SEMGET(2) Linux Programmer's Manual SEMGET(2)
semget - get a semaphore set identifier
int semget(key_t key, int nsems, int semflg);
This function 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 to key
and IPC_CREAT is asserted in semflg (i.e. semflg & IPC_CREAT isn't
The presence in semflg of the fields IPC_CREAT and IPC_EXCL plays the
same role, with respect to the existence of the semaphore set, as the
presence of O_CREAT and O_EXCL in the mode argument of the open(2) sys-
tem call: i.e. the semget function fails if semflg asserts both
IPC_CREAT and IPC_EXCL and a semaphore set already exists for key.
Upon creation, the low-order 9 bits of the argument semflg define the
access permissions (for owner, group and others) for the semaphore set.
These bits have the same format, and the same meaning, as the mode
argument in the open(2) or creat(2) system calls (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 semaphore
set's associated data structure semid_ds 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 low-order 9 bits of sem_perm.mode are set to the low-order 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 access permissions are veri-
If successful, the return value will be the semaphore set identifier (a
nonnegative 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.
EEXIST A semaphore set exists for key and semflg was asserting both
IPC_CREAT and IPC_EXCL.
ENOENT No semaphore set exists for key and semflg wasn't asserting
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.
ENOMEM A semaphore set has to be created but the system has not
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 low-order 9
bits of semflg and creates a new semaphore set (on success).
The followings are limits on semaphore set resources affecting a semget
SEMMNI System wide maximum number of semaphore sets: policy depen-
SEMMSL Maximum number of semaphores per semid: implementation
dependent (500 currently).
SEMMNS System wide maximum number of semaphores: policy dependent.
Values greater than SEMMSL * SEMMNI makes it irrelevant.
Use of IPC_PRIVATE doesn't inhibit to other processes the access to the
allocated semaphore set.
There is currently no intrinsic way for a process to ensure exclusive
access to a semaphore set. Asserting both IPC_CREAT and IPC_EXCL in
semflg only ensures (on success) that a new semaphore set will be cre-
ated, it doesn't imply exclusive access to the semaphore set.
The data structure associated with each semaphore in the set isn't ini-
tialized by the system call. In order to initialize those data struc-
tures, one has to execute a subsequent call to semctl(2) to perform a
SETVAL or a SETALL command on the semaphore set.
SVr4, SVID. SVr4 documents additional error conditions EFBIG, E2BIG,
EAGAIN, ERANGE, EFAULT.
ftok(3), ipc(5), semctl(2), semop(2)
Linux 2.5 2002-01-04 SEMGET(2)