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.