semctl(2)							     semctl(2)

NAME    [Toc]    [Back]

     semctl - semaphore	control	operations

SYNOPSIS    [Toc]    [Back]

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

     union semun {
	  int val;
	  struct semid_ds *buf;
	  ushort *array;
     };

     int semctl(int semid, int semnum, int cmd,	... /* union semun arg */);

DESCRIPTION    [Toc]    [Back]

     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
		     command is	successfully executed, the semadj value
		     corresponding to the specified semaphore in all processes
		     is	cleared.

	  GETPID     Return the	value of (int) 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}



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semctl(2)							     semctl(2)



	  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 access permission bits */

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

	  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 be
		     executed only by a	process	that has an effective user ID
		     equal to the value	of sem_perm.cuid or sem_perm.uid in
		     the data structure	associated with	semid or to a process
		     that has the super-user privilege.

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

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

     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.

     EINVAL	    cmd	is IPC_SET and sem_perm.uid or sem_perm.gid is not
		    valid.

     EOVERFLOW	    cmd	is IPC_STAT and	uid or gid is too large	to be stored
		    in the structure pointed to	by arg.buf.

     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 the value	of
		    sem_perm.cuid or sem_perm.uid in the data structure
		    associated with semid and the calling process does not
		    have super-user privilege.

     EFAULT	    arg<b>.buf points to an illegal address.







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semctl(2)							     semctl(2)



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

DIAGNOSTICS    [Toc]    [Back]

     Upon successful completion, the value returned depends on cmd as follows:
	  GETVAL    the	value of semval
	  GETPID    the	value of (int) 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.


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