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MQ_SEND(2)               DragonFly System Calls Manual              MQ_SEND(2)


NAME

     mq_send, mq_timedsend -- send a message to a 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);

     #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 *abs_timeout);


DESCRIPTION

     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 mes-
     sage will be inserted after other messages in the queue, if any, with
     equal msg_prio.  The value of msg_prio will 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() 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 mes-
     sage will not be queued and mq_send() will return an error.

     The mq_timedsend() function will add a message to the message queue spec-
     ified by mqdes in the manner defined for the mq_send() function.  How-
     ever, if the specified message queue is full and O_NONBLOCK is not set in
     the message queue description associated with mqdes, the wait for suffi-
     cient room in the queue will be terminated when the specified timeout
     expires.  If O_NONBLOCK is set in the message queue description, 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 valid-
     ity of the abs_timeout parameter need not be checked when there is suffi-
     cient room in the queue.


RETURN VALUES

     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.


ERRORS

     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 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, or
                        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.


SEE ALSO

     mq_close(2), mq_getattr(2), mq_notify(2), mq_open(2), mq_receive(2),
     mq_setattr(2), mq_unlink(2)


STANDARDS

     These functions are expected to conform to the IEEE Std 1003.1-2001
     (``POSIX.1'') standard.


HISTORY

     The mq_send() and mq_timedsend() functions first appeared in
     DragonFly 2.5.


COPYRIGHT

     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 Spec-
     ifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electri-
     cal 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 ref-
     eree document.  The original Standard can be obtained online at
     http://www.opengroup.org/unix/online.html .

DragonFly 3.5                   August 19, 2009                  DragonFly 3.5

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