Ubuntu Manpage: mq_notify - register for notification when a message is available

NAME

       mq_notify - register for notification when a message is available

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

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

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

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(3type).

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(3type) 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(3type) 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).
       ┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                                                                   │ Attribute     │ Value   │
       ├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │ mq_notify()                                                                 │ Thread safety │ MT-Safe │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

VERSIONS

   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

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

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 <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)
               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 */
       }