.\" Copyright (c) 2005 David Xu .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice(s), this list of conditions and the following disclaimer as .\" the first lines of this file unmodified other than the possible .\" addition of one or more copyright notices. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice(s), this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY .\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE .\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR .\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, .\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE .\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, .\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" 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. .\" .\" $FreeBSD: releng/12.2/lib/libc/sys/mq_notify.2 302899 2016-07-15 15:12:56Z jhb $ .\" .Dd July 15, 2016 .Dt MQ_NOTIFY 2 .Os .Sh NAME .Nm mq_notify .Nd "notify process that a message is available (REALTIME)" .Sh LIBRARY .Lb librt .Sh SYNOPSIS .In mqueue.h .Ft int .Fn mq_notify "mqd_t mqdes" "const struct sigevent *notification" .Sh DESCRIPTION If the argument notification is not .Dv NULL , this system call will register the calling process to be notified of message arrival at an empty message queue associated with the specified message queue descriptor, .Fa mqdes . The notification specified by the .Fa notification argument will be sent to the process when the message queue transitions from empty to non-empty. At any time, only one process may be registered for notification by a message queue. If the calling process or any other process has already registered for notification of message arrival at the specified message queue, subsequent attempts to register for that message queue will fail. .Pp The .Fa notification argument points to a .Vt sigevent structure that defines how the calling process will be notified. If .Fa notification->sigev_notify is .Dv SIGEV_NONE , then no signal will be posted, but the error status and the return status for the operation will be set appropriately. For .Dv SIGEV_SIGNO and .Dv SIGEV_THREAD_ID notifications, the signal specified in .Fa notification->sigev_signo will be sent to the calling process .Pq Dv SIGEV_SIGNO or to the thread whose LWP ID is .Fa notification->sigev_notify_thread_id .Pq Dv SIGEV_THREAD_ID . The information for the queued signal will include: .Bl -column ".Va si_value" .It Sy Member Ta Sy Value .It Va si_code Ta Dv SI_MESGQ .It Va si_value Ta the value stored in .Fa notification->sigev_value .It Va si_mqd Ta Fa mqdes .El .Pp If .Fa notification is .Dv NULL and the process is currently registered for notification by the specified message queue, the existing registration will be removed. .Pp When the notification is sent to the registered process, its registration is removed. The message queue then is available for registration. .Pp If a process has registered for notification of message arrival at a message queue and some thread is blocked in .Fn mq_receive waiting to receive a message when a message arrives at the queue, the arriving message will satisfy the appropriate .Fn mq_receive . The resulting behavior is as if the message queue remains empty, and no notification will be sent. .Sh RETURN VALUES .Rv -std .Sh ERRORS The .Fn mq_notify system call will fail if: .Bl -tag -width Er .It Bq Er EBADF The .Fa mqdes argument is not a valid message queue descriptor. .It Bq Er EBUSY Process is already registered for notification by the message queue. .It Bq Er EINVAL The asynchronous notification method in .Fa notification->sigev_notify is invalid or not supported. .El .Sh SEE ALSO .Xr mq_open 2 , .Xr mq_send 2 , .Xr mq_timedsend 2 , .Xr sigevent 3 , .Xr siginfo 3 .Sh STANDARDS The .Fn mq_notify system call conforms to .St -p1003.1-2004 . .Sh HISTORY Support for .Tn POSIX message queues first appeared in .Fx 7.0 . .Sh 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.