Controlling Semaphores (System Interface Guide)

System Interface Guide

Controlling Semaphores

semctl(2) changes permissions and other characteristics of a semaphore set. It must be called with a valid semaphore ID. The semnum value selects a semaphore within an array by its index. The cmd argument is one of the following control flags.

`GETVAL`	Return the value of a single semaphore.
`SETVAL`	Set the value of a single semaphore. In this case, arg is taken as `arg.val`, an int.
`GETPID`	Return the `PID` of the process that performed the last operation on the semaphore or array.
`GETNCNT`	Return the number of processes waiting for the value of a semaphore to increase.
`GETZCNT`	Return the number of processes waiting for the value of a particular semaphore to reach zero.
`GETALL`	Return the values for all semaphores in a set. In this case, arg is taken as `arg.array`, a pointer to an array of `unsigned short`s.
`SETALL`	Set values for all semaphores in a set. In this case, arg is taken as `arg.array`, a pointer to an array of unsigned shorts.
`IPC_STAT`	Return the status information from the control structure for the semaphore set and place it in the data structure pointed to by `arg.buf`, a pointer to a buffer of type `semid_d`s.
`IPC_SET`	Set the effective user and group identification and permissions. In this case, arg is taken as `arg.buf`.
`IPC_RMID`	Remove the specified semaphore set.

A process must have an effective user identification of owner, creator, or superuser to perform an IPC_SET or IPC_RMID command. Read and write permission is required as for the other control commands.

The following code illustrates semctl(2):

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
...
 	register int				i;
 ...
 	i = semctl(semid, semnum, cmd, arg);
 	if (i == -1) {
 		perror("semctl: semctl failed");
 		exit(1);
...