SEMGET(2) — SYSTEM CALLS

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:

structipc_perm sem_perm; /∗ operation permission struct ∗/
ushortsem_nsems;/∗ number of sems in set ∗/
time_tsem_otime;/∗ last operation time ∗/
time_tsem_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:

ushortcuid;/∗ creator user id ∗/
ushortcgid;/∗ creator group id ∗/
ushortuid;/∗ user id ∗/
ushortgid;/∗ group id ∗/
ushortmode;/∗ 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:

ushortsemval;/∗ semaphore value ∗/
shortsempid;/∗ pid of last operation ∗/
ushortsemncnt;/∗ # awaiting semval > cval ∗/
ushortsemzcnt;/∗ # 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.

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