mq_notify(3) Library Functions Manual mq_notify(3)
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 defin-
ition 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 notifi-
cation 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 mes-
sage queue.
If sevp is NULL, and the calling process is currently registered to re-
ceive notifications for this message queue, then the registration is
removed; another process can then register to receive a message notifi-
cation 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 ef-
fect.
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 nonblocking mode is useful for empty-
ing 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 attrib-
utes(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 imple-
mented on top of the system call of the same name. When sevp is NULL,
or specifies a notification mechanism other than SIGEV_THREAD, the li-
brary 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 mes-
sage 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 */
}
SEE ALSO
mq_close(3), mq_getattr(3), mq_open(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7), sigevent(3type)
Linux man-pages 6.7 2023-10-31 mq_notify(3)
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