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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.

     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 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 5.4		      September 12, 1995		   FreeBSD 5.4

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