| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON | The Linux Programming Interface |
MQ_NOTIFY(3) Linux Programmer's Manual MQ_NOTIFY(3)
mq_notify - register for notification when a message is available
#include <mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *sevp);
Link with -lrt.
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 sevp argument is a pointer to a sigevent structure. For the definition
and general details of this structure, see sigevent(7).
If sevp 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 sevp 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. 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 message
queue.
If sevp 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 nonblocking mode is useful for emptying the queue of messages
without blocking once it is empty.)
On success mq_notify() returns 0; on error, -1 is returned, with errno set to
indicate the error.
EBADF The descriptor specified in mqdes is invalid.
EBUSY Another process has already registered to receive notification for this
message queue.
EINVAL sevp->sigev_notify is not one of the permitted values; or
sevp->sigev_notify is SIGEV_SIGNAL and sevp->sigev_signo is not a valid
signal number.
ENOMEM Insufficient memory.
POSIX.1-2008 says that an implementation may generate an EINVAL error if sevp
is NULL, and the caller is not currently registered to receive notifications
for the queue mqdes.
POSIX.1-2001.
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 <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 sev;
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");
sev.sigev_notify = SIGEV_THREAD;
sev.sigev_notify_function = tfunc;
sev.sigev_notify_attributes = NULL;
sev.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */
if (mq_notify(mqdes, &sev) == -1)
handle_error("mq_notify");
pause(); /* Process will be terminated by thread function */
}
mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7), sigevent(7)
This page is part of release 3.32 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 2010-10-04 MQ_NOTIFY(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface