mq_receive(3) - NetBSD Manual Pages

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MQ_RECEIVE(3)           NetBSD Library Functions Manual          MQ_RECEIVE(3)


NAME

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


LIBRARY

     POSIX Real-time Library (librt, -lrt)


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 receives 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 fails and returns
     an error.  Otherwise, the selected message will be removed from the queue
     and copied to the buffer pointed to by the msg_ptr argument.

     If the argument msg_prio is not NULL, the priority of the selected mes-
     sage will 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() blocks
     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, then the thread of
     highest priority that has been waiting the longest will be selected to
     receive the message.  If the specified message queue is empty and
     O_NONBLOCK is set in the message queue description associated with mqdes,
     no message will be removed from the queue, and mq_receive() returns an
     error.

     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.

     The resolution of the timeout is based on the CLOCK_REALTIME clock.  The
     timespec argument is defined in the <time.h> header.

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


RETURN VALUES

     Upon successful completion, the mq_receive() and mq_timedreceive() func-
     tions return the length of the selected message in bytes, and the message
     is removed from the queue.  Otherwise, no message will be removed from
     the queue, the functions return a value of -1, and set the global vari-
     able errno to indicate the error.


ERRORS

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

     [EAGAIN]           O_NONBLOCK was set in the message description associ-
                        ated with mqdes, and the specified message queue is
                        empty.

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

     [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.

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

     [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.


SEE ALSO

     mq(3), mq_send(3)


STANDARDS

     These functions are expected to conform to the IEEE Std 1003.1-2001
     (``POSIX.1'') standard.


HISTORY

     The mq_receive() and mq_timedreceive() functions first appeared in
     NetBSD 5.0.


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 Spec-
     ifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electri-
     cal 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 ref-
     eree document.  The original Standard can be obtained online at
           http://www.opengroup.org/unix/online.html.

NetBSD 10.1                    November 18, 2015                   NetBSD 10.1