semget(S)

semget -- get set of semaphores

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/sem.h>
   
   int semget(key_t key, int nsems, int semflg);

Description

semget returns the semaphore identifier associated with key. This identifier is accessible by any process in the system, subject to normal access restrictions and the permissions set with semflg.

A semaphore identifier and associated data structure and set containing nsems semaphores [see intro(S)] are created for key if one of the following is true:

key is equal to IPC_PRIVATE.
key does not already have a semaphore identifier associated with it, and (semflg&IPC_CREAT) is true.

On creation, the data structure associated with the new semaphore identifier is initialized as follows:

sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.
The access permission bits of sem_perm.mode are set equal to the access permission bits of semflg.
sem_nsems is set equal to the value of nsems.
sem_otime is set equal to 0 and sem_ctime is set equal to the current time.

Return values

On success, semget returns a non-negative integer, namely a semaphore identifier. On failure, semget returns -1 and sets errno to identify the error.

Errors

In the following conditions, semget fails and sets errno to:

EINVAL: nsems is either less than or equal to zero or greater than the system-imposed limit.
EACCES: A semaphore identifier exists for key, but operation permission [see intro(S)] as specified by the low-order 9 bits of semflg would not be granted.
EINVAL: A semaphore identifier exists for key, but the number of semaphores in the set associated with it is less than nsems, and nsems is not equal to zero.
ENOENT: A semaphore identifier does not exist for key and (semflg&IPC_CREAT) is false.
ENOSPC: A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores or semaphore identifiers system wide would be exceeded.
ENOSPC: A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system wide would be exceeded.
EEXIST: A semaphore identifier exists for key but both (semflg&IPC_CREAT) and (semflg&IPC_EXCL) are both true.

References

intro(S), semctl(S), semop(S), stdipc(S)