NAME
semget — get set
of XSI semaphores
SYNOPSIS
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
The semget () function operates on XSI
semaphores (see XBD V1_chap04(7)). It is unspecified
whether this function interoperates with the realtime interprocess
communication facilities defined in V2_chap02(3).
The semget () function shall return the
semaphore identifier associated with key.
A semaphore identifier with its associated semid_ds data structure and its associated set of nsems semaphores (see <sys/sem.h>) is created for key if one of the following is true:
- The argument key is equal to IPC_PRIVATE.
- The argument key does not already have a semaphore identifier associated with it and ( semflg &IPC_CREAT) is non-zero.
Upon creation, the semid_ds data structure associated with the new semaphore identifier is initialized as follows:
- In the operation permissions structure sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid shall be set to the effective user ID and effective group ID, respectively, of the calling process.
- The low-order 9 bits of sem_perm.mode shall be set to the low-order 9 bits of semflg.
- The variable sem_nsems shall be set to the value of nsems.
- The variable sem_otime shall be set to 0 and sem_ctime shall be set to the current time, as described in V2_chap02(3).
Upon creation, the value of the semval, sempid, semncnt, and semzcnt members of all nsems semaphores in the associated semaphore set shall be set to zero.
RETURN VALUE
Upon successful completion, semget ()
shall return a non-negative integer, namely a semaphore identifier;
otherwise, it shall return -1 and set
errno to
indicate the error.
ERRORS
The semget () function shall fail if:
- [EACCES]
- A semaphore identifier exists for key , but operation permission as specified by the low-order 9 bits of semflg would not be granted; see 2.7 XSI Interprocess Communication .
- [EEXIST]
- A semaphore identifier exists for the argument key but (( semflg &IPC_CREAT) &&( semflg &IPC_EXCL)) is non-zero.
- [EINVAL]
- The value of nsems is either less than or equal to 0 or greater than the system-imposed limit, or a semaphore identifier exists for the argument key , but the number of semaphores in the set associated with it is less than nsems and nsems is not equal to 0.
- [ENOENT]
- A semaphore identifier does not exist for the argument key and ( semflg &IPC_CREAT) is equal to 0.
- [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.
EXAMPLES
Refer to semop(3).
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in V2_chap02(3) can be easily modified to use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
V2_chap02(3), V2_chap02(3), ftok(3), semctl(3), semop(3), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_trywait(3), sem_unlink(3)
XBD V1_chap04(7), <sys/sem.h>
CHANGE HISTORY
First released in Issue 2. Derived from Issue 2 of the SVID.
Issue 5
The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.
Issue 6
IEEE Std 1003.1-2001 (“POSIX.1”)/Cor 2-2004, item XSH/TC2/D6/122 is applied, updating the DESCRIPTION from "each semaphore in the set shall not be initialized" to "each semaphore in the set need not be initialized".
Issue 7
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0536 [335,439] and XSH/TC1-2008/0537 [344] are applied.
Issue 8
Austin Group Defect 377 is applied, adding a requirement that the
value of the semval, sempid,
semncnt, and semzcnt members of all
semaphores in a semaphore set be initialized to zero when a call to
semget () creates a semaphore set.