MQ_NOTIFY(3P) POSIX Programmer's Manual MQ_NOTIFY(3P)
PROLOG
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.
NAME
mq_notify -- notify process that a message is available (REALTIME)
SYNOPSIS
#include <mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
DESCRIPTION
If the argument notification is not NULL, this function shall register
the calling process to be notified of message arrival at an empty mes-
sage 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 notifi-
cation by a message queue. If the calling process or any other process
has already registered for notification of message arrival at the spec-
ified 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 registra-
tion 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_time-
dreceive() 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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
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 mes-
sage 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 */
}
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
mq_open(), mq_send(), mq_receive(), msgctl(), msgget(), msgrcv(),
msgsnd()
The Base Definitions volume of POSIX.1-2008, <mqueue.h>
COPYRIGHT
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 Electri-
cal 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MQ_NOTIFY(3P)
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