Manpage of MQ_NOTIFY

MQ_NOTIFY

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

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 sevpargument is a pointer to a sigeventstructure. For the definition and general details of this structure, see sigevent(7).

If sevpis a non-null pointer, then mq_notify() registers the calling process to receive message notification. The sigev_notifyfield of the sigeventstructure to which sevppoints 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_codefield of the siginfo_tstructure will be set to SI_MESGQ. In addition, si_pidwill be set to the PID of the process that sent the message, and si_uidwill be set to the real user ID of the sending process.
SIGEV_THREAD
Upon message delivery, invoke sigev_notify_functionas 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 sevpis 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 errnoset to indicate the error.  

ERRORS

EBADF
The message queue descriptor specified in mqdesis invalid.
EBUSY
Another process has already registered to receive notification for this message queue.
EINVAL
sevp->sigev_notifyis not one of the permitted values; or sevp->sigev_notifyis SIGEV_SIGNALand sevp->sigev_signois not a valid signal number.
ENOMEM
Insufficient memory.

POSIX.1-2008 says that an implementation maygenerate an EINVALerror if sevpis 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).
InterfaceAttributeValue
mq_notify() Thread safetyMT-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 sevpis 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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO
NOTES
C library/kernel differences
EXAMPLE
Program source
SEE ALSO

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