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(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).

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

STANDARDS

       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 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

       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 */
       }