mq_send, mq_timedsend — send a message to a message queue


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

#include <time.h>
#include <mqueue.h>
int mq_timedsend( mqd_t mqdes,
  const char *msg_ptr,
  size_t msg_len,
  unsigned int msg_prio,
  const struct timespec *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_send() adds the message pointed to by msg_ptr to the message queue referred to by the message queue descriptor mqdes. The msg_len argument specifies the length of the message pointed to by msg_ptr; this length must be less than or equal to the queue's mq_msgsize attribute. Zero-length messages are allowed.

The msg_prio argument is a nonnegative integer that specifies the priority of this message. Messages are placed on the queue in decreasing order of priority, with newer messages of the same priority being placed after older messages with the same priority. See mq_overview(7) for details on the range for the message priority.

If the message queue is already full (i.e., the number of messages on the queue equals the queue's mq_maxmsg attribute), then, by default, mq_send() blocks until sufficient space becomes available to allow the message to be queued, or until 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_timedsend() behaves just like mq_send(), except that if the queue is full 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 the message queue is full, and the timeout has already expired by the time of the call, mq_timedsend() returns immediately.


On success, mq_send() and mq_timedsend() return zero; on error, −1 is returned, with errno set to indicate the error.



The queue was full, 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 writing.


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 greater 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_send(), mq_timedsend() Thread safety MT-Safe


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


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


mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_receive(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 <>

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