MQ_SEND(3) BSD Library Functions Manual MQ_SEND(3)NAME
mq_send, mq_timedsend — send a message to a message queue (REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
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 will 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 will be less than or equal to the mq_msgsize
attribute of the message queue, or mq_send() will fail.
If the specified message queue is not full, mq_send() behaves 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
will be inserted before messages with lower values of msg_prio. A mes‐
sage will be inserted after other messages in the queue, if any, with
equal msg_prio. The value of msg_prio will 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() blocks until
space becomes available to enqueue the message, or until mq_send() is
interrupted by a signal. If more than one thread is waiting to send when
space becomes available in the message queue, then the thread of the
highest priority that has been waiting the longest will be unblocked to
send its message. If the specified message queue is full and O_NONBLOCK
is set in the message queue description associated with mqdes, the mes‐
sage will not be queued and mq_send() will return an error.
The mq_timedsend() function will add a message to the message queue spec‐
ified by mqdes in the manner defined for the mq_send() function. How‐
ever, if the specified message queue is full and O_NONBLOCK is not set in
the message queue description associated with mqdes, the wait for suffi‐
cient room in the queue will be terminated when the specified timeout
expires. If O_NONBLOCK is set in the message queue description, this
function will be equivalent to mq_send().
The timeout will expire when the absolute time specified by abs_timeout
passes, as measured by the clock on which timeouts 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.
The resolution of the timeout is based on the CLOCK_REALTIME clock. The
timespec argument is defined in the <time.h> header.
Under no circumstance will the operation fail with a timeout if there is
sufficient room in the queue to add the message immediately. The valid‐
ity of the abs_timeout parameter need not be checked when there is suffi‐
cient room in the queue.
RETURN VALUES
Upon successful completion, the mq_send() and mq_timedsend() functions
return a value of zero. Otherwise, no message will be enqueued, the
functions will return -1, and the global variable errno will be set to
indicate the error.
ERRORS
The mq_send() and mq_timedsend() functions 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, or
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.
SEE ALSOmq(3), mq_receive(3)STANDARDS
These functions are expected to conform to the IEEE Std 1003.1-2001
(“POSIX.1”) standard.
HISTORY
The mq_send() and mq_timedsend() functions first appeared in NetBSD 5.0.
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 Spec‐
ifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electri‐
cal 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 ref‐
eree document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
BSD June 7, 2010 BSD
