mq_send(3) — Linux manual pages

Linux manual pages	Section 3

Name

mq_send, mq_timedsend — send a message to a message queue

Synopsis

#include <mqueue.h>

`mqd_t mq_send(`	mqd_t `mqdes`,
	const char *`msg_ptr`,
	size_t `msg_len`,
	unsigned `msg_prio)`;


#define _XOPEN_SOURCE 600
#include <time.h>
#include <mqueue.h>

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

	Note
	Link with `−lrt`.

DESCRIPTION

mq_send() adds the message pointed to by msg_ptr to the message queue referred to by the 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.

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 a ceiling on the time for which the call will block. This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC), and it is 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.

RETURN VALUE

On success, mq_send() and mq_timedsend() return zero; 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; see signal(7).
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 greater 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.

NOTES

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

COLOPHON

This page is part of release 3.24 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>

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