mq_notify(2) — Linux manual page


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

NAME         top

       mq_notify - register for notification when a message is available

SYNOPSIS         top

       #include <mqueue.h>

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

       Link with -lrt.

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 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

              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

       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

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 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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top


NOTES         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.

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 <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 */

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)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-06-09                     MQ_NOTIFY(3)

Pages that refer to this page: syscalls(2)