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 noti-
fication is to be performed. This field has one of the following val-
ues:
SIGEV_NONE
A "null" notification: the calling process is registered as the
target for notification, but when a message arrives, no notifi-
cation 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
Only one process can be registered to receive notification from a mes-
sage queue.
If notification is NULL, and the calling process is currently regis-
tered to receive notifications for this message queue, then the regis-
tration is removed; another process can then register to receive a mes-
sage 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 noti-
fication 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 notifi-
cation. This should be done before emptying all unread messages from
the queue. (Placing the queue in non-blocking mode is useful for emp-
tying 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 notifica-
tion->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.
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 mes-
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)
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