SEMCTL(2)                            SysV                            SEMCTL(2)



NAME
     semctl - semaphore control operations

SYNOPSIS
     #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 a variety of semaphore control operations as specified by
     cmd.

     The following cmds are executed with respect to 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 cmd is
                successfully executed, the semadj value corresponding to the
                specified 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 semval 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 cmd is successfully executed the semadj
                values corresponding to each specified semaphore in all
                processes are cleared.

     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 contents 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 cmd 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.cuid or 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 cmd 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.cuid or sem_perm.uid in
                the data structure associated with semid.

ERRORS
     semctl fails if one or more of the following are true:

     [EINVAL]   semid is not a valid semaphore identifier.

     [EINVAL]   semnum is less than 0 or greater than sem_nsems.

     [EINVAL]   cmd is not a valid command.

     [EACCES]   Operation permission is denied to the calling process (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 effective user ID
                of the calling process is not equal to that of super-user or
                to the value of sem_perm.cuid or sem_perm.uid in the data
                structure associated with semid.

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

SEE ALSO
     intro(2), semget(2), semop(2).

DIAGNOSTICS
     Upon successful completion, the value returned depends on cmd as follows:

     GETVAL      The value of semval.
     GETPID      The value of sempid.
     GETNCNT     The value of semncnt.
     GETZCNT     The value of semzcnt.
     All others  A value of 0.

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