MQ_SEND(3)                 POSIX Programmer's Manual                MQ_SEND(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 *abstime);

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 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 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
       timeout 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 abstime
       passes, as measured by the clock on which timeouts are based (that is,
       when the value of that clock equals or exceeds abstime), or if the
       absolute time specified by abstime has already been passed at the time
       of the call.

       The timeout shall be based on the CLOCK_REALTIME clock.  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 abstime 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 message 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 abstime
              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 priorities range from 0
       to {MQ_PRIO_MAX}-1.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       mq__open(), mq__receive(), mq__setattr(), time()

       The Base Definitions volume of POSIX.1-2017, <mqueue.h>, <time.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           MQ_SEND(3)
