NAME
semctl — control operations on a semaphore setLIBRARY
Standard C Library (libc, -lc)SYNOPSIS
#include <sys/types.h>#include <sys/ipc.h>
#include <sys/sem.h> int
semctl(int semid, int semnum, int cmd, ...);
DESCRIPTION
The semctl() system call performs the operation indicated by cmd on the semaphore set indicated by semid. A fourth argument, a union semun arg, is required for certain values of cmd. For the commands that use the arg argument, union semun must be 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 */
};
_WANT_SEMUN
before including sys/sem.h to use the system
definition of union semun.
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 process'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. Outstanding adjust on exit values for this semaphore in any process are cleared.
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. Outstanding adjust on exit values for all semaphores in this set, in any process are cleared.
struct semid_ds {
struct ipc_perm sem_perm; /* operation permission struct */
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 ofGETVAL,
GETPID,
GETNCNT or
GETZCNT,
semctl() returns the corresponding value;
otherwise, 0 is returned. On failure, -1 is returned, and
errno is set to indicate the error.
ERRORS
The semctl() system call will fail if:- [
EINVAL] - No semaphore set corresponds to semid.
- [
EINVAL] - The semnum argument 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.
- [
ERANGE] -
SETVALorSETALLattempted to set a semaphore outside the allowable range [0 ..SEMVMX].
SEE ALSO
semget(2), semop(2)BUGS
SETALL may update some semaphore elements
before returning an error.| July 9, 2020 | Debian |