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 (con-
       sult 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 priority 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 argument.

       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 message 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  sig-
       nal.  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 timeout shall be based on the system clock as returned by the time() function.

       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 message can be removed from the message queue immedi-
       ately.  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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                MQ_RECEIVE(3P)