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+.\" Copyright (c) 2005 David Xu <davidxu@FreeBSD.org>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer as
+.\"    the first lines of this file unmodified other than the possible
+.\"    addition of one or more copyright notices.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer in
+.\"    the documentation and/or other materials provided with the
+.\"    distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
+.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd November 29, 2005
+.Dt MQ_RECEIVE 2
+.Os
+.Sh NAME
+.Nm mq_receive , mq_timedreceive
+.Nd "receive a message from message queue (REALTIME)"
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In mqueue.h
+.Ft ssize_t
+.Fo mq_receive
+.Fa "mqd_t mqdes"
+.Fa "char *msg_ptr"
+.Fa "size_t msg_len"
+.Fa "unsigned *msg_prio"
+.Fc
+.Ft ssize_t
+.Fo mq_timedreceive
+.Fa "mqd_t mqdes"
+.Fa "char *msg_ptr"
+.Fa "size_t msg_len"
+.Fa "unsigned *msg_prio"
+.Fa "const struct timespec *abs_timeout"
+.Fc
+.Sh DESCRIPTION
+The
+.Fn mq_receive
+system call receives oldest of the highest priority message(s) from the
+message queue specified by
+.Fa mqdes .
+If the size of the buffer in bytes, specified by the
+.Fa msg_len
+argument, is less than the
+.Va mq_msgsize
+attribute of the message queue, the system call will fail and return an
+error.
+Otherwise, the selected message will be removed from the queue
+and copied to the buffer pointed to by the
+.Fa msg_ptr
+argument.
+.Pp
+If the argument
+.Fa msg_prio
+is not
+.Dv NULL ,
+the priority of the selected message will be stored in the
+location referenced by
+.Fa msg_prio .
+If the specified message queue is empty and
+.Dv O_NONBLOCK
+is not set in the message queue description associated with
+.Fa mqdes ,
+.Fn mq_receive
+will block until a message is enqueued on the message queue or until
+.Fn 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 will be selected to receive the message.
+Otherwise, it is unspecified which waiting thread receives the message.
+If the specified message queue is empty and
+.Dv O_NONBLOCK
+is set in the message queue description associated with
+.Fa mqdes ,
+no message
+will be removed from the queue, and
+.Fn mq_receive
+will return an error.
+.Pp
+The
+.Fn mq_timedreceive
+system call will receive the oldest of the highest priority messages from the
+message queue specified by
+.Fa mqdes
+as described for the
+.Fn mq_receive
+system call.
+However, if
+.Dv O_NONBLOCK
+was not specified when the message queue was opened via the
+.Fn mq_open
+system call, and no message exists on the queue to satisfy the receive, the wait
+for such a message will be terminated when the specified timeout expires.
+If
+.Dv O_NONBLOCK
+is set, this system call is equivalent to
+.Fn mq_receive .
+.Pp
+The timeout expires when the absolute time specified by
+.Fa abs_timeout
+passes, as measured by the clock on which timeouts are based (that is, when
+the value of that clock equals or exceeds
+.Fa abs_timeout ) ,
+or if the absolute time specified by
+.Fa abs_timeout
+has already been passed at the time of the call.
+.Pp
+The timeout is based on the
+.Dv CLOCK_REALTIME
+clock.
+.Sh RETURN VALUES
+Upon successful completion, the
+.Fn mq_receive
+and
+.Fn mq_timedreceive
+system calls return the length of the selected message in bytes and the
+message is removed from the queue.
+Otherwise, no message is removed
+from the queue, the system call returns a value of \-1,
+and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+The
+.Fn mq_receive
+and
+.Fn mq_timedreceive
+system calls
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+.Dv O_NONBLOCK
+flag is set in the message queue description associated with
+.Fa mqdes ,
+and the specified message queue is empty.
+.It Bq Er EBADF
+The
+.Fa mqdes
+argument is not a valid message queue descriptor open for reading.
+.It Bq Er EMSGSIZE
+The specified message buffer size,
+.Fa msg_len ,
+is less than the message size attribute of the message queue.
+.It Bq Er EINTR
+The
+.Fn mq_receive
+or
+.Fn mq_timedreceive
+operation was interrupted by a signal.
+.It Bq Er EINVAL
+The process or thread would have blocked, and the
+.Fa abs_timeout
+parameter specified a nanoseconds field value less than zero or greater
+than or equal to 1000 million.
+.It Bq Er ETIMEDOUT
+The
+.Dv O_NONBLOCK
+flag was not set when the message queue was opened, but no message arrived
+on the queue before the specified timeout expired.
+.El
+.Sh SEE ALSO
+.Xr mq_open 2 ,
+.Xr mq_send 2 ,
+.Xr mq_timedsend 2
+.Sh STANDARDS
+The
+.Fn mq_receive
+and
+.Fn mq_timedreceive
+system calls conform to
+.St -p1003.1-2004 .
+.Sh HISTORY
+Support for
+.Tn POSIX
+message queues first appeared in
+.Fx 7.0 .