SEMGET(2)

NAME
semget - get set of semaphores

LIBRARY
libc

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

DESCRIPTION
The semget() system call returns the semaphore identifier associated with key.

A new set containing nsems semaphores is created if either key is equal to IPC_PRIVATE, or key does not have a semaphore set associated with it and the IPC_CREAT bit is set in semflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in semflg, and key has a semaphore set associated with it already, the operation will fail.

If a new set of semaphores is created, the data structure associated with it (the semid_ds structure, see semctl(2)) is initialized as follows:

•   sem_perm.cuid and sem_perm.uid are set to the effective uid of the calling process.

•   sem_perm.gid and sem_perm.cgid are set to the effective gid of the calling process.

•   sem_perm.mode is set to the lower 9 bits of semflg.

•   sem_nsems is set to the value of nsems.

•   sem_ctime is set to the current time.

•   sem_otime is set to 0.

RETURN VALUES
semget() returns a non-negative semaphore identifier if successful. Otherwise, -1 is returned and errno is set to reflect the error.

ERRORS

[EACCES]
    The caller has no permission to access a semaphore set already associated with key.

[EEXIST]
    Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key.

[EINVAL]
    nsems is less than 0 or greater than the system limit for the number in a semaphore set.

    A semaphore set associated with key exists, but has fewer semaphores than the number specified in nsems.

[ENOSPC]
    A new set of semaphores could not be created because the system limit for the number of semaphores or the number of semaphore sets has been reached.

[ENOENT]
    IPC_CREAT is not set in semflg and no semaphore set associated with key was found.

SEE ALSO
ipcs(1), semctl(2), semop(2), ftok(3)

STANDARDS
The semget system call conforms to xsh5.

HISTORY
Semaphores appeared in the first release of AT&T UNIX.