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 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 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 */
               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, 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                                         September 12, 1995                                      SEMCTL(2)