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

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

NAME
       mq_receive, mq_timedreceive - receive a message from  a	message	 queue
       (REALTIME)

SYNOPSIS
       #include <mqueue.h>

       ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
	      unsigned *msg_prio);

       #include <mqueue.h>
       #include <time.h>
       ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
	      size_t msg_len, unsigned *restrict msg_prio,
	      const struct timespec *restrict abs_timeout);

DESCRIPTION
       The  mq_receive() function shall receive the oldest of the highest pri‐
       ority message(s) from the message queue specified by mqdes. If the size
       of the buffer in bytes, specified by the msg_len argument, is less than
       the mq_msgsize attribute of the message queue, the function shall  fail
       and  return  an error. Otherwise, the selected message shall be removed
       from the queue and copied to the buffer pointed to by the msg_ptr argu‐
       ment.

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

       If the argument msg_prio is not NULL, the priority of the selected mes‐
       sage shall be stored in the location referenced by msg_prio.

       If  the	specified  message queue is empty and O_NONBLOCK is not set in
       the message queue description associated with mqdes, mq_receive() shall
       block  until  a	message	 is  enqueued  on  the	message queue or until
       mq_receive() is interrupted by a signal. If more	 than  one  thread  is
       waiting	to  receive a message when a message arrives at an empty queue
       and the Priority Scheduling option is supported,	 then  the  thread  of
       highest priority that has been waiting the longest shall be selected to
       receive the message. Otherwise, it is unspecified which waiting	thread
       receives	 the  message.	If  the	 specified  message queue is empty and
       O_NONBLOCK is set in the	 message  queue	 description  associated  with
       mqdes,  no  message  shall  be removed from the queue, and mq_receive()
       shall return an error.

       The mq_timedreceive() function shall receive the oldest of the  highest
       priority	 messages  from	 the  message  queue  specified	 by  mqdes  as
       described for the mq_receive() function. However, if O_NONBLOCK was not
       specified when the message queue was opened via the mq_open() function,
       and no message exists on the queue to satisfy the receive, the wait for
       such  a message shall be terminated when the specified timeout expires.
       If O_NONBLOCK is set, this function is equivalent to mq_receive().

       The timeout expires when the absolute  time  specified  by  abs_timeout
       passes,	as measured by the clock on which timeouts are based (that is,
       when the value of that clock equals or exceeds abs_timeout), or if  the
       absolute	 time  specified by abs_timeout has already been passed at the
       time of the call.

       If the Timers option is supported, the timeout shall be	based  on  the
       CLOCK_REALTIME  clock; if the Timers option is not supported, the time‐
       out shall be based on the system clock as returned by the time()	 func‐
       tion.

       The  resolution	of the timeout shall be the resolution of the clock on
       which it is based. The timespec argument is  defined  in	 the  <time.h>
       header.

       Under no circumstance shall the operation fail with a timeout if a mes‐
       sage can be removed from the message queue immediately.	 The  validity
       of  the	abs_timeout  parameter need not be checked if a message can be
       removed from the message queue immediately.

RETURN VALUE
       Upon successful completion,  the	 mq_receive()	and  mq_timedreceive()
       functions  shall return the length of the selected message in bytes and
       the message shall be removed from  the  queue.  Otherwise,  no  message
       shall  be removed from the queue, the functions shall return a value of
       -1, and set errno to indicate the error.

ERRORS
       The mq_receive()	 and mq_timedreceive() functions shall fail if:

       EAGAIN O_NONBLOCK was set in the message	 description  associated  with
	      mqdes, and the specified message queue is empty.

       EBADF  The  mqdes argument is not a valid message queue descriptor open
	      for reading.

       EMSGSIZE
	      The specified message buffer size, msg_len,  is  less  than  the
	      message size attribute of the message queue.

       EINTR  The mq_receive()	or mq_timedreceive() operation was interrupted
	      by a signal.

       EINVAL The process or thread would have blocked,	 and  the  abs_timeout
	      parameter	 specified a nanoseconds field value less than zero or
	      greater than or equal to 1000 million.

       ETIMEDOUT
	      The O_NONBLOCK flag was not  set	when  the  message  queue  was
	      opened, but no message arrived on the queue before the specified
	      timeout expired.

       The mq_receive()	 and mq_timedreceive() functions may fail if:

       EBADMSG
	      The implementation has detected a data corruption	 problem  with
	      the message.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       mq_open(),  mq_send(),  mq_timedsend(),	msgctl(),  msgget(), msgrcv(),
       msgsnd(), time(), the Base Definitions volume of	 IEEE Std 1003.1-2001,
       <mqueue.h>, <time.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 original Standard can be  obtained	online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003			MQ_RECEIVE(3P)
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