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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017, mqueue.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 MQ_NOTIFY(3P)
Pages that refer to this page: mqueue.h(0p), mq_getattr(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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