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PROLOG

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

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 .