NAME
mq_notify —
notify process that a message is
available (REALTIME)
LIBRARY
library “librt”
SYNOPSIS
#include
<mqueue.h>
int
mq_notify(mqd_t
mqdes, const struct
sigevent *notification);
DESCRIPTION
If the argument notification is notNULL, 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,
mqdes. The notification specified by the
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.
The notification argument points to a
sigevent structure that defines how the calling
process will be notified. If
notification->sigev_notify is
SIGEV_NONE, then no signal will be posted, but the
error status and the return status for the operation will be set
appropriately. For SIGEV_SIGNO and
SIGEV_THREAD_ID notifications, the signal specified
in notification->sigev_signo will be sent to the
calling process (SIGEV_SIGNO) or to the thread whose
LWP ID is notification->sigev_notify_thread_id
(SIGEV_THREAD_ID). The information for the queued
signal will include:
| Member | Value |
| si_code | SI_MESGQ |
| si_value | the value stored in notification->sigev_value |
| si_mqd | mqdes |
If notification is
NULL and the process is currently registered for
notification by the specified message queue, the existing registration will
be removed.
When the notification is sent to the registered process, its registration is removed. The message queue then is available for registration.
If a process has registered for notification of
message arrival at a message queue and some thread is blocked in
mq_receive()
waiting to receive a message when a message arrives at the queue, the
arriving message will satisfy the appropriate
mq_receive(). The resulting behavior is as if the
message queue remains empty, and no notification will be sent.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
The mq_notify() system call will fail
if:
- [
EBADF] - The mqdes argument is not a valid message queue descriptor.
- [
EBUSY] - Process is already registered for notification by the message queue.
- [
EINVAL] - The asynchronous notification method in notification->sigev_notify is invalid or not supported.
SEE ALSO
mq_open(2), mq_send(2), mq_timedsend(2), sigevent(3), siginfo(3)
STANDARDS
The mq_notify() system call conforms to
IEEE Std 1003.1-2004 (“POSIX.1”).
HISTORY
Support for POSIX message queues first appeared in FreeBSD 7.0.
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.