SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semget(key_t 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 IPC_PRIVATE or if no existing semaphore set is associated with key and IPC_CREAT is specified in semflg.

If semflg specifies both IPC_CREAT and IPC_EXCL 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 O_CREAT | O_EXCL 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 meaningful for semaphores, and write permissions mean permission to alter semaphore values).

When creating a new semaphore set, semget() initializes the set's associated data structure, semid_ds (see semctl(2)), as follows:

sem_perm.cuid and sem_perm.uid are set to the effective user ID of the calling process.

sem_perm.cgid and sem_perm.gid are set to the effective group ID of the calling process.

The least significant 9 bits of sem_perm.mode are set to the least significant 9 bits of semflg.

sem_nsems is set to the value of nsems.

sem_otime is set to 0.

sem_ctime 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.

CONFORMING TO

SVr4, POSIX.1-2001.

NOTES

The inclusion of <sys/types.h> and <sys/ipc.h> 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.

IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores all but the least significant 9 bits of semflg and creates a new semaphore set (on success).

Semaphore initialization

The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 and POSIX.1-2008 are explicit on this point, although POSIX.1-2008 notes that a future version of the standard may require an implementation to initialize the semaphores to 0.) Although Linux, like many other implementations, initializes the semaphore values to 0, a portable application cannot rely on this: it should explicitly initialize the semaphores to the desired values.

Initialization can be done using semctl(2) SETVAL or SETALL operation. Where multiple peers do not know who will be the first to initialize the set, checking for a nonzero sem_otime in the associated data structure retrieved by a semctl(2) IPC_STAT operation can be used to avoid races.

Semaphore limits

The following limits on semaphore set resources affect the semget() call:

SEMMNI

System-wide limit on the number of semaphore sets. On Linux systems before version 3.19, the default value for this limit was 128. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via the fourth field of /proc/sys/kernel/sem.

SEMMSL

Maximum number of semaphores per semaphore ID. On Linux systems before version 3.19, the default value for this limit was 250. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via the first field of /proc/sys/kernel/sem.

SEMMNS

System-wide limit on the number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of /proc/sys/kernel/sem). Note that the number of semaphores system-wide is also limited by the product of SEMMSL and SEMMNI.

COLOPHON

This page is part of release 4.06 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.