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

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 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:

       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_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.

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

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

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚mq_notify()                                                                                                                           β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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 sevp is NULL, or specifies a noti‐
       fication 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 li‐
       brary POSIX threads implementation.)  The implementation involves the use of a raw netlink(7) socket and creates a new thread for each notification that  is  de‐
       livered to the process.

EXAMPLES
       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)

Linux                                                                          2021-03-22                                                                   MQ_NOTIFY(3)