SEMGET(2)

NAME
semget - get set of semaphores

SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semget(key, nsems, semflg)
key_t key;
int nsems, semflg;

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

A semaphore identifier and associated data structure and set containing nsems semaphores (see intro(2)) are created for key if one of the following are true:

•   key is equal to IPC_PRIVATE.

•   key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT) is ``true''.

Upon 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 low-order 9 bits of sem_perm.mode are set equal to the low-order 9 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.

A semaphore identifier (semid) is a unique positive integer created by a semget(2) system call. Each semid has a set of semaphores and a data structure associated with it. The data structure is referred to as semid_ds and contains the following members:

    struct	ipc_perm sem_perm;	/* operation permission struct */
    ushort	sem_nsems;	/* number of sems in set */
    time_t	sem_otime;	/* last operation time */
    time_t	sem_ctime;	/* last change time */
    		/* Times measured in secs since */
    		/* 00:00:00 GMT, Jan. 1, 1970 */

sem_perm is an ipc_perm structure that specifies the semaphore operation permission (see below). This structure includes the following members:

    ushort	cuid;	/* creator user id */
    ushort	cgid;	/* creator group id */
    ushort	uid;	/* user id */
    ushort	gid;	/* group id */
    ushort	mode;	/* r/a permission */

The value of sem_nsems is equal to the number of semaphores in the set. Each semaphore in the set is referenced by a positive integer referred to as a sem_num. sem_num values run sequentially from 0 to the value of sem_nsems minus 1. sem_otime is the time of the last semop(2) operation, and sem_ctime is the time of the last semctl(2) operation that changed a member of the above structure.

A semaphore is a data structure that contains the following members:

    ushort	semval;	/* semaphore value */
    short	sempid;	/* pid of last operation  */
    ushort	semncnt;	/* # awaiting semval > cval */
    ushort	semzcnt;	/* # awaiting semval = 0 */

semval is a non-negative integer. sempid is equal to the process ID of the last process that performed a semaphore operation on this semaphore. semncnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become greater than its current value. semzcnt is a count of the number of processes that are currently suspended awaiting this semaphore's semval to become zero.

RETURN VALUES
semget() returns a non-negative semaphore identifier on success. On failure, it returns -1 and sets errno to indicate the error.

ERRORS

EACCES
    A semaphore identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits of semflg would not be granted.

EEXIST
    A semaphore identifier exists for key but ( (semflg & IPC_CREAT) and (semflg & IPC_EXCL)) is ``true''.

EINVAL
    nsems is either less than or equal to zero or greater than the system-imposed limit.

    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 semaphore identifiers system wide would be exceeded.

    A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system wide would be exceeded.

SEE ALSO
ipcrm(1), ipcs(1), intro(2), semctl(2), semop(2)