semget() - Unix, Linux System Call
semget - get a semaphore set identifier
int semget(key_t key, int nsems, int semflg);
semget() system call returns the 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
IPC_CREAT is specified in
semflg specifies both
IPC_EXCL and a semaphore set already exists for
semget() fails with
errno set to
EEXIST. (This is analogous to the effect of the combination
O_CREAT | O_EXCL for
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
(though the execute permissions are
not meaningful 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,
initialises the semaphore values to 0,
a portable application cannot rely on this:
it should explicitly initialise the semaphores to the desired values.
When creating a new semaphore set,
semget() initialises the sets associated data structure,
nsems can be 0
(a dont care)
when a semaphore set is not being created.
nsems must be greater than 0
and less than or equal to the maximum number of semaphores per semaphore set
sem_perm.uid are set to the effective user ID of the calling process.
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
sem_nsems is set to the value of
sem_otime is set to 0.
sem_ctime is set to the current time.
If the semaphore set already exists, the permissions are
If successful, the return value will be the semaphore set identifier
(a nonnegative integer), otherwise -1
is returned, with
errno indicating the error.
errno will be set to one of the following:
A semaphore set exists for
key, but the calling process does not have permission to access the set,
and does not have the
A semaphore set exists for
semflg specified both
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.
No semaphore set exists for
semflg did not specify
A semaphore set has to be created but the system does not have
enough memory for the new data structure.
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.
IPC_PRIVATE isnt a flag field but a
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
System wide maximum number of semaphore sets: policy dependent
(on Linux, this limit can be read and modified via the fourth field of
Maximum number of semaphores per semid: implementation dependent
(on Linux, this limit can be read and modified via the first field of
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 function.
The semaphores in a set are not initialised by
semget(). In order to initialise the semaphores,
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
initialise the set, checking for a non-zero
sem_otime in the associated data structure retrieved by a
semctl() IPC_STAT operation can be used to avoid races.)