set, checking for a nonzero semotime in the associated data structure retrieved by a sem‐ ctl(2) IPCSTAT operation can be used to avoid races.) SEE ALSO semctl(2), semop(2), ftok(3), capabilities(7), semoverview(7), svipc(7) COLOPHON This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-05-31 SEMGET(2)