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 semop(2)							    semop(2)




 NAME
      semop - semaphore operations

 SYNOPSIS
      #include <&lt&lt&lt;sys/sem.h>&gt&gt&gt;

      int semop(
	   int semid,
	   struct sembuf *sops,
	   size_t nsops
      );

 DESCRIPTION
      semop() is used to atomically perform an array of semaphore operations
      on the set of semaphores associated with the semaphore identifier
      specified by semid.  sops is a pointer to the array of semaphore-
      operation structures.  nsops is the number of such structures in the
      array.  The contents of each structure includes the following members:

	   ushort    sem_num;	   /* semaphore number */
	   short     sem_op	   /* semaphore operation */
	   short     sem_flg;	   /* operation flags */

      Each semaphore operation specified by sem_op is performed on the
      corresponding semaphore specified by semid and sem_num.  Semaphore
      array operations are atomic in that none of the semaphore operations
      are performed until blocking conditions on all of the semaphores in
      the array have been removed.

      sem_op specifies one of three semaphore operations as follows:

		If sem_op is a negative integer, one of the following
		occurs:

		     If semval (see semaphore identifier in glossary(9)) is
		     greater than or equal to the absolute value of sem_op,
		     the absolute value of sem_op is subtracted from semval.
		     Also, if (sem_flg & SEM_UNDO) is ``true'', the absolute
		     value of sem_op is added to the calling process's
		     semadj value (see glossary(9) and exit(2)) for the
		     specified semaphore.

		     If semval is less than the absolute value of sem_op and
		     (sem_flg & IPC_NOWAIT) is ``true'', semop() returns
		     immediately.

		     If semval is less than the absolute value of sem_op and
		     (sem_flg & IPC_NOWAIT) is ``false'', semop() increments
		     the semncnt associated with the specified semaphore and
		     suspend execution of the calling process until one of
		     the following conditions occur:



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 semop(2)							    semop(2)




			  semval becomes greater than or equal to the
			  absolute value of sem_op.  When this occurs, the
			  value of semncnt associated with the specified
			  semaphore is decremented, the absolute value of
			  sem_op is subtracted from semval and, if (sem_flg
			  & SEM_UNDO) is ``true'', the absolute value of
			  sem_op is added to the calling process's semadj
			  value for the specified semaphore.

			  The semid for which the calling process is
			  awaiting action is removed from the system (see
			  semctl(2)).  When this occurs, errno is set equal
			  to EIDRM, and a value of -1 is returned.

			  The calling process receives a signal that is to
			  be caught.  When this occurs, the value of semncnt
			  associated with the specified semaphore is
			  decremented, and the calling process resumes
			  execution in the manner prescribed in signal(5).

		If sem_op is a positive integer, the value of sem_op is
		added to semval and, if (sem_flg & SEM_UNDO) is ``true'',
		the value of sem_op is subtracted from the calling process's
		semadj value for the specified semaphore.

		If sem_op is zero, one of the following occurs:

		     If semval is zero, semop() proceeds to the next
		     semaphore operation specified by sops, or returns
		     immediately if this is the last operation.

		     If semval is not equal to zero and (sem_flg &
		     IPC_NOWAIT) is ``true'', semop() returns immediately.

		     If semval is not equal to zero and (sem_flg &
		     IPC_NOWAIT) is ``false'', semop() increments the
		     semzcnt associated with the specified semaphore and
		     suspends execution of the calling process until one of
		     the following occurs:

			  semval becomes zero, at which time the value of
			  semzcnt associated with the specified semaphore is
			  decremented.

			  The semid for which the calling process is
			  awaiting action is removed from the system.  When
			  this occurs, errno is set equal to EIDRM, and a
			  value of -1 is returned.

			  The calling process receives a signal that is to
			  be caught.  When this occurs, the value of semzcnt



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 semop(2)							    semop(2)




			  associated with the specified semaphore is
			  decremented, and the calling process resumes
			  execution in the manner prescribed in signal(5).

 EXAMPLES
      The following call to semop() atomically performs a "P" or "get"
      operation on the second semaphore in the semaphore set and a "V" or
      "release" operation on the third semaphore in the set.  This example
      assumes the process has a valid semid which represents a set of 4
      semaphores as shown on the semget(2) manual page.	 It also assumes
      that the semvals of the semaphores in the set have been initialized as
      shown in the semctl(2) manual entry.

	   struct sembuf sops[4];

	   sops[0].sem_num = 1;
	   sops[0].sem_op  = -1;      /* P (get) */
	   sops[0].sem_flg = 0;
	   sops[1].sem_num = 2;
	   sops[1].sem_op  =  1;      /* V (release) */
	   sops[1].sem_flg = 0;

	   semop (mysemid, sops, 2);

 RETURN VALUE
      If semop() returns due to the receipt of a signal, a value of -1 is
      returned to the calling process and errno is set to EINTR.  If it
      returns due to the removal of a semid from the system, a value of -1
      is returned and errno is set to EIDRM.

      Upon successful completion, a non-negative value is returned.
      Otherwise, a value of -1 is returned and errno is set to indicate the
      error.

 ERRORS
      semop() fails if one or more of the following is true for any of the
      semaphore operations specified by sops:

	   [EINVAL]	  semid is not a valid semaphore identifier.

	   [EFBIG]	  sem_num is less than zero or greater than or equal
			  to the number of semaphores in the set associated
			  with semid.

	   [E2BIG]	  nsops is greater than the system-imposed maximum.

	   [EACCES]	  Operation permission is denied to the calling
			  process (see glossary(9)).

	   [EAGAIN]	  The operation would result in suspension of the
			  calling process but (sem_flg & IPC_NOWAIT) is



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 semop(2)							    semop(2)




			  ``true''.

	   [ENOSPC]	  The limit on the number of individual	 processes
			  requesting an SEM_UNDO would be exceeded.

	   [EINVAL]	  The number of individual semaphores for which the
			  calling process requests a SEM_UNDO would exceed
			  the limit.

	   [ERANGE]	  An operation would cause a semval to overflow the
			  system-imposed limit.

	   [ERANGE]	  An operation would cause a semadj value to
			  overflow the system-imposed limit.

	   [EFAULT]	  sops points to an illegal address.  The reliable
			  detection of this error will be implementation
			  dependent.

      Upon successful completion, the value of sempid for each semaphore
      specified in the array pointed to by sops is set equal to the process
      ID of the calling process.  The value of sem_otime in the data
      structure associated with the semaphore identifier will be set to the
      current time.

 WARNINGS
      Check all references to signal(5) for appropriateness on systems that
      support sigvector(2).  sigvector(2) can affect the behavior described
      on this page.

 SEE ALSO
      ipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C),
      signal(5).

 STANDARDS CONFORMANCE
      semop(): SVID2, SVID3, XPG2, XPG3, XPG4


















 Hewlett-Packard Company	    - 4 -   HP-UX Release 11i: November 2000