semget(2) -- Linux man page



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);  


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 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 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 verified.  


If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicating the error.  


On failure 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.
A semaphore set exists for key and semflg was asserting both IPC_CREAT and IPC_EXCL.
No semaphore set exists for key and semflg wasn't asserting IPC_CREAT.
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.
A semaphore set has to be created but the system has not 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 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 call:

System wide maximum number of semaphore sets: policy dependent.
Maximum number of semaphores per semid: implementation dependent (500 currently).
System wide maximum number of semaphores: policy dependent. 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 data structure associated with each semaphore in the set isn't initialized by the system call. In order to initialize 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 EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT.  


ftok(3), ipc(5), semctl(2), semop(2)