SEMCTL(2) BSD Programmer's Manual SEMCTL(2)NAME
semctl - control operations on a semaphore set
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semctl(int semid, int semnum, int cmd, union semun arg);
DESCRIPTION
The semctl() function performs the operation indicated by cmd on the
semaphore set indicated by semid. For the commands that use the arg pa-
rameter, union semun is defined as follows:
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */
u_short *array; /* array for GETALL & SETALL */
};
Commands are performed as follows:
IPC_STAT Fetch the semaphore set's struct semid_ds, storing it in the
memory pointed to by arg.buf.
IPC_SET Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode
members of the semaphore set's struct semid_ds to match
those of the struct pointed to by arg.buf. The calling pro-
cess's effective uid must match either sem_perm.uid or
sem_perm.cuid, or it must have superuser privileges.
IPC_RMID Immediately removes the semaphore set from the system. The
calling process's effective uid must equal the semaphore
set's sem_perm.uid or sem_perm.cuid, or the process must
have superuser privileges.
GETVAL Return the value of semaphore number semnum.
SETVAL Set the value of semaphore number semnum to arg.val.
GETPID Return the pid of the last process to perform an operation
on semaphore number semnum.
GETNCNT Return the number of processes waiting for semaphore number
semnum's value to become greater than its current value.
GETZCNT Return the number of processes waiting for semaphore number
semnum's value to become 0.
GETALL Fetch the value of all of the semaphores in the set into the
array pointed to by arg.array.
SETALL Set the values of all of the semaphores in the set to the
values in the array pointed to by arg.array.
The struct semid_ds structure is defined as follows:
struct semid_ds {
struct ipc_perm sem_perm; /* operation permission struct */
struct sem *sem_base; /* pointer to first semaphore in set */
u_short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
time_t sem_ctime; /* last change time */
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
};
RETURN VALUES
On success, when cmd is one of GETVAL, GETNCNT, or GETZCNT, semctl() re-
turns the corresponding value; otherwise, 0 is returned. On failure, -1
is returned, and errno is set to indicate the error.
ERRORS
The semctl() function will fail if:
[EINVAL] No semaphore set corresponds to semid.
[EINVAL] semnum is not in the range of valid semaphores for given
semaphore set.
[EPERM] The calling process's effective uid does not match the uid
of the semaphore set's owner or creator.
[EACCES] Permission denied due to mismatch between operation and
mode of semaphore set.
SEE ALSOsemget(2), semop(2)4th Berkeley Distribution July 3, 1996 2