Name

mq_receive, mq_timedreceive — receive a message from a message queue

Synopsis

#include <mqueue.h>
ssize_t mq_receive( mqd_t   mqdes,
  char *  msg_ptr,
  size_t   msg_len,
  unsigned *  msg_prio);

#define _XOPEN_SOURCE 600
#include <time.h>
#include <mqueue.h>
ssize_t mq_timedreceive( mqd_t   mqdes,
  char *  msg_ptr,
  size_t   msg_len,
  unsigned *  msg_prio,
  const struct timespec *  abs_timeout);
[Note] Note

Link with −lrt.

DESCRIPTION

mq_receive() removes the oldest message with the highest priority from the message queue referred to by the descriptor mqdes, and places it in the buffer pointed to by msg_ptr. The msg_len argument specifies the size of the buffer pointed to by msg_ptr; this must be greater than the mq_msgsize attribute of the queue (see mq_getattr(3)). If prio is not NULL, then the buffer to which it points is used to return the priority associated with the received message.

If the queue is empty, then, by default, mq_receive() blocks until a message becomes available, or the call is interrupted by a signal handler. If the O_NONBLOCK flag is enabled for the message queue description, then the call instead fails immediately with the error EAGAIN.

mq_timedreceive() behaves just like mq_receive(), except that if the queue is empty and the O_NONBLOCK flag is not enabled for the message queue description, then abs_timeout points to a structure which specifies a ceiling on the time for which the call will block. This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch (midnight on the morning of 1 January 1970), specified in the following structure:

struct timespec {
  time_t   tv_sec;
/* seconds */
  long   tv_nsec;
/* nanoseconds */
};

If no message is available, and the timeout has already expired by the time of the call, mq_timedreceive() returns immediately.

RETURN VALUE

On success, mq_receive() and mq_timedreceive() return the number of bytes in the received message; on error, −1 is returned, with errno set to indicate the error.

ERRORS

EAGAIN

The queue was empty, and the O_NONBLOCK flag was set for the message queue description referred to by mqdes.

EBADF

The descriptor specified in mqdes was invalid.

EINTR

The call was interrupted by a signal handler.

EINVAL

The call would have blocked, and abs_timeout was invalid, either because tv_sec was less than zero, or because tv_nsec was less than zero or greater than 1000 million.

EMSGSIZE

msg_len was less than the mq_msgsize attribute of the message queue.

ETIMEDOUT

The call timed out before a message could be transferred.

CONFORMING TO

POSIX.1-2001.

SEE ALSO

mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_send(3), mq_unlink(3), feature_test_macros(7), mq_overview(7)

COLOPHON

This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


  t

Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.