// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \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 were not allowed to do in the Unix signal handler.

    \snippet snippets/code/doc_src_unix-signal-handlers.cpp 4
*/
