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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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mq_notify(3) Library Functions Manual mq_notify(3)
mq_notify - register for notification when a message is available
Real-time library (librt, -lrt)
#include <mqueue.h>
#include <signal.h> /* Definition of SIGEV_* constants */
int mq_notify(mqd_t mqdes, const struct sigevent *sevp);
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(3type).
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(3type) 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(3type) 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 │
└─────────────────────────────────────┴───────────────┴─────────┘
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.
POSIX.1-2008.
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.
Program source
#include <mqueue.h>
#include <pthread.h>
#include <signal.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(3type)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 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), sigevent(3type), mq_overview(7), user_namespaces(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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