NAME¶
mq_notify - register for notification when a message is available
SYNOPSIS¶
#include <mqueue.h>
int mq_notify(mqd_t mqdes, const struct sigevent *sevp);
Link with
-lrt.
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 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.)
RETURN VALUE¶
On success
mq_notify() returns 0; on error, -1 is returned, with
errno set to indicate the error.
ERRORS¶
- 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.
ATTRIBUTES¶
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.
NOTES¶
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.
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.
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 */
}
SEE ALSO¶
mq_close(3),
mq_getattr(3),
mq_open(3),
mq_receive(3),
mq_send(3),
mq_unlink(3),
mq_overview(7),
sigevent(7)
COLOPHON¶
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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/.