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MQ_SEND(3P) POSIX Programmer's Manual MQ_SEND(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
NAME
mq_send, mq_timedsend - send a message to a message queue (REALTIME)
SYNOPSIS
#include <mqueue.h>
int mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len,
unsigned msg_prio);
#include <mqueue.h>
#include <time.h>
int mq_timedsend(mqd_t mqdes, const char *msg_ptr, size_t msg_len,
unsigned msg_prio, const struct timespec *abs_timeout);
DESCRIPTION
The mq_send() function shall add the message pointed to by the argument msg_ptr to the message queue specified by mqdes.
The msg_len argument specifies the length of the message, in bytes, pointed to by msg_ptr. The value of msg_len shall be
less than or equal to the mq_msgsize attribute of the message queue, or mq_send() shall fail.
If the specified message queue is not full, mq_send() shall behave as if the message is inserted into the message queue
at the position indicated by the msg_prio argument. A message with a larger numeric value of msg_prio shall be inserted
before messages with lower values of msg_prio. A message shall be inserted after other messages in the queue, if any,
with equal msg_prio. The value of msg_prio shall be less than {MQ_PRIO_MAX}.
If the specified message queue is full and O_NONBLOCK is not set in the message queue description associated with mqdes,
mq_send() shall block until space becomes available to enqueue the message, or until mq_send() is interrupted by a sig-
nal. If more than one thread is waiting to send when space becomes available in the message queue and the Priority Sched-
uling option is supported, then the thread of the highest priority that has been waiting the longest shall be unblocked
to send its message. Otherwise, it is unspecified which waiting thread is unblocked. If the specified message queue is
full and O_NONBLOCK is set in the message queue description associated with mqdes, the message shall not be queued and
mq_send() shall return an error.
The mq_timedsend() function shall add a message to the message queue specified by mqdes in the manner defined for the
mq_send() function. However, if the specified message queue is full and O_NONBLOCK is not set in the message queue
description associated with mqdes, the wait for sufficient room in the queue shall be terminated when the specified time-
out expires. If O_NONBLOCK is set in the message queue description, this function shall be equivalent to mq_send().
The timeout shall expire when the absolute time specified by abs_timeout passes, as measured by the clock on which time-
outs are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified
by abs_timeout has already been passed at the time of the call.
If the Timers option is supported, the timeout shall be based on the CLOCK_REALTIME clock; if the Timers option is not
supported, the timeout shall be based on the system clock as returned by the time() function.
The resolution of the timeout shall be the resolution of the clock on which it is based. The timespec argument is defined
in the <time.h> header.
Under no circumstance shall the operation fail with a timeout if there is sufficient room in the queue to add the message
immediately. The validity of the abs_timeout parameter need not be checked when there is sufficient room in the queue.
RETURN VALUE
Upon successful completion, the mq_send() and mq_timedsend() functions shall return a value of zero. Otherwise, no mes-
sage shall be enqueued, the functions shall return -1, and errno shall be set to indicate the error.
ERRORS
The mq_send() and mq_timedsend() functions shall fail if:
EAGAIN The O_NONBLOCK flag is set in the message queue description associated with mqdes, and the specified message queue
is full.
EBADF The mqdes argument is not a valid message queue descriptor open for writing.
EINTR A signal interrupted the call to mq_send() or mq_timedsend().
EINVAL The value of msg_prio was outside the valid range.
EINVAL The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less
than zero or greater than or equal to 1000 million.
EMSGSIZE
The specified message length, msg_len, exceeds the message size attribute of the message queue.
ETIMEDOUT
The O_NONBLOCK flag was not set when the message queue was opened, but the timeout expired before the message
could be added to the queue.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The value of the symbol {MQ_PRIO_MAX} limits the number of priority levels supported by the application. Message priori-
ties range from 0 to {MQ_PRIO_MAX}-1.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
mq_open(), mq_receive(), mq_setattr(), mq_timedreceive(), time(), the Base Definitions volume of IEEE Std 1003.1-2001,
<mqueue.h>, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be obtained online at http://www.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 MQ_SEND(3P)
