NAME

semget - get set of semaphores

 SYNOPSIS

#include <sys/sem.h>

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

 DESCRIPTION

The semget() function returns 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>, are 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 initialised as follows:

• In the operation permissions structure 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.

• The variable sem_nsems is set equal to the value of nsems.

• The variable sem_otime is set equal to 0 and sem_ctime is set equal to the current time.

• The data structure associated with each semaphore in the set is not initialised. The semctl() function with the command SETVAL or SETALL can be used to initialise each semaphore.

 RETURN VALUE

Upon successful completion, semget() returns a non-negative integer, namely a semaphore identifier; otherwise, it returns -1 and errno will be set to indicate the error.

 ERRORS

The semget() function will 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 IPC.

[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

None.

 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 IPC can be easily modified to use the alternative interfaces.

 FUTURE DIRECTIONS

None.

 SEE ALSO

semctl(), semop(), sem_close(), sem_destroy(), sem_getvalue(), sem_init(), sem_open(), sem_post(), sem_unlink(), sem_wait(), <sys/sem.h>, IPC.

DERIVATION

Derived from Issue 2 of the SVID.