MQ_RECEIVE(3)

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

LIBRARY
librt

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 message 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() functions 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 variable errno to indicate the error.

ERRORS
The mq_receive() and mq_timedreceive() functions 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.

[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 p1003.1-2001 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 Specifications Issue 6, Copyright (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.opengroup.org/unix/online.html.