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/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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** accordance with the commercial license agreement provided with the
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** a written agreement between you and The Qt Company. For licensing terms
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** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
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****************************************************************************/
/*!
\page unix-signals.html
\title Calling Qt Functions From Unix Signal Handlers
\brief You can't. But don't despair, there is a way...
\ingroup best-practices
You \e can't call Qt functions from Unix signal handlers. The
standard POSIX rule applies: You can only call async-signal-safe
functions from signal handlers. See \l
{http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_04.html#tag_02_04_01}
{Signal Actions} for the complete list of functions you can call
from Unix signal handlers.
But don't despair, there is a way to use Unix signal handlers with
Qt. The strategy is to have your Unix signal handler do something
that will eventually cause a Qt signal to be emitted, and then you
simply return from your Unix signal handler. Back in your Qt
program, that Qt signal gets emitted and then received by your Qt
slot function, where you can safely do whatever Qt stuff you
weren't allowed to do in the Unix signal handler.
One simple way to make this happen is to declare a socket pair in
your class for each Unix signal you want to handle. The socket
pairs are declared as static data members. You also create a
QSocketNotifier to monitor the \e read end of each socket pair,
declare your Unix signal handlers to be static class methods, and
declare a slot function corresponding to each of your Unix signal
handlers. In this example, we intend to handle both the SIGHUP and
SIGTERM signals. Note: You should read the socketpair(2) and the
sigaction(2) man pages before plowing through the following code
snippets.
\snippet snippets/code/doc_src_unix-signal-handlers.cpp 0
In the MyDaemon constructor, use the socketpair(2) function to
initialize each file descriptor pair, and then create the
QSocketNotifier to monitor the \e read end of each pair. The
activated() signal of each QSocketNotifier is connected to the
appropriate slot function, which effectively converts the Unix
signal to the QSocketNotifier::activated() signal.
\snippet snippets/code/doc_src_unix-signal-handlers.cpp 1
Somewhere else in your startup code, you install your Unix signal
handlers with sigaction(2).
\snippet snippets/code/doc_src_unix-signal-handlers.cpp 2
In your Unix signal handlers, you write a byte to the \e write end
of a socket pair and return. This will cause the corresponding
QSocketNotifier to emit its activated() signal, which will in turn
cause the appropriate Qt slot function to run.
\snippet snippets/code/doc_src_unix-signal-handlers.cpp 3
In the slot functions connected to the
QSocketNotifier::activated() signals, you \e read the byte. Now
you are safely back in Qt with your signal, and you can do all the
Qt stuff you weren'tr allowed to do in the Unix signal handler.
\snippet snippets/code/doc_src_unix-signal-handlers.cpp 4
*/
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