mirror of https://github.com/mkerrisk/man-pages
271 lines
7.8 KiB
Groff
271 lines
7.8 KiB
Groff
'\" t
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH MQ_NOTIFY 3 2006-02-25 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mq_notify \- register for notification when a message is available
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.SH SYNOPSIS
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.nf
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.B #include <mqueue.h>
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.sp
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.BI "mqd_t mq_notify(mqd_t " mqdes ", const struct sigevent *" notification );
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.fi
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.sp
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Link with \fI\-lrt\fP.
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.SH DESCRIPTION
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.BR mq_notify ()
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allows the calling process to register or unregister for delivery of
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an asynchronous notification when a new message arrives on
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the empty message queue referred to by the descriptor
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.IR mqdes .
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The
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.I notification
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argument is a pointer to a
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.I sigevent
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structure that is defined something like the following:
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.in +4n
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.nf
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union sigval { /* Data passed with notification */
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int sival_int; /* Integer value */
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void *sival_ptr; /* Pointer value */
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};
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struct sigevent {
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int sigev_notify; /* Notification method */
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int sigev_signo; /* Notification signal */
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union sigval sigev_value; /* Data passed with
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notification */
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void (*sigev_notify_function) (union sigval);
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/* Function for thread
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notification */
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void *sigev_notify_attributes;
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/* Thread function attributes */
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};
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.fi
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.in
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.PP
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If
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.I notification
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is a non-NULL pointer, then
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.BR mq_notify ()
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registers the calling process to receive message notification.
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The
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.I sigev_notify
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field of the
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.I sigevent
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to which
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.I notification
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points specifies how notification is to be performed.
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This field has one of the following values:
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.TP
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.B SIGEV_NONE
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A "null" notification: the calling process is registered as the target
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for notification, but when a message arrives, no notification is sent.
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.\" When is SIGEV_NONE useful?
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.TP
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.B SIGEV_SIGNAL
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Notify the process by sending the signal specified in
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.IR sigev_signo .
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If the signal is caught with a signal handler that
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was registered using the
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.BR sigaction (2)
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.B SA_SIGINFO
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flag, then the following fields are set in the
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.I siginfo_t
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structure that is passed as the second argument of the handler:
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.I si_code
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is set to
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.BR SI_MESGQ ;
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.I si_signo
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is set to the signal number;
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.I si_value
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is set to the value specified in
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.IR notification\->sigev_value ;
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.\" I don't know of other implementations that set
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.\" si_pid and si_uid -- MTK
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.I si_pid
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is set to the PID of the process that sent the message; and
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.I si_uid
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is set to the real user ID of the sending process.
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The same information is available if the signal is accepted using
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.BR sigwaitinfo (2).
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.TP
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.B SIGEV_THREAD
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Deliver notification by invoking
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.I notification\->sigev_thread_function
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as the start function of a new thread.
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The function is invoked with
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.I notification\->sigev_value
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as its sole argument.
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If
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.I notification\->sigev_notify_attributes
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is not NULL, then it should point to a
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.I pthread_attr_t
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structure that defines attributes for the thread.
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.\" FIXME . add a cross reference to pthread_attr_init(3), when
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.\" that page is one day written.
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.PP
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Only one process can be registered to receive notification
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from a message queue.
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If
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.I notification
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is NULL, and the calling process is currently registered to receive
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notifications for this message queue, then the registration is removed;
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another process can then register to receive a message notification
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for this queue.
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Message notification only occurs when a new message arrives and
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the queue was previously empty.
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If the queue was not empty at the time
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.BR mq_notify ()
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was called, then a notification will only occur after
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the queue is emptied and a new message arrives.
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If another process or thread is waiting to read a message
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from an empty queue using
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.BR mq_receive (3),
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then any message notification registration is ignored:
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the message is delivered to the process or thread calling
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.BR mq_receive (3),
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and the message notification registration remains in effect.
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Notification occurs once: after a notification is delivered,
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the notification registration is removed,
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and another process can register for message notification.
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If the notified process wishes to receive the next notification,
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it can use
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.BR mq_notify ()
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to request a further notification.
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This should be done before emptying all unread messages from the queue.
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(Placing the queue in non-blocking mode is useful for emptying
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the queue of messages without blocking once it is empty.)
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.SH RETURN VALUE
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On success
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.BR mq_notify ()
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returns 0; on error, \-1 is returned, with
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.I errno
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set to indicate the error.
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.SH ERRORS
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.TP
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.B EBADF
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The descriptor specified in
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.I mqdes
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is invalid.
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.TP
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.B EBUSY
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Another process has already registered to receive notification
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for this message queue.
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.TP
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.B EINVAL
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.I notification\->sigev_notify
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is not one of the permitted values; or
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.I notification\->sigev_notify
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is
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.B SIGEV_SIGNAL
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and
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.I notification\->sigev_signo
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is not a valid signal number.
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.TP
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.B ENOMEM
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Insufficient memory.
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH EXAMPLE
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The following program registers a notification request for the
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message queue named in its command-line argument.
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Notification is performed by creating a thread.
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The thread executes a function which reads one message from the
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queue and then terminates the process.
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.nf
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#include <pthread.h>
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#include <mqueue.h>
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#include <assert.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#define handle_error(msg) \\
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do { perror(msg); exit(EXIT_FAILURE); } while (0)
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static void /* Thread start function */
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tfunc(union sigval sv)
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{
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struct mq_attr attr;
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ssize_t nr;
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void *buf;
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mqd_t mqdes = *((mqd_t *) sv.sival_ptr);
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/* Determine max. msg size; allocate buffer to receive msg */
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if (mq_getattr(mqdes, &attr) == \-1)
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handle_error("mq_getattr");
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buf = malloc(attr.mq_msgsize);
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if (buf == NULL)
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handle_error("malloc");
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nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
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if (nr == \-1)
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handle_error("mq_receive");
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printf("Read %ld bytes from MQ\\n", (long) nr);
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free(buf);
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exit(EXIT_SUCCESS); /* Terminate the process */
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}
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int
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main(int argc, char *argv[])
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{
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mqd_t mqdes;
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struct sigevent not;
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assert(argc == 2);
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mqdes = mq_open(argv[1], O_RDONLY);
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if (mqdes == (mqd_t) \-1)
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handle_error("mq_open");
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not.sigev_notify = SIGEV_THREAD;
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not.sigev_notify_function = tfunc;
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not.sigev_notify_attributes = NULL;
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not.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */
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if (mq_notify(mqdes, ¬) == \-1)
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handle_error("mq_notify");
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pause(); /* Process will be terminated by thread function */
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}
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.fi
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.SH "SEE ALSO"
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.BR mq_close (3),
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.BR mq_getattr (3),
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.BR mq_open (3),
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.BR mq_receive (3),
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.BR mq_send (3),
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.BR mq_unlink (3),
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.BR mq_overview (7)
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