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\page unix-signals.html
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\title Calling Qt Functions From Unix Signal Handlers
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\brief You can't. But don't despair, there is a way...
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\ingroup platform-specific
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\ingroup best-practices
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You \e can't call Qt functions from Unix signal handlers. The
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standard POSIX rule applies: You can only call async-signal-safe
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functions from signal handlers. See \l
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{http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_04.html#tag_02_04_01}
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{Signal Actions} for the complete list of functions you can call
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from Unix signal handlers.
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But don't despair, there is a way to use Unix signal handlers with
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Qt. The strategy is to have your Unix signal handler do something
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that will eventually cause a Qt signal to be emitted, and then you
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simply return from your Unix signal handler. Back in your Qt
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program, that Qt signal gets emitted and then received by your Qt
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slot function, where you can safely do whatever Qt stuff you
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weren't allowed to do in the Unix signal handler.
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One simple way to make this happen is to declare a socket pair in
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your class for each Unix signal you want to handle. The socket
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pairs are declared as static data members. You also create a
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QSocketNotifier to monitor the \e read end of each socket pair,
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declare your Unix signal handlers to be static class methods, and
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declare a slot function corresponding to each of your Unix signal
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handlers. In this example, we intend to handle both the SIGHUP and
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SIGTERM signals. Note: You should read the socketpair(2) and the
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sigaction(2) man pages before plowing through the following code
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\snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 0
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In the MyDaemon constructor, use the socketpair(2) function to
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initialize each file descriptor pair, and then create the
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QSocketNotifier to monitor the \e read end of each pair. The
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activated() signal of each QSocketNotifier is connected to the
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appropriate slot function, which effectively converts the Unix
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signal to the QSocketNotifier::activated() signal.
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\snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 1
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Somewhere else in your startup code, you install your Unix signal
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handlers with sigaction(2).
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\snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 2
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In your Unix signal handlers, you write a byte to the \e write end
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of a socket pair and return. This will cause the corresponding
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QSocketNotifier to emit its activated() signal, which will in turn
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cause the appropriate Qt slott function to run.
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\snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 3
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In the slot functions connected to the
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QSocketNotifier::activated() signals, you \e read the byte. Now
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you are safely back in Qt with your signal, and you can do all the
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Qt stuff you weren'tr allowed to do in the Unix signal handler.
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\snippet doc/src/snippets/code/doc_src_unix-signal-handlers.qdoc 4