SEMGET(2) Linux Programmer's Manual SEMGET(2)
semget - get a semaphore set identifier
int semget(key_t key, int nsems, int semflg);
The semget() system call returns the semaphore set identifier associ-
ated 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
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, 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.
If successful, the return value will be the semaphore set identifier (a
non-negative integer), otherwise -1 is returned, with errno indicating
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
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 semaphore 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
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
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 sig-
nificant 9 bits of semflg and creates a new semaphore set (on success).
The following limits on semaphore set resources affect the semget()
SEMMNI System wide maximum number of semaphore sets: policy depen-
dent (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 sec-
ond 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 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
non-zero sem_otime in the associated data structure retrieved by a sem-
ctl(2) IPC_STAT operation can be used to avoid races.)
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
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be found at http://www.kernel.org/doc/man-pages/.
Linux 2004-05-27 SEMGET(2)