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 msgop(2)							    msgop(2)




 NAME
      msgsnd, msgrcv - message operations

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

      int msgsnd(
	  int	      msqid,
	  const void *msgp,
	  size_t      msgsz,
	  int	      msgflg
      );

      int msgrcv(
	  int	  msqid,
	  void	 *msgp,
	  size_t  msgsz,
	  long	  msgtyp,
	  int	  msgflg
      );

 DESCRIPTION
      The msgsnd() system call sends a message to the queue associated with
      the message queue identifier specified by msqid.

      msgp points to a user-defined buffer that must contain first a field
      of type long that specifies the type of the message, followed by a
      data portion that will hold the data bytes of the message.  The
      structure below is an example of what this user-defined buffer might
      look like:

	   long	 mtype;	    /* message type */
	   char	 mtext[];   /* message text */

      mtype is a positive integer that can be used by the receiving process
      for message selection (see msgrcv() below).  mtext is any text of
      length msgsz bytes.  msgsz can range from 0 to a system-imposed
      maximum.

      msgflg specifies the action to be taken if one or more of the
      following is true:

	+  The number of bytes already on the queue is equal to msg_qbytes
	   (see message queue identifier in glossary(9)).

	+  The total number of messages on all queues system-wide is equal
	   to the system-imposed limit.

      These actions are as follows:





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 msgop(2)							    msgop(2)




	   If (msgflg  &&amp&amp&amp; IPC_NOWAIT) is true, the message is not sent and
	   the calling process returns immediately.

	   If (msgflg  &&amp&amp&amp; IPC_NOWAIT) is false, the calling process suspends
	   execution until one of the following occurs:

	     +	The condition responsible for the suspension no longer
		exists, in which case the message is sent.

	     +	msqid is removed from the system (see msgctl(2)).  When this
		occurs, errno is set to [EIDRM] and a value of -1 is
		returned.

	     +	The calling process receives a signal to be caught.  In this
		case, the message is not sent and the calling process
		resumes execution in the manner prescribed in signal(5).

      Upon successful completion, the following actions are taken with
      respect to the data structure associated with msqid:

	   msg_qnum is incremented by 1.

	   msg_lspid is set to the process ID of the calling process.

	   msg_stime is set to the current time.

      The msgrcv() system call reads a message from the queue associated
      with the message queue identifier specified by msqid and places it in
      the structure pointed to by msgp.	 This structure is composed of the
      following members:

	   long	 mtype;	    /* message type */
	   char	 mtext[];   /* message text */

      mtype is the received message's type as specified by the sending
      process.	mtext is the text of the message.  msgsz specifies the size
      in bytes of mtext . The received message is truncated to msgsz bytes
      if it is larger than msgsz and (msgflg  &&amp&amp&amp; MSG_NOERROR) is true.  The
      truncated part of the message is lost and no indication of the
      truncation is given to the calling process.

      msgtyp specifies the type of message requested as follows:

	   msgtyp = 0  First message on the queue is received.

	   msgtyp > 0  First message of type msgtyp is received.

	   msgtyp < 0  First message of the lowest type that is less than or
		       equal to the absolute value of msgtyp is received.





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 msgop(2)							    msgop(2)




      msgflg specifies the action to be taken if a message of the desired
      type is not on the queue.	 These are as follows:

	   If (msgflg  &&amp&amp&amp; IPC_NOWAIT) is true, the calling process returns
	   immediately with a value of -1 and errno set to [ENOMSG].

	   If (msgflg  &&amp&amp&amp; IPC_NOWAIT) is false, the calling process suspends
	   execution until one of the following occurs:

	     +	A message of the desired type is placed on the queue.

	     +	msqid is removed from the system.  When this occurs, errno
		is set to [EIDRM] and a value of -1 is returned.

	     +	The calling process receives a signal that is to be caught.
		In this case, a message is not received and the calling
		process resumes execution in the manner prescribed in
		signal(5)).

      Upon successful completion, the following actions are taken with
      respect to the data structure associated with msqid.

	   msg_qnum is decremented by 1.

	   msg_lrpid is set to the process ID of the calling process.

	   msg_rtime is set to the current time.

 RETURN VALUES
      Upon successful completion, the return value is as follows:

	   msgsnd() returns a value of 0.

	   msgrcv() returns a value equal to the number of bytes actually
	   placed into mtext.

      Otherwise, a value of -1 is returned and errno is set to indicate the
      error.

 ERRORS
      If msgrcv() fails, errno is set to one of the following values.

	   [E2BIG]	  mtext is greater than msgsz and (msgflg  &&amp&amp&amp;
			  MSG_NOERROR) is false.

	   [EACCES]	  Operation permission is denied to the calling
			  process.

	   [EFAULT]	  msgp points to an illegal address.  The reliable
			  detection of this error is implementation
			  dependent.



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 msgop(2)							    msgop(2)




	   [EIDRM]	  The message queue identifier msqid has been
			  removed from the system.

	   [EINTR]	  The function msgrcv() was interrupted by a signal.

	   [EINVAL]	  msqid is not a valid message queue identifier.

	   [EINVAL]	  msgsz is less than 0.

	   [ENOMSG]	  The queue does not contain a message of the
			  desired type and (msgflg  &&amp&amp&amp; IPC_NOWAIT) is true.

      If msgsnd() fails, errno is set to one of the following values.

	   [EACCES]	  Operation permission is denied to the calling
			  process.

	   [EAGAIN]	  The message cannot be sent for one of the reasons
			  cited above and (msgflg  &&amp&amp&amp; IPC_NOWAIT) is true.

	   [EFAULT]	  msgp points to an illegal address.  The reliable
			  detection of this error is implementation
			  dependent.

	   [EIDRM]	  The message queue identifier msqid has been
			  removed from the system.

	   [EINTR]	  msgsnd() was interrupted by a signal.

	   [EINVAL]	  msqid is not a valid message queue identifier.

	   [EINVAL]	  mtype is less than 1.

	   [EINVAL]	  msgsz is less than zero or greater than the
			  system-imposed limit.

 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), msgctl(2), msgget(2), stdipc(3C), signal(5).

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

      msgsnd(): SVID2, SVID3, XPG2, XPG3, XPG4






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