man-pages/man3/mq_notify.3

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.TH MQ_NOTIFY 3 2008-08-13 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_notify \- register for notification when a message is available
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.sp
.BI "mqd_t mq_notify(mqd_t " mqdes ", const struct sigevent *" notification );
.fi
.sp
Link with \fI\-lrt\fP.
.SH DESCRIPTION
.BR 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
.IR mqdes .

The
.I notification
argument is a pointer to a
.I sigevent
structure that is defined something like the following:
.in +4n
.nf

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 */
};
.fi
.in
.PP
If
.I notification
is a non-NULL pointer, then
.BR mq_notify ()
registers the calling process to receive message notification.
The
.I sigev_notify
field of the
.I sigevent
to which
.I notification
points specifies how notification is to be performed.
This field has one of the following values:
.TP
.B SIGEV_NONE
A "null" notification: the calling process is registered as the target
for notification, but when a message arrives, no notification is sent.
.\" When is SIGEV_NONE useful?
.TP
.B SIGEV_SIGNAL
Notify the process by sending the signal specified in
.IR sigev_signo .
If the signal is caught with a signal handler that
was registered using the
.BR sigaction (2)
.B SA_SIGINFO
flag, then the following fields are set in the
.I siginfo_t
structure that is passed as the second argument of the handler:
.I si_code
is set to
.BR SI_MESGQ ;
.I si_signo
is set to the signal number;
.I si_value
is set to the value specified in
.IR notification\->sigev_value ;
.\" I don't know of other implementations that set
.\" si_pid and si_uid -- MTK
.I si_pid
is set to the PID of the process that sent the message; and
.I si_uid
is set to the real user ID of the sending process.
The same information is available if the signal is accepted using
.BR sigwaitinfo (2).
.TP
.B SIGEV_THREAD
Deliver notification by invoking
.I notification\->sigev_notify_function
as the start function of a new thread.
The function is invoked with
.I notification\->sigev_value
as its sole argument.
If
.I notification\->sigev_notify_attributes
is not NULL, then it should point to a
.I pthread_attr_t
structure that defines attributes for the thread.
.\" FIXME . add a cross reference to pthread_attr_init(3), when
.\" that page is one day written.
.PP
Only one process can be registered to receive notification
from a message queue.

If
.I 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
.BR 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
.BR mq_receive (3),
then any message notification registration is ignored:
the message is delivered to the process or thread calling
.BR 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
.BR 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.)
.SH RETURN VALUE
On success
.BR mq_notify ()
returns 0; on error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EBADF
The descriptor specified in
.I mqdes
is invalid.
.TP
.B EBUSY
Another process has already registered to receive notification
for this message queue.
.TP
.B EINVAL
.I notification\->sigev_notify
is not one of the permitted values; or
.I notification\->sigev_notify
is
.B SIGEV_SIGNAL
and
.I notification\->sigev_signo
is not a valid signal number.
.TP
.B ENOMEM
Insufficient memory.
.SH CONFORMING TO
POSIX.1-2001.
.SH 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.
.nf

#include <pthread.h>
#include <mqueue.h>
#include <assert.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 %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)
        handle_error("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, &not) == \-1)
        handle_error("mq_notify");

    pause();    /* Process will be terminated by thread function */
}
.fi
.SH "SEE ALSO"
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_open (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR mq_overview (7)