Name
mq_notify — register for notification when a message is available
Synopsis
#include <mqueue.h>
mqd_t
mq_notify( |
mqd_t | mqdes, |
| const struct sigevent * | notification); |
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DESCRIPTION
mq_notify() allows the
calling process to register or unregister for delivery of an
asynchronous notification when a new message arrives on the
empty message queue referred to by the descriptor mqdes.
The notification
argument is a pointer to a sigevent structure that is defined
something like the following:
union sigval { int sival_int;void * sival_ptr;}; struct sigevent { int sigev_notify;int sigev_signo;union sigval sigev_value;
notification */void (* sigev_notify_function)(union sigval);
notification */void * sigev_notify_attributes;};
If notification is
a non-NULL pointer, then mq_notify() registers the calling process
to receive message notification. The sigev_notify field of the
sigevent to which notification points specifies
how notification is to be performed. This field has one of
the following values:
SIGEV_NONE-
A "null" notification: the calling process is registered as the target for notification, but when a message arrives, no notification is sent.
SIGEV_SIGNAL-
Notify the process by sending the signal specified in
sigev_signo. If the signal is caught with a signal handler that was registered using the sigaction(2)SA_SIGINFOflag, then the following fields are set in the siginfo_t structure that is passed as the second argument of the handler:si_codeis set toSI_MESGQ;si_signois set to the signal number;si_valueis set to the value specified innotification−>sigev_value;si_pidis set to the PID of the process that sent the message; andsi_uidis set to the real user ID of the sending process. The same information is available if the signal is accepted using sigwaitinfo(2). SIGEV_THREAD-
Deliver notification by invoking
notification−>sigev_thread_functionas the start function of a new thread. The function is invoked withnotification−>sigev_valueas its sole argument. Ifnotification−>sigev_notify_attributesis not NULL, then it should point to a pthread_attr_t structure that defines attributes for the thread.
Only one process can be registered to receive notification from a message queue.
If notification is
NULL, and the calling process is currently registered to
receive notifications for this message queue, then the
registration is removed; another process can then register to
receive a message notification for this queue.
Message notification only occurs when a new message
arrives and the queue was previously empty. If the queue was
not empty at the time mq_notify() was called, then a notification
will only occur after the queue is emptied and a new message
arrives.
If another process or thread is waiting to read a message from an empty queue using mq_receive(3), then any message notification registration is ignored: the message is delivered to the process or thread calling mq_receive(3), and the message notification registration remains in effect.
Notification occurs once: after a notification is
delivered, the notification registration is removed, and
another process can register for message notification. If the
notified process wishes to receive the next notification, it
can use mq_notify() to request
a further notification. This should be done before emptying
all unread messages from the queue. (Placing the queue in
non-blocking mode is useful for emptying the queue of
messages without blocking once it is empty.)
RETURN VALUE
On success mq_notify()
returns 0; on error, −1 is returned, with errno set to indicate the error.
ERRORS
- EBADF
-
The descriptor specified in
mqdesis invalid. - EBUSY
-
Another process has already registered to receive notification for this message queue.
- EINVAL
-
notification−>sigev_notifyis not one of the permitted values; ornotification−>sigev_notifyisSIGEV_SIGNALandnotification−>sigev_signois not a valid signal number. - ENOMEM
-
Insufficient memory.
EXAMPLE
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>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
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 max. msg size; allocate buffer to receive msg */
if (mq_getattr(mqdes, &attr) == −1)
handle_error("mq_getattr");
buf = malloc(attr.mq_msgsize);
if (buf == NULL)
handle_error("malloc");
nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
if (nr == −1)
handle_error("mq_receive");
printf("Read %ld bytes from MQ\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)
handle_error("mq_open");
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)
handle_error("mq_notify");
pause(); /* Process will be terminated by thread function */
}
SEE ALSO
mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
COLOPHON
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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t Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |