mq_notify - register for notification when a message is available


#include <mqueue.h>

mqd_t mq_notify(mqd_t mqdes, const struct sigevent *notification);

Link with -lrt.


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 descriptor mqdes.

The notification argument is a pointer to a sigevent structure that is defined something like the following:

union sigval { /* Data passed with notification */ int sival_int; /* Integer value */ void *sival_ptr; /* Pointer value */ };

struct sigevent { int sigev_notify; /* Notification method */ int sigev_signo; /* Notification signal */ union sigval sigev_value; /* Data passed with notification */ void (*sigev_notify_function) (union sigval); /* Function for thread notification */ void *sigev_notify_attributes; /* Thread function attributes */ };

If notification is a non-NULL pointer, then mq_notify() registers the calling process to receive message notification. The sigev_notify field of the sigevent to which notification points specifies how notification is to be performed. This field has one of the following values:
 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. If the signal is caught with a signal handler that was registered using the sigaction(2) SA_SIGINFO flag, then the following fields are set in the siginfo_t structure that is passed as the second argument of the handler: si_code is set to SI_MESGQ; si_signo is set to the signal number; si_value is set to the value specified in notification->sigev_value; si_pid is set to the PID of the process that sent the message; and si_uid is set to the real user ID of the sending process. The same information is available if the signal is accepted using sigwaitinfo(2).
 Deliver notification by invoking notification->sigev_notify_function as the start function of a new thread. The function is invoked with notification->sigev_value as its sole argument. If notification->sigev_notify_attributes is not NULL, then it should point to a pthread_attr_t structure that defines attributes for the thread (see pthread_attr_init(3)).
Only one process can be registered to receive notification from a message queue.

If notification 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 only occurs 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 only occur 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 non-blocking mode is useful for emptying the queue of messages without blocking once it is empty.)


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


EBADF The descriptor specified in mqdes is invalid.
EBUSY Another process has already registered to receive notification for this message queue.
EINVAL notification->sigev_notify is not one of the permitted values; or notification->sigev_notify is SIGEV_SIGNAL and notification->sigev_signo is not a valid signal number.
ENOMEM Insufficient memory.
POSIX.1-2008 says that an implementation may generate an EINVAL error if notification is NULL, and the caller is not currently registered to receive notifications for the queue mqdes.




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.

#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 %ld bytes from MQ\n", (long) nr); free(buf); exit(EXIT_SUCCESS); /* Terminate the process */ }

int main(int argc, char *argv[]) { mqd_t mqdes; struct sigevent not;

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");

not.sigev_notify = SIGEV_THREAD; not.sigev_notify_function = tfunc; not.sigev_notify_attributes = NULL; not.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */ if (mq_notify(mqdes, ¬) == -1) handle_error("mq_notify");

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)


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