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



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

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 abstime);

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

       The timeout shall be based on the CLOCK_REALTIME clock.  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 immediately. The validity
       of the abstime 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
       These 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 abstime
              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.

       These 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(), msgctl(), msgget(), msgrcv(), msgsnd(), time()

       The Base Definitions volume of POSIX.1-2017, <mqueue.h>, <time.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                        MQ_RECEIVE(3)
