SEMCTL(2)                DOMAIN/IX SYS5                 SEMCTL(2)



NAME
     semctl - semaphore control operations

USAGE
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/sem.h>

     int semctl(semid, semnum, cmd, arg)
     int semid, cmd;
     int semnum;
     union semun {
          int val;
          struct semid_ds *buf;
          ushort *array;
     } arg;


DESCRIPTION
     Semctl provides semaphore control operations in the form of
     commands, denoted cmd in USAGE above.

     The following cmds are executed on the semaphore specified
     by semid and semnum

          GETVAL    Return the value of semval (see intro(2)).
                    {READ}

          SETVAL    Set the value of semval to arg.val.  {ALTER}
                    When this command is successfully executed,
                    the semadj value corresponding to the speci-
                    fied semaphore in all processes is cleared.

          GETPID    Return the value of sempid.  {READ}

          GETNCNT   Return the value of semncnt.  {READ}

          GETZCNT   Return the value of semzcnt.  {READ}

     The following cmds return and set, respectively, every sem-
     val in the set of semaphores.

          GETALL    Place semvals into array pointed to by
                    arg.array.  {READ}

          SETALL    Set semvals according to the array pointed to
                    by arg.array.  {ALTER} When this command is
                    successfully executed, the semadj values
                    corresponding to each specified semaphore in
                    all processes are cleared.



Printed 12/4/86                                          SEMCTL-1

SEMCTL(2)                DOMAIN/IX SYS5                 SEMCTL(2)



     The following cmds are also available:

          IPC_STAT  Place the current value of each member of the
                    data structure associated with semid into the
                    structure pointed to by arg.buf.  The con-
                    tents of this structure are defined in
                    intro(2).  {READ}

          IPC_SET   Set the value of the following members of the
                    data structure associated with semid to the
                    corresponding value found in the structure
                    pointed to by arg.buf:

                      sem_perm.uid
                      sem_perm.gid
                      sem_perm.mode /* only low 9 bits */

                    This command can only be executed by a pro-
                    cess that has an effective user ID equal to
                    either that of super-user or to the value of
                    sem_perm.uid in the data structure associated
                    with semid.

          IPC_RMID  Remove the semaphore identifier specified by
                    semid from the system and destroy the set of
                    semaphores and data structure associated with
                    it.  This command can only be executed by a
                    process that has an effective user ID equal
                    to either that of super-user or to the value
                    of sem_perm.uid in the data structure associ-
                    ated with semid.

RETURN VALUE
     The value returned upon successful completion depends on
     cmd, as shown here.

          GETVAL    Returns the value of semval.

          GETPID    Returns the value of sempid.

          GETNCNT   Returns the value of semncnt.

          GETZCNT   Returns the value of semzcnt.

          All others return a value of 0.

     Otherwise, a value of -1 is returned and errno is set to
     indicate the error.





SEMCTL-2                                          Printed 12/4/86

SEMCTL(2)                DOMAIN/IX SYS5                 SEMCTL(2)



ERRORS
     Semctl will fail if one or more of the following is true:

     [EINVAL]  Semid is not a valid semaphore identifier.

     [EINVAL]  Semnum is less than zero or greater than
               sem_nsems.

     [EINVAL]  Cmd is not a valid command.

     [EACCES]  Operation permission is denied to the calling pro-
               cess (see intro(2)).

     [ERANGE]  Cmd is SETVAL or SETALL and the value to which
               semval is to be set is greater than the system-
               imposed maximum.

     [EPERM]   Cmd is equal to IPC_RMID or IPC_SET and the effec-
               tive user ID of the calling process is not equal
               to that of super-user and it is not equal to the
               value of sem_perm.uid in the data structure asso-
               ciated with semid.

     [EFAULT]  Arg.buf points to an illegal address.

RELATED INFORMATION
     semget(2), semop(2)


























Printed 12/4/86                                          SEMCTL-3