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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
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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 message queue
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 occurs only 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 occur only 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 message queue 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.
For an explanation of the terms used in this section, see
attributes(7).
┌────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────┼───────────────┼─────────┤
│mq_notify() │ Thread safety │ MT-Safe │
└────────────┴───────────────┴─────────┘
POSIX.1-2001.
C library/kernel differences
In the glibc implementation, the mq_notify() library function is
implemented on top of the system call of the same name. When sevp is
NULL, or specifies a notification mechanism other than SIGEV_THREAD,
the library function directly invokes the system call. For
SIGEV_THREAD, much of the implementation resides within the library,
rather than the kernel. (This is necessarily so, since the thread
involved in handling the notification is one that must be managed by
the C library POSIX threads implementation.) The implementation
involves the use of a raw netlink(7) socket and creates a new thread
for each notification that is delivered to the process.
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.
Program source
#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 %zd bytes from MQ\n", 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 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 MQ_NOTIFY(3)
Pages that refer to this page: sigaction(2), syscalls(2), mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7), sigevent(7), user_namespaces(7)
Copyright and license for this manual page
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