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MQ_NOTIFY(3P) POSIX Programmer's Manual MQ_NOTIFY(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
mq_notify — notify process that a message is available (REALTIME)
#include <mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
If the argument notification is not NULL, this function shall
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 shall 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 shall fail.
If notification is NULL and the process is currently registered for
notification by the specified message queue, the existing
registration shall be removed.
When the notification is sent to the registered process, its
registration shall be removed. The message queue shall then be
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() or
mq_timedreceive() waiting to receive a message when a message arrives
at the queue, the arriving message shall satisfy the appropriate
mq_receive() or mq_timedreceive(), respectively. The resulting
behavior is as if the message queue remains empty, and no
notification shall be sent.
Upon successful completion, the mq_notify() function shall return a
value of zero; otherwise, the function shall return a value of −1 and
set errno to indicate the error.
The mq_notify() function shall fail if:
EBADF The mqdes argument is not a valid message queue descriptor.
EBUSY A process is already registered for notification by the
message queue.
The mq_notify() function may fail if:
EINVAL The notification argument is NULL and the process is currently
not registered.
The following sections are informative.
The following program registers a notification request for the
message queue named in its command-line argument. Notification is
performed by creating a thread. The thread executes a function which
reads one message from the queue and then terminates the process.
#include <pthread.h>
#include <mqueue.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static void /* Thread start function */
tfunc(union sigval sv)
{
struct mq_attr attr;
ssize_t nr;
void *buf;
mqd_t mqdes = *((mqd_t *) sv.sival_ptr);
/* Determine maximum msg size; allocate buffer to receive msg */
if (mq_getattr(mqdes, &attr) == -1) {
perror("mq_getattr");
exit(EXIT_FAILURE);
}
buf = malloc(attr.mq_msgsize);
if (buf == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
if (nr == -1) {
perror("mq_receive");
exit(EXIT_FAILURE);
}
printf("Read %ld bytes from message queue\n", (long) nr);
free(buf);
exit(EXIT_SUCCESS); /* Terminate the process */
}
int
main(int argc, char *argv[])
{
mqd_t mqdes;
struct sigevent not;
assert(argc == 2);
mqdes = mq_open(argv[1], O_RDONLY);
if (mqdes == (mqd_t) -1) {
perror("mq_open");
exit(EXIT_FAILURE);
}
not.sigev_notify = SIGEV_THREAD;
not.sigev_notify_function = tfunc;
not.sigev_notify_attributes = NULL;
not.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */
if (mq_notify(mqdes, ¬) == -1) {
perror("mq_notify");
exit(EXIT_FAILURE);
}
pause(); /* Process will be terminated by thread function */
}
None.
None.
None.
mq_open(3p), mq_send(3p), mq_receive(3p), msgctl(3p), msgget(3p),
msgrcv(3p), msgsnd(3p)
The Base Definitions volume of POSIX.1‐2008, mqueue.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MQ_NOTIFY(3P)
Pages that refer to this page: mqueue.h(0p), mq_getattr(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p)