MQ_SEND(3) BSD Library Functions Manual MQ_SEND(3)
mq_send, mq_timedsend -- send a message to a message queue (REALTIME)
POSIX Real-time Library (librt, -lrt)
mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len, unsigned msg_prio);
mq_timedsend(mqd_t mqdes, const char *msg_ptr, size_t msg_len, unsigned msg_prio, const struct timespec *abs_timeout);
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 message will be inserted after other messages in the queue, if any, with equal msg_prio. The value of msg_prio will be less than
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 message will not be queued and mq_send() will return an error.
The mq_timedsend() function will 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 will be terminated when the specified timeout expires. If O_NONBLOCK is set in the message queue descrip-
tion, 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
validity of the abs_timeout parameter need not be checked when there is sufficient room in the queue.
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.
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
[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 param-
eter 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.
These functions are expected to conform to the IEEE Std 1003.1-2001 (``POSIX.1'') standard.
The mq_send() and mq_timedsend() functions first appeared in NetBSD 5.0.
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, Copyright (C) 2001-2003 by the Institute of Elec-
trical 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
June 7, 2010 BSD