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SEMCTL(2)			     BSD 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 indi-
     cated 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 sema-
		  phore 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 */
	     long    sem_pad1;	     /* SVABI/386 says I need this here */
	     time_t  sem_ctime;      /* last change time */
				     /* Times measured in secs since */
				     /* 00:00:00 GMT, Jan. 1, 1970 */
	     long    sem_pad2;	     /* SVABI/386 says I need this here */
	     long    sem_pad3[4];    /* SVABI/386 says I need this here */
     };

RETURN VALUES
     On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT, semctl() returns the cor-
     responding 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 sema-
			phore 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.

BSD					September 12, 1995				      BSD
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