mq_notify(3) — Linux manual page


mq_notify(3)            Library Functions Manual            mq_notify(3)

NAME         top

       mq_notify - register for notification when a message is available

LIBRARY         top

       Real-time library (librt, -lrt)

SYNOPSIS         top

       #include <mqueue.h>
       #include <signal.h>           /* Definition of SIGEV_* constants */

       int mq_notify(mqd_t mqdes, const struct sigevent *sevp);

DESCRIPTION         top

       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 sevp argument is a pointer to a sigevent structure.  For the
       definition and general details of this structure, see

       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:

              A "null" notification: the calling process is registered
              as the target for notification, but when a message
              arrives, no notification is sent.

              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.

              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         top

       On success mq_notify() returns 0; on error, -1 is returned, with
       errno set to indicate the error.

ERRORS         top

       EBADF  The message queue descriptor specified in mqdes is

       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         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ mq_notify()                         │ Thread safety │ MT-Safe │

VERSIONS         top

   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.

STANDARDS         top


HISTORY         top


EXAMPLES         top

       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 <mqueue.h>
       #include <pthread.h>
       #include <signal.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)
           buf = malloc(attr.mq_msgsize);
           if (buf == NULL)

           nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
           if (nr == -1)

           printf("Read %zd bytes from MQ\n", nr);
           exit(EXIT_SUCCESS);         /* Terminate the process */

       main(int argc, char *argv[])
           mqd_t mqdes;
           struct sigevent sev;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <mq-name>\n", argv[0]);

           mqdes = mq_open(argv[1], O_RDONLY);
           if (mqdes == (mqd_t) -1)

           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)

           pause();    /* Process will be terminated by thread function */

SEE ALSO         top

       mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3),
       mq_send(3), mq_unlink(3), mq_overview(7), sigevent(7)

Linux man-pages (unreleased)     (date)                     mq_notify(3)

Pages that refer to this page: sigaction(2)syscalls(2)mq_close(3)mq_getattr(3)mq_open(3)mq_receive(3)mq_send(3)mq_unlink(3)mq_overview(7)sigevent(7)system_data_types(7)user_namespaces(7)