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 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
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
When creating a new semaphore set, semget() initializes the set's asso-
ciated 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
If the semaphore set already exists, the permissions are verified.
If successful, the return value will be the semaphore set identifier (a
nonnegative integer), otherwise, -1 is returned, with errno indicating
EINVAL A semaphore set corresponding to key already exists, but nsems
is larger than the number of semaphores in that set.
ENOENT No semaphore set exists for key and semflg did not specify
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 all but the least significant
9 bits of semflg and creates a new semaphore set (on success).
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 ini-
tialize 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 struc-
ture retrieved by a semctl(2) IPC_STAT operation can be used to avoid
The following limits on semaphore set resources affect the semget()
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
SEMMSL Maximum number of semaphores per semaphore ID. On Linux systems
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
clearly show its function.
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
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Linux 2015-02-21 SEMGET(2)
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