MQ_NOTIFY(3) Linux Programmer's Manual MQ_NOTIFY(3)NAMEmq_notify - register for notification when a message is available
SYNOPSIS
#include <mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
Link with -lrt.
DESCRIPTIONmq_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. For
the definition and general details of this structure, see sigevent(7).
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 structure to which notification points specifies
how notification is to be performed. This field has one of the follow‐
ing values:
SIGEV_NONE
A "null" notification: the calling process is registered as the
target for notification, but when a message arrives, no notifi‐
cation is sent.
SIGEV_SIGNAL
Notify the process by sending the signal specified in
sigev_signo. See sigevent(7) for general details. The si_code
field of the siginfo_t structure will be set to SI_MESGQ. In
addition, si_pid will be set to the PID of the process that sent
the message, and si_uid will be set to the real user ID of the
sending process.
SIGEV_THREAD
Upon message delivery, invoke sigev_notify_function as if it
were the start function of a new thread. See sigevent(7) for
details.
Only one process can be registered to receive notification from a mes‐
sage queue.
If notification is NULL, and the calling process is currently regis‐
tered to receive notifications for this message queue, then the regis‐
tration is removed; another process can then register to receive a mes‐
sage 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 noti‐
fication 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 notifi‐
cation. This should be done before emptying all unread messages from
the queue. (Placing the queue in nonblocking mode is useful for empty‐
ing 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 mqdes is invalid.
EBUSY Another process has already registered to receive notification
for this message queue.
EINVAL notification->sigev_notify is not one of the permitted values;
or notification->sigev_notify is SIGEV_SIGNAL and notifica‐
tion->sigev_signo is not a valid signal number.
ENOMEM Insufficient memory.
POSIX.1-2008 says that an implementation may generate an EINVAL error
if notification is NULL, and the caller is not currently registered to
receive notifications for the queue mqdes.
CONFORMING TO
POSIX.1-2001.
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 mes‐
sage from the queue and then terminates the process.
#include <pthread.h>
#include <mqueue.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;
if (argc != 2) {
fprintf(stderr, "Usage: %s <mq-name>\n", argv[0]);
exit(EXIT_FAILURE);
}
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 ALSOmq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7), sigevent(7)COLOPHON
This page is part of release 3.27 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/.
Linux 2010-09-19 MQ_NOTIFY(3)
