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    41 

    42 /*!

    43     \page unix-signals.html

    44     \title Calling Qt Functions From Unix Signal Handlers

    45     \brief You can't. But don't despair, there is a way...

    46 

    47     \ingroup platform-specific

    48     \ingroup best-practices

    49 

    50     You \e can't call Qt functions from Unix signal handlers. The

    51     standard POSIX rule applies: You can only call async-signal-safe

    52     functions from signal handlers. See \l

    53     {http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_04.html#tag_02_04_01}

    54     {Signal Actions} for the complete list of functions you can call

    55     from Unix signal handlers.

    56 

    57     But don't despair, there is a way to use Unix signal handlers with

    58     Qt. The strategy is to have your Unix signal handler do something

    59     that will eventually cause a Qt signal to be emitted, and then you

    60     simply return from your Unix signal handler. Back in your Qt

    61     program, that Qt signal gets emitted and then received by your Qt

    62     slot function, where you can safely do whatever Qt stuff you

    63     weren't allowed to do in the Unix signal handler.

    64 

    65     One simple way to make this happen is to declare a socket pair in

    66     your class for each Unix signal you want to handle. The socket

    67     pairs are declared as static data members. You also create a

    68     QSocketNotifier to monitor the \e read end of each socket pair,

    69     declare your Unix signal handlers to be static class methods, and

    70     declare a slot function corresponding to each of your Unix signal

    71     handlers. In this example, we intend to handle both the SIGHUP and

    72     SIGTERM signals. Note: You should read the socketpair(2) and the

    73     sigaction(2) man pages before plowing through the following code

    74     snippets.

    75     

    76     \snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 0

    77 

    78     In the MyDaemon constructor, use the socketpair(2) function to

    79     initialize each file descriptor pair, and then create the

    80     QSocketNotifier to monitor the \e read end of each pair. The

    81     activated() signal of each QSocketNotifier is connected to the

    82     appropriate slot function, which effectively converts the Unix

    83     signal to the QSocketNotifier::activated() signal.

    84 

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

    86 

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

    88     handlers with sigaction(2).

    89 

    90     \snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 2

    91 

    92     In your Unix signal handlers, you write a byte to the \e write end

    93     of a socket pair and return. This will cause the corresponding

    94     QSocketNotifier to emit its activated() signal, which will in turn

    95     cause the appropriate Qt slott function to run.

    96 

    97     \snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 3

    98 

    99     In the slot functions connected to the

   100     QSocketNotifier::activated() signals, you \e read the byte. Now

   101     you are safely back in Qt with your signal, and you can do all the

   102     Qt stuff you weren'tr allowed to do in the Unix signal handler.

   103 

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

   105 */