MQ_NOTIFY(3)              (2020-06-09)               MQ_NOTIFY(3)

          mq_notify - register for notification when a message is

          #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 unreg-
          ister 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 regis-
               tered as the target for notification, but when a mes-
               sage 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 regis-
          tered to receive notifications for this message queue, then

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          the registration is removed; another process can then regis-
          ter 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 notifica-
          tion 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 deliv-
          ered 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 pro-
          cess 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 empty-
          ing the queue of messages without blocking once it is

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

               The message queue descriptor specified in mqdes is

               Another process has already registered to receive noti-
               fication for this message queue.

               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.

               Insufficient memory.

          POSIX.1-2008 says that an implementation may generate an
          EINVAL error if sevp is NULL, and the caller is not cur-
          rently registered to receive notifications for the queue


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          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lb lb lb l l l.
          Interface Attribute Value T{ mq_notify() T}   Thread
          safety  MT-Safe


        C library/kernel differences
          In the glibc implementation, the mq_notify() library func-
          tion is implemented on top of the system call of the same
          name.  When sevp is NULL, or specifies a notification mecha-
          nism 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 implemen-
          tation involves the use of a raw netlink(7) socket and cre-
          ates 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.  Noti-
          fication is performed by creating a thread.  The thread exe-
          cutes 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)

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              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)

          This page is part of release 5.10 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

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