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SEMOP(2)		       System Calls Manual			SEMOP(2)

NAME
     semop -- atomic array of 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
     semop(int semid, struct sembuf *array, size_t nops);

DESCRIPTION
     The  semop()  system call atomically performs the array of operations indi-
     cated by array on the semaphore set indicated  by	semid.	 The  length  of
     array  is indicated by nops.  Each operation is encoded in a struct sembuf,
     which is defined as follows:

     struct sembuf {
	     u_short sem_num;	     /* semaphore # */
	     short   sem_op;	     /* semaphore operation */
	     short   sem_flg;	     /* operation flags */
     };

     For each element in array, sem_op and sem_flg determine an operation to  be
     performed	on semaphore number sem_num in the set.  The values SEM_UNDO and
     IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify the  be-
     havior of the given operation.

     The operation performed depends as follows on the value of sem_op:

     *	 When sem_op is positive and the process has alter permission, the sema-
	 phore's  value is incremented by sem_op's value.  If SEM_UNDO is speci-
	 fied, the semaphore's adjust on exit value is decremented  by	sem_op's
	 value.   A positive value for sem_op generally corresponds to a process
	 releasing a resource associated with the semaphore.

     *	 The behavior when sem_op is negative and the process has alter  permis-
	 sion, depends on the current value of the semaphore:

	 *   If  the  current value of the semaphore is greater than or equal to
	     the absolute value of sem_op, then the value is decremented by  the
	     absolute  value  of  sem_op.   If	SEM_UNDO is specified, the sema-
	     phore's adjust on exit value is incremented by the  absolute  value
	     of sem_op.

	 *   If  the  current  value  of the semaphore is less than the absolute
	     value of sem_op, one of the following happens:

	     *	 If IPC_NOWAIT was specified, then semop()  returns  immediately
		 with a return value of EAGAIN.

	     *	 Otherwise, the calling process is put to sleep until one of the
		 following conditions is satisfied:

		 *   Some  other process removes the semaphore with the IPC_RMID
		     option of semctl(2).  In this case, semop() returns immedi-
		     ately with a return value of EIDRM.

		 *   The process receives a signal that is  to	be  caught.   In
		     this  case, the process will resume execution as defined by
		     sigaction(2).

		 *   The semaphore's value is greater than or equal to	the  ab-
		     solute  value of sem_op.  When this condition becomes true,
		     the semaphore's value is decremented by the absolute  value
		     of  sem_op,  the semaphore's adjust on exit value is incre-
		     mented by the absolute value of sem_op.

	 A negative value for sem_op generally means that a process  is  waiting
	 for a resource to become available.

     *	 When  sem_op  is  zero  and the process has read permission, one of the
	 following will occur:

	 *   If the current value of the semaphore is equal to zero then semop()
	     can return immediately.

	 *   If IPC_NOWAIT was specified, then semop() returns immediately  with
	     a return value of EAGAIN.

	 *   Otherwise,  the  calling  process	is put to sleep until one of the
	     following conditions is satisfied:

	     *	 Some other process removes the semaphore with the IPC_RMID  op-
		 tion  of  semctl(2).  In this case, semop() returns immediately
		 with a return value of EIDRM.

	     *	 The process receives a signal that is to be  caught.	In  this
		 case,	the  process  will resume execution as defined by sigac-
		 tion(2).

	     *	 The semaphore's value becomes zero.

     For each semaphore a process has in use, the kernel maintains an "adjust on
     exit" value, as alluded to earlier.  When a process exits, either voluntar-
     ily or involuntarily, the adjust on exit value for each semaphore is  added
     to  the  semaphore's  value.  This can be used to ensure that a resource is
     released if a process terminates unexpectedly.

RETURN VALUES
     The semop() function returns the  value  0  if  successful;  otherwise  the
     value  -1	is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The semop() system call will fail if:

     [EINVAL]		No semaphore set corresponds to semid,	or  the  process
			would  exceed the system-defined limit for the number of
			per-process SEM_UNDO structures.

     [EACCES]		Permission denied due to mismatch between operation  and
			mode of semaphore set.

     [EAGAIN]		The semaphore's value would have resulted in the process
			being put to sleep and IPC_NOWAIT was specified.

     [E2BIG]		Too many operations were specified.  [SEMOPM]

     [EFBIG]		sem_num was not in the range of valid semaphores for the
			set.

     [EIDRM]		The semaphore set was removed from the system.

     [EINTR]		The semop() system call was interrupted by a signal.

     [ENOSPC]		The system SEM_UNDO pool [SEMMNU] is full.

     [ERANGE]		The  requested	operation  would  cause either the sema-
			phore's current value [SEMVMX] or  its	adjust	on  exit
			value [SEMAEM] to exceed the system-imposed limits.

SEE ALSO
     semctl(2), semget(2), sigaction(2)

BUGS
     The semop() system call may block waiting for memory even if IPC_NOWAIT was
     specified.

FreeBSD ports 15.quarterly     September 22, 1995			SEMOP(2)

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