MQ_NOTIFY(3)                Linux Programmer's Manual               MQ_NOTIFY(3)

       mq_notify - register for notification when a message is available

       #include <mqueue.h>

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

       Link with -lrt.

       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

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

       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

       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

       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.)

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

       EBADF  The message queue descriptor specified in mqdes is invalid.

       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.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                     Attribute     Value   │
       │mq_notify()                                   │ Thread safety │ MT-Safe │


   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.

       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)
           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 */

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

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       latest version of this page, can be found at

Linux                              2021-03-22                       MQ_NOTIFY(3)