notify the calling process when the message queue isn't empty
#include <mqueue.h> int mq_notify( mqd_t mqdes, const struct sigevent *notification );
If notification isn't NULL, this function requests the server to notify the calling process when the queue makes the transition from empty to nonempty. The means by which the server is to notify the process is passed in notification. Once the message queue server has notified the process of the transition, the notification is removed.
The sigevent structure pointed to by notification contains at least the following member:
Under normal operation, only one process may register for notification at a time. If a process attempts to attach a notification, and another process is already attached, an error is returned, and errno is set to EBUSY. However, if the QNX extended option MQ_MULT_NOTIFY is set in the mq_flags for the specified queue, multiple processes may be registered for notification. They're notified in FIFO order.
If a process has registered for notification, and another process is blocked on an mq_receive(), the mq_receive() call is satisfied by any arriving message. The resulting behavior is as if the message queue remained empty.
If notification is NULL, and the current process is currently registered for notification, the existing registration is removed.
If the MQ_NOTIFY_ALWAYS flag is set in the mq_flags field ( or the server is started with the -a option ), the caller is notified when a message is written to the queue even if the queue currently contains some messages.
POSIX 1003.4 with extensions
|Signal handler||Yes, but modifies errno|
In order to use the mq_... functions, you must:
errno, mq_close(), mq_getattr(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink()
chapter on POSIX.4 message queues