mq_notify —
notify process that a message is available
(REALTIME)
POSIX Real-time Library (librt, -lrt)
#include
<mqueue.h>
int
mq_notify(
mqd_t
mqdes,
const
struct sigevent *notification);
If the argument notification is not
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,
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.
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.
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.
mq_open(2),
mq_send(2),
mq_timedsend(2),
sigevent(3),
siginfo(3)
The
mq_notify() system call conforms to
IEEE Std 1003.1-2004
(“POSIX.1”).
Support for POSIX message queues first appeared in
FreeBSD
7.0.
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.