NAME
mq_notify - register for notification when a message is available
SYNOPSIS
#include <mqueue.h>
I mqd_t mq_notify(mqd_t mqdes , const struct sigevent * notification );
DESCRIPTION
R 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
R mqdes .
The
notification
argument is a pointer to a
sigevent
structure that is defined something like the following:
union sigval { /* Data passed with notification */
int sival_int; /* Integer value */
void *sival_ptr; /* Pointer value */
};
struct sigevent {
int sigev_notify; /* Notification method */
int sigev_signo; /* Notification signal */
union sigval sigev_value; /* Data passed with notification */
void (*sigev_notify_function) (union sigval);
/* Function for thread notification */
void *sigev_notify_attributes;
/* Thread function attributes */
};
If
notification
is a non-NULL pointer, then
R mq_notify ()
registers the calling process to receive message notification.
The
sigev_notify
field of the
sigevent
to which
notification
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
R sigev_signo .
If the signal is caught with a signal handler that
was registered using the
sigaction(2)
SA_SIGINFO
flag, then the following fields are set in the
siginfo_t
structure that is passed as the second argument of the handler:
si_code
is set to
R SI_MESGQ ;
si_signo
is set to the signal number;
si_value
is set to the value specified in
R notification->sigev_value ;
si_pid
is set to the PID of the process that sent the message; and
si_uid
is set to the real user ID of the sending process.
The same information is available if the signal is accepted using
sigwaitinfo(2).
SIGEV_THREAD
Deliver notification by invoking
notification->sigev_thread_function
as the start function of a new thread.
The function is invoked with
R notification->sigev_value
as its sole argument.
If
R notification->sigev_notify_attributes
is not NULL, then it should point to a
pthread_attr_t
structure that defines attributes for the thread.
Only one process can be registered to receive notification
from a message queue.
If
notification
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
R 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
R mq_notify ()
to request a further notification.
This should be done before emptying all unread messages from the queue.
(Placing the queue in non-blocking mode is useful for emptying
the queue of messages without blocking once it is empty.)
RETURN VALUE
On success
R 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
notification->sigev_notify
is not one of the permitted values; or
notification->sigev_notify
is
SIGEV_SIGNAL
and
notification->sigev_signo
is not a valid signal number.
ENOMEM
Insufficient memory.
CONFORMING TO
POSIX.1-2001.
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.
#include <pthread.h>
#include <mqueue.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define die(msg) { perror(msg); exit(EXIT_FAILURE); }
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)
die("mq_getattr");
buf = malloc(attr.mq_msgsize);
if (buf == NULL)
die("malloc");
nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
if (nr == -1)
die("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 not;
assert(argc == 2);
mqdes = mq_open(argv[1], O_RDONLY);
if (mqdes == (mqd_t) -1)
die("mq_open");
not.sigev_notify = SIGEV_THREAD;
not.sigev_notify_function = tfunc;
not.sigev_notify_attributes = NULL;
not.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */
if (mq_notify(mqdes, ¬) == -1)
die("mq_notify");
pause(); /* Process will be terminated by thread function */
}
SEE ALSO
ČLÁNKY DO MAILU