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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 platform-specific

    \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 doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 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 doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 1

    Somewhere else in your startup code, you install your Unix signal

    handlers with sigaction(2).

    \snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 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 slott function to run.

    \snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 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 doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 4

*/