man3/mq_notify.3

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.TH MQ_NOTIFY 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_notify \- register for notification when a message is available
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.PP
.BI "int mq_notify(mqd_t " mqdes ", const struct sigevent *" sevp );
.fi
.PP
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 message queue descriptor
.IR mqdes .
.PP
The
.I sevp
argument is a pointer to a
.I sigevent
structure.
For the definition and general details of this structure, see
.BR sigevent (7).
.PP
If
.I sevp
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
structure to which
.I sevp
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 .
See
.BR sigevent (7)
for general details.
The
.I si_code
field of the
.I siginfo_t
structure will be set to
.BR SI_MESGQ .
In addition,
.\" I don't know of other implementations that set
.\" si_pid and si_uid -- MTK
.I si_pid
will be set to the PID of the process that sent the message, and
.I si_uid
will be set to the real user ID of the sending process.
.TP
.B SIGEV_THREAD
Upon message delivery, invoke
.I sigev_notify_function
as if it were the start function of a new thread.
See
.BR sigevent (7)
for details.
.PP
Only one process can be registered to receive notification
from a message queue.
.PP
If
.I 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.
.PP
Message notification occurs only 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 occur only after
the queue is emptied and a new message arrives.
.PP
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.
.PP
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 nonblocking 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 message queue 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 sevp\->sigev_notify
is not one of the permitted values; or
.I sevp\->sigev_notify
is
.B SIGEV_SIGNAL
and
.I sevp\->sigev_signo
is not a valid signal number.
.TP
.B ENOMEM
Insufficient memory.
.PP
POSIX.1-2008 says that an implementation
.I may
generate an
.B EINVAL
.\" Linux does not do this
error if
.I sevp
is NULL, and the caller is not currently registered to receive
notifications for the queue
.IR mqdes .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR mq_notify ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
.\"
.SS C library/kernel differences
In the glibc implementation, the
.BR mq_notify ()
library function is implemented on top of the system call of the same name.
When
.I sevp
is NULL, or specifies a notification mechanism other than
.BR SIGEV_THREAD ,
the library function directly invokes the system call.
For
.BR 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
.BR netlink (7)
socket and creates a new thread for each notification that is
delivered to the process.
.SH EXAMPLES
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.
.SS Program source
.EX
#include <pthread.h>
#include <mqueue.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define handle_error(msg) \e
    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\en", 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>\en", 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 */
}
.EE
.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),
.BR sigevent (7)