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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH mq_send 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
mq_send, mq_timedsend \- send a message to a message queue
.SH LIBRARY
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.PP
.BI "int mq_send(mqd_t " mqdes ", const char " msg_ptr [. msg_len ],
.BI "              size_t " msg_len ", unsigned int " msg_prio );
.PP
.B #include <time.h>
.B #include <mqueue.h>
.PP
.BI "int mq_timedsend(mqd_t " mqdes ", const char " msg_ptr [. msg_len ],
.BI "              size_t " msg_len ", unsigned int " msg_prio ,
.BI "              const struct timespec *" abs_timeout );
.fi
.PP
.ad l
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR mq_timedsend ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.BR mq_send ()
adds the message pointed to by
.I msg_ptr
to the message queue referred to by the message queue descriptor
.IR mqdes .
The
.I msg_len
argument specifies the length of the message pointed to by
.IR msg_ptr ;
this length must be less than or equal to the queue's
.I mq_msgsize
attribute.
Zero-length messages are allowed.
.PP
The
.I 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
.BR mq_overview (7)
for details on the range for the message priority.
.PP
If the message queue is already full
(i.e., the number of messages on the queue equals the queue's
.I mq_maxmsg
attribute), then, by default,
.BR 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
.B O_NONBLOCK
flag is enabled for the message queue description,
then the call instead fails immediately with the error
.BR EAGAIN .
.PP
.BR mq_timedsend ()
behaves just like
.BR mq_send (),
except that if the queue is full and the
.B O_NONBLOCK
flag is not enabled for the message queue description, then
.I 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 a
.BR timespec (3)
structure.
.PP
If the message queue is full,
and the timeout has already expired by the time of the call,
.BR mq_timedsend ()
returns immediately.
.SH RETURN VALUE
On success,
.BR mq_send ()
and
.BR mq_timedsend ()
return zero; on error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
The queue was full, and the
.B O_NONBLOCK
flag was set for the message queue description referred to by
.IR mqdes .
.TP
.B EBADF
The descriptor specified in
.I mqdes
was invalid or not opened for writing.
.TP
.B EINTR
The call was interrupted by a signal handler; see
.BR signal (7).
.TP
.B EINVAL
The call would have blocked, and
.I abs_timeout
was invalid, either because
.I tv_sec
was less than zero, or because
.I tv_nsec
was less than zero or greater than 1000 million.
.TP
.B EMSGSIZE
.I msg_len
was greater than the
.I mq_msgsize
attribute of the message queue.
.TP
.B ETIMEDOUT
The call timed out before a message could be transferred.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR mq_send (),
.BR mq_timedsend ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH VERSIONS
On Linux,
.BR mq_timedsend ()
is a system call, and
.BR mq_send ()
is a library function layered on top of that system call.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH SEE ALSO
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_open (3),
.BR mq_receive (3),
.BR mq_unlink (3),
.BR timespec (3),
.BR mq_overview (7),
.BR time (7)