NAME
mq_notify - register for notification when a message is available
SYNOPSIS
#include <mqueue.h>
mqd_t mq_notify(mqd_t mqdes, const struct sigevent *notification);
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 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:
| 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. 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). | |
| SIGEV_THREAD | |
| 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)). | |
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.)
RETURN VALUE
On success mq_notify() returns 0; on error, -1 is returned, with errno set to indicate the error.
ERRORS
| 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. |
CONFORMING TO
POSIX.1-2001.
EXAMPLE
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 */
}
SEE ALSO
mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
COLOPHON
This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
