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Linux 2.6 - man page for msgrcv (linux section 3posix)

MSGRCV(P)			    POSIX Programmer's Manual				MSGRCV(P)

NAME
       msgrcv - XSI message receive operation

SYNOPSIS
       #include <sys/msg.h>

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

DESCRIPTION
       The  msgrcv()  function operates on XSI message queues (see the Base Definitions volume of
       IEEE Std 1003.1-2001, Section 3.224, Message Queue). It is unspecified whether this  func-
       tion  interoperates  with  the  realtime  interprocess communication facilities defined in
       Realtime .

       The msgrcv() function shall read a message from the  queue  associated  with  the  message
       queue  identifier specified by msqid and place it in the user-defined buffer pointed to by
       msgp.

       The application shall ensure that the argument msgp points to a user-defined  buffer  that
       contains  first	a  field of type long specifying the type of the message, and then a data
       portion that holds the data bytes of the message. The structure below  is  an  example  of
       what this user-defined buffer might look like:

	      struct mymsg {
		  long	  mtype;     /* Message type. */
		  char	  mtext[1];  /* Message text. */
	      }

       The  structure  member  mtype  is  the received message's type as specified by the sending
       process.

       The structure member mtext is the text of the message.

       The argument msgsz specifies the size in bytes of mtext.  The received  message	shall  be
       truncated  to  msgsz  bytes  if it is larger than msgsz and (msgflg & MSG_NOERROR) is non-
       zero. The truncated part of the message shall be lost and no indication of the  truncation
       shall be given to the calling process.

       If the value of msgsz is greater than {SSIZE_MAX}, the result is implementation-defined.

       The argument msgtyp specifies the type of message requested as follows:

	* If msgtyp is 0, the first message on the queue shall be received.

	* If msgtyp is greater than 0, the first message of type msgtyp shall be received.

	* If  msgtyp  is  less	than 0, the first message of the lowest type that is less than or
	  equal to the absolute value of msgtyp shall be received.

       The argument 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 & IPC_NOWAIT) is non-zero, the calling thread shall return immediately with
	  a return value of -1 and errno set to [ENOMSG].

	* If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend execution until one  of
	  the following occurs:

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

	   * The  message  queue  identifier  msqid is removed from the system; when this occurs,
	     errno shall be set equal to [EIDRM] and -1 shall be returned.

	   * The calling thread receives a signal that is to be caught; in this case a message is
	     not  received  and  the calling thread resumes execution in the manner prescribed in
	     sigaction() .

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

	* msg_qnum shall be decremented by 1.

	* msg_lrpid shall be set equal to the process ID of the calling process.

	* msg_rtime shall be set equal to the current time.

RETURN VALUE
       Upon  successful  completion,  msgrcv()	shall return a value equal to the number of bytes
       actually placed into the buffer mtext. Otherwise, no message shall be  received,  msgrcv()
       shall return (ssize_t)-1, and errno shall be set to indicate the error.

ERRORS
       The msgrcv() function shall fail if:

       E2BIG  The value of mtext is greater than msgsz and (msgflg & MSG_NOERROR) is 0.

       EACCES Operation  permission is denied to the calling process; see XSI Interprocess Commu-
	      nication .

       EIDRM  The message queue identifier msqid is removed from the system.

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

       EINVAL msqid is not a valid message queue identifier.

       ENOMSG The queue does not contain a message of the desired type and (msgflg &  IPC_NOWAIT)
	      is non-zero.

       The following sections are informative.

EXAMPLES
   Receiving a Message
       The  following  example receives the first message on the queue (based on the value of the
       msgtyp argument, 0). The queue is identified by the  msqid  argument  (assuming	that  the
       value has previously been set). This call specifies that an error should be reported if no
       message is available, but not if the message is too large. The message size is  calculated
       directly using the sizeof operator.

	      #include <sys/msg.h>
	      ...
	      int result;
	      int msqid;
	      struct message {
		  long type;
		  char text[20];
	      } msg;
	      long msgtyp = 0;
	      ...
	      result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
		       msgtyp, MSG_NOERROR | IPC_NOWAIT);

APPLICATION USAGE
       The POSIX Realtime Extension defines alternative interfaces for interprocess communication
       (IPC). Application developers who need to use IPC should design their applications so that
       modules	using  the IPC routines described in XSI Interprocess Communication can be easily
       modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       XSI Interprocess Communication , Realtime , mq_close()  ,  mq_getattr()	,  mq_notify()	,
       mq_open()  , mq_receive() , mq_send() , mq_setattr() , mq_unlink() , msgctl() , msgget() ,
       msgsnd() , sigaction() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/msg.h>

COPYRIGHT
       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					MSGRCV(P)


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