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SEMCTL(2)		  FreeBSD System Calls Manual		     SEMCTL(2)

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.	Out-
		  standing 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 sema-
			phores 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.

FreeBSD	9.2		      September	12, 1995		   FreeBSD 9.2

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | BUGS

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