mq_receive man page on FreeBSD

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MQ_RECEIVE(2)		    BSD System Calls Manual		 MQ_RECEIVE(2)

NAME
     mq_receive, mq_timedreceive — receive a message from message queue (REAL‐
     TIME)

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);

     ssize_t
     mq_timedreceive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
	 unsigned *msg_prio, const struct timespec *abs_timeout);

DESCRIPTION
     The mq_receive() system call receives oldest of the highest priority mes‐
     sage(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 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 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 spec‐
     ified message queue is empty and O_NONBLOCK is not set in the message
     queue description associated with mqdes, mq_receive() will block until a
     message is enqueued on the message queue or until mq_receive() is inter‐
     rupted by a signal.  If more than one thread is waiting to receive a mes‐
     sage 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 O_NONBLOCK is set in the message
     queue description associated with mqdes, no message will be removed from
     the queue, and mq_receive() will return an error.

     The mq_timedreceive() system call will receive the oldest of the highest
     priority messages from the message queue specified by mqdes as described
     for the mq_receive() system call.	However, if O_NONBLOCK was not speci‐
     fied when the message queue was opened via the 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
     O_NONBLOCK is set, this system call 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.

     The timeout is based on the CLOCK_REALTIME clock.

RETURN VALUES
     Upon successful completion, the mq_receive() and 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
     errno is set to indicate the error.

ERRORS
     The mq_receive() and mq_timedreceive() system calls will fail if:

     [EAGAIN]		O_NONBLOCK flag is set in the message queue descrip‐
			tion 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.

SEE ALSO
     mq_open(2), mq_send(2), mq_timedsend(2)

STANDARDS
     The mq_receive() and mq_timedreceive() system calls conform to IEEE Std
     1003.1-2004 (“POSIX.1”).

HISTORY
     Support for POSIX message queues first appeared in FreeBSD 7.0.

COPYRIGHT
     Portions of this text are reprinted and reproduced in electronic form
     from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology
     -- Portable Operating System Interface (POSIX), The Open Group Base Spec‐
     ifications Issue 6, Copyright (C) 2001-2004 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.

BSD			       November 29, 2005			   BSD
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