NAME

mq_send, mq_timedsend —

send a message to 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);

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() 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 {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() 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.

RETURN VALUES

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 error.

ERRORS

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 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.

STANDARDS

The mq_send() and mq_timedsend() system calls conform to IEEE Std 1003.1-2004 (“POSIX.1”).

HISTORY

Support for POSIX message queues first appeared in FreeBSD 7.0.

COPYRIGHT

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 http://www.opengroup.org/unix/online.html.