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copied!<p>I used to wonder a lot about the same!</p> <p>A GUI main loop looks like this, in pseudo-code:</p> <pre><code>void App::exec() { for(;;) { vector<Waitable> waitables; waitables.push_back(m_networkSocket); waitables.push_back(m_xConnection); waitables.push_back(m_globalTimer); Waitable* whatHappened = System::waitOnAll(waitables); switch(whatHappened) { case &m_networkSocket: readAndDispatchNetworkEvent(); break; case &m_xConnection: readAndDispatchGuiEvent(); break; case &m_globalTimer: readAndDispatchTimerEvent(); break; } } } </code></pre> <p>What is a "Waitable"? Well, it's system dependant. On UNIX it's called a "file descriptor" and "waitOnAll" is the ::select system call. The so-called <code>vector<Waitable></code> is a <code>::fd_set</code> on UNIX, and "whatHappened" is actually queried via <code>FD_ISSET</code>. The actual waitable-handles are acquired in various ways, for example <code>m_xConnection</code> can be taken from ::XConnectionNumber(). X11 also provides a high-level, portable API for this -- ::XNextEvent() -- but if you were to use that, you wouldn't be able to wait on several event sources <em>simultaneously</em>.</p> <p>How does the blocking work? "waitOnAll" is a syscall that tells the OS to put your process on a "sleep list". This means you are not given any CPU time until an event occurs on one of the waitables. This, then, means your process is idle, consuming 0% CPU. When an event occurs, your process will briefly react to it and then return to idle state. GUI apps spend almost <em>all</em> their time idling.</p> <p>What happens to all the CPU cycles while you're sleeping? Depends. Sometimes another process will have a use for them. If not, your OS will busy-loop the CPU, or put it into temporary low-power mode, etc.</p> <p>Please ask for further details!</p>

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