NAME

       semctl — control operations on a semaphore set

LIBRARY

       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 */
       };
       Non-portable software may define _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.

       The struct semid_ds is defined as follows:

       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 of GETVAL, 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]           SETVAL or SETALL attempted 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.

Debian                                            July 9, 2020                                         SEMCTL(2)