MSGRCV(3P)						     POSIX Programmer's Manual							MSGRCV(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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 function 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 exe-
	     cution 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 Communication .

       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 modi-
       fied 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 Technol-
       ogy -- 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 original Standard can  be  obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								MSGRCV(3P)