mq_receive, mq_timedreceive — receive a message from a message queue


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

#include <time.h>
#include <mqueue.h>
ssize_t mq_timedreceive( mqd_t mqdes,
  char *restrict msg_ptr,
  size_t msg_len,
  unsigned int *restrict msg_prio,
  const struct timespec *restrict abs_timeout);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_POSIX_C_SOURCE >= 200112L
[Note] Note

Link with −lrt.


mq_receive() removes the oldest message with the highest priority from the message queue referred to by the message queue 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 or equal to the mq_msgsize attribute of the queue (see mq_getattr(3)). If msg_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 how long the call will block. This value is an absolute timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC), 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.


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.



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


The descriptor specified in mqdes was invalid or not opened for reading.


The call was interrupted by a signal handler; see signal(7).


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.


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


The call timed out before a message could be transferred.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
mq_receive(), mq_timedreceive() Thread safety MT-Safe


POSIX.1-2001, POSIX.1-2008.


On Linux, mq_timedreceive() is a system call, and mq_receive() is a library function layered on top of that system call.


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


This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (C) 2006 Michael Kerrisk <>

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.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

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