mq_send(3) — Linux manual page


mq_send(3)              Library Functions Manual              mq_send(3)

NAME         top

       mq_send, mq_timedsend - send a message to a message queue

LIBRARY         top

       Real-time library (librt, -lrt)

SYNOPSIS         top

       #include <mqueue.h>

       int mq_send(mqd_t mqdes, const char msg_ptr[.msg_len],
                     size_t msg_len, unsigned int msg_prio);

       #include <time.h>
       #include <mqueue.h>

       int mq_timedsend(mqd_t mqdes, const char msg_ptr[.msg_len],
                     size_t msg_len, unsigned int msg_prio,
                     const struct timespec *abs_timeout);

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 200112L

DESCRIPTION         top

       mq_send() adds the message pointed to by msg_ptr to the message
       queue referred to by the message queue descriptor mqdes.  The
       msg_len argument specifies the length of the message pointed to
       by msg_ptr; this length must be less than or equal to the queue's
       mq_msgsize attribute.  Zero-length messages are allowed.

       The msg_prio argument is a nonnegative integer that specifies the
       priority of this message.  Messages are placed on the queue in
       decreasing order of priority, with newer messages of the same
       priority being placed after older messages with the same
       priority.  See mq_overview(7) for details on the range for the
       message priority.

       If the message queue is already full (i.e., the number of
       messages on the queue equals the queue's mq_maxmsg attribute),
       then, by default, mq_send() blocks until sufficient space becomes
       available to allow the message to be queued, or until the call is
       interrupted by a signal handler.  If the O_NONBLOCK flag is
       enabled for the message queue description, then the call instead
       fails immediately with the error EAGAIN.

       mq_timedsend() behaves just like mq_send(), except that if the
       queue is full and the O_NONBLOCK flag is not enabled for the
       message queue description, then abs_timeout points to a structure
       which specifies how long the call will block.  This value is an
       absolute timeout in seconds and nanoseconds since the Epoch,
       1970-01-01 00:00:00 +0000 (UTC), specified in a timespec(3)

       If the message queue is full, and the timeout has already expired
       by the time of the call, mq_timedsend() returns immediately.

RETURN VALUE         top

       On success, mq_send() and mq_timedsend() return zero; on error,
       -1 is returned, with errno set to indicate the error.

ERRORS         top

       EAGAIN The queue was full, and the O_NONBLOCK flag was set for
              the message queue description referred to by mqdes.

       EBADF  The descriptor specified in mqdes was invalid or not
              opened for writing.

       EINTR  The call was interrupted by a signal handler; see

       EINVAL The call would have blocked, and abs_timeout was invalid,
              either because tv_sec was less than zero, or because
              tv_nsec was less than zero or greater than 1000 million.

              msg_len was greater than the mq_msgsize attribute of the
              message queue.

              The call timed out before a message could be transferred.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ mq_send(), mq_timedsend()           │ Thread safety │ MT-Safe │

VERSIONS         top

       On Linux, mq_timedsend() is a system call, and mq_send() is a
       library function layered on top of that system call.

STANDARDS         top


HISTORY         top


SEE ALSO         top

       mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3),
       mq_receive(3), mq_unlink(3), timespec(3), mq_overview(7), time(7)

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-05-02                     mq_send(3)

Pages that refer to this page: syscalls(2)mq_close(3)mq_getattr(3)mq_notify(3)mq_open(3)mq_receive(3)mq_unlink(3)mq_overview(7)signal(7)