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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 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 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 │
└────────────┴───────────────┴─────────┘
CONFORMING TO
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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