SEMGET(2) Linux Programmer's Manual SEMGET(2)
semget - get a System V semaphore set identifier
int semget(key_t key, int nsems, int semflg);
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 sem-
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 val-
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 portable 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 associated data struc-
ture, semid_ds (see semctl(2)), as follows:
sem_perm.cuid and sem_perm.uid are set to the effective user ID of the calling
sem_perm.cgid and sem_perm.gid are set to the effective group ID of the calling
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.
If successful, the return value will be the semaphore set identifier (a nonnegative inte-
ger), otherwise -1 is returned, with errno indicating the error.
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 CAP_IPC_OWNER capability.
EEXIST A semaphore set exists for key and semflg specified both IPC_CREAT and IPC_EXCL.
EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per sem-
aphore set (SEMMSL), or a semaphore set corresponding 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 IPC_CREAT.
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.
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 everything but the least significant 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.
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its
The semaphores in a set are not initialized by semget(). In order to initialize the sema-
phores, 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 sem_otime in the associated data structure retrieved by a semctl(2)
IPC_STAT operation can be used to avoid races.)
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), svipc(7)
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Linux 2012-05-31 SEMGET(2)