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mq_send, mq_timedsend — send a message to message queue (REALTIME)
POSIX Real-time Library (librt, -lrt)
mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len,
mq_timedsend(mqd_t mqdes, const char *msg_ptr, size_t msg_len,
unsigned msg_prio, const struct timespec *abs_timeout);
The mq_send() system call adds 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 should 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() will 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 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 should 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() will block
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 and the Priority
Scheduling option is supported, then the thread of the highest priority
that has been waiting the longest will 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 will not be queued
and mq_send() will return an error.
The mq_timedsend() system call will add a message to the message queue
specified by mqdes in the manner defined for the mq_send() system call.
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 description,
this system call is 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 timeout is based on the CLOCK_REALTIME clock.
Upon successful completion, the mq_send() and mq_timedsend() system calls
return a value of zero. Otherwise, no message will be enqueued, the
system calls return -1, and the global variable errno is set to indicate
The mq_send() and mq_timedsend() system calls will 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
[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
[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.
mq_open(2), mq_receive(2), mq_setattr(2), mq_timedreceive(2)
The mq_send() and mq_timedsend() system calls conform to IEEE Std
Support for POSIX message queues first appeared in FreeBSD 7.0.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2004 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