sigsem(S)

sigsem -- signals a process waiting on a semaphore

Syntax

cc . . . -lx

int sigsem(sem_num);
int sem_num;

Description

sigsem signals a process that is waiting on the semaphore sem_num that it may proceed and use the resource governed by the semaphore. sigsem is used in conjunction with waitsem(S) to allow synchronization of processes wishing to access a resource. One or more processes may waitsem on the given semaphore and is put to sleep until the process which currently has access to the resource issues a sigsem call. If there are any waiting processes, sigsem causes the process which is next in line on the semaphore's queue to be rescheduled for execution. The semaphore's queue is organized in first in first out (FIFO) order.

System compatibility

sigsem can only be used to signal semaphores created under UNIX Version 3.0, not for UNIX System V semaphores.

Diagnostics

sigsem returns the value (int) -1 if an error occurs. If sem_num does not refer to a semaphore type file, errno is set to ENOTNAM. If sem_num has not been previously opened by opensem, errno is set to EBADF. If the process issuing a sigsem call is not the current ``owner'' of the semaphore (i.e., if the process has not issued a waitsem call before the sigsem), errno is set to ENAVAIL.

Notes

This feature is a XENIX specific enhancement and may not be present in all UNIX implementations.

Standards conformance

sigsem is not part of any currently supported standard; it is an extension of AT&T System V provided by the Santa Cruz Operation.