semget - get a semaphore set identifier
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/sem.h>
int semget ( key_t key, int nsems, int semflg )
The function returns the semaphore set identifier associated
to the value of the argument key. A new set of nsems sema-
phores is created if key has value IPC_PRIVATE or key isn't
IPC_PRIVATE, no existing message queue is associated to key,
and IPC_CREAT is asserted in semflg (i.e. semflg&IPC_CREAT
isn't zero). 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) system call:
i.e. the msgget function fails if semflg asserts both
IPC_CREAT and IPC_EXCL and a semaphore set already exists
Upon creation, the lower 9 bits of the argument semflg
define the access permissions (for owner, group and others)
to the semaphore set in the same format, and with the same
meaning, as for the access permissions parameter in the
open(2) or creat(2) system calls (though the execute permis-
sions are not used by the system, and write permissions, for
a semaphore set, effectively means alter permissions).
Furthermore, while creating, the system call initializes the
system semaphore set 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 lowest order 9 bits of sem_perm.mode are set to the
lowest order 9 bit 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 the system
call isn't a create one. Otherwise nsems must be greater
than 0 and less or equal to the maximum number of semaphores
per semid, (SEMMSL).
If the semaphore set already exists, the access permissions
are verified, and a check is made to see if it is marked for
If successful, the return value will be the semaphore set
identifier (a positive integer), otherwise -1 with errno
indicating the error.
For a failing return, errno will be set to one among the
EACCES A semaphore set exists for key, but the calling
process has no access permissions to the set.
EEXIST A semaphore set exists for key and semflg was
asserting both IPC_CREAT and IPC_EXCL.
EIDRM The semaphore set is marked as to be deleted.
ENOENT No semaphore set exists for key and semflg wasn't
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 exceeded.
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 lowest order 9 bits of semflg and creates
a new semaphore set (on success).
The followings are limits on semaphore set resources affect-
ing a semget call:
SEMMNI System wide maximum number of semaphore sets:
SEMMSL Maximum number of semaphores per semid: implemen-
tation 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.
As for the files, 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 created, it
doesn't imply exclusive access to the semaphore set.
The data structure associated with each semaphore in the set
isn't initialized by the system call. In order to initial-
ize those data structures, 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 EIN-
VAL, EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT.
ftok(3), ipc(5), semctl(2), semop(2).