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diff --git a/doc/src/examples/semaphores.qdoc b/doc/src/examples/semaphores.qdoc deleted file mode 100644 index b6b4d68f9b..0000000000 --- a/doc/src/examples/semaphores.qdoc +++ /dev/null @@ -1,145 +0,0 @@ -/**************************************************************************** -** -** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies). -** Contact: http://www.qt-project.org/legal -** -** This file is part of the documentation of the Qt Toolkit. -** -** $QT_BEGIN_LICENSE:FDL$ -** Commercial License Usage -** Licensees holding valid commercial Qt licenses may use this file in -** accordance with the commercial license agreement provided with the -** Software or, alternatively, in accordance with the terms contained in -** a written agreement between you and Digia. For licensing terms and -** conditions see http://qt.digia.com/licensing. For further information -** use the contact form at http://qt.digia.com/contact-us. -** -** GNU Free Documentation License Usage -** Alternatively, this file may be used under the terms of the GNU Free -** Documentation License version 1.3 as published by the Free Software -** Foundation and appearing in the file included in the packaging of -** this file. Please review the following information to ensure -** the GNU Free Documentation License version 1.3 requirements -** will be met: http://www.gnu.org/copyleft/fdl.html. -** $QT_END_LICENSE$ -** -****************************************************************************/ - -/*! - \example threads/semaphores - \title Semaphores Example - - The Semaphores example shows how to use QSemaphore to control - access to a circular buffer shared by a producer thread and a - consumer thread. - - The producer writes data to the buffer until it reaches the end - of the buffer, at which point it restarts from the beginning, - overwriting existing data. The consumer thread reads the data as - it is produced and writes it to standard error. - - Semaphores make it possible to have a higher level of concurrency - than mutexes. If accesses to the buffer were guarded by a QMutex, - the consumer thread couldn't access the buffer at the same time - as the producer thread. Yet, there is no harm in having both - threads working on \e{different parts} of the buffer at the same - time. - - The example comprises two classes: \c Producer and \c Consumer. - Both inherit from QThread. The circular buffer used for - communicating between these two classes and the semaphores that - protect it are global variables. - - An alternative to using QSemaphore to solve the producer-consumer - problem is to use QWaitCondition and QMutex. This is what the - \l{threads/waitconditions}{Wait Conditions} example does. - - \section1 Global Variables - - Let's start by reviewing the circular buffer and the associated - semaphores: - - \snippet examples/threads/semaphores/semaphores.cpp 0 - - \c DataSize is the amout of data that the producer will generate. - To keep the example as simple as possible, we make it a constant. - \c BufferSize is the size of the circular buffer. It is less than - \c DataSize, meaning that at some point the producer will reach - the end of the buffer and restart from the beginning. - - To synchronize the producer and the consumer, we need two - semaphores. The \c freeBytes semaphore controls the "free" area - of the buffer (the area that the producer hasn't filled with data - yet or that the consumer has already read). The \c usedBytes - semaphore controls the "used" area of the buffer (the area that - the producer has filled but that the consumer hasn't read yet). - - Together, the semaphores ensure that the producer is never more - than \c BufferSize bytes ahead of the consumer, and that the - consumer never reads data that the producer hasn't generated yet. - - The \c freeBytes semaphore is initialized with \c BufferSize, - because initially the entire buffer is empty. The \c usedBytes - semaphore is initialized to 0 (the default value if none is - specified). - - \section1 Producer Class - - Let's review the code for the \c Producer class: - - \snippet examples/threads/semaphores/semaphores.cpp 1 - \snippet examples/threads/semaphores/semaphores.cpp 2 - - The producer generates \c DataSize bytes of data. Before it - writes a byte to the circular buffer, it must acquire a "free" - byte using the \c freeBytes semaphore. The QSemaphore::acquire() - call might block if the consumer hasn't kept up the pace with the - producer. - - At the end, the producer releases a byte using the \c usedBytes - semaphore. The "free" byte has successfully been transformed into - a "used" byte, ready to be read by the consumer. - - \section1 Consumer Class - - Let's now turn to the \c Consumer class: - - \snippet examples/threads/semaphores/semaphores.cpp 3 - \snippet examples/threads/semaphores/semaphores.cpp 4 - - The code is very similar to the producer, except that this time - we acquire a "used" byte and release a "free" byte, instead of - the opposite. - - \section1 The main() Function - - In \c main(), we create the two threads and call QThread::wait() - to ensure that both threads get time to finish before we exit: - - \snippet examples/threads/semaphores/semaphores.cpp 5 - \snippet examples/threads/semaphores/semaphores.cpp 6 - - So what happens when we run the program? Initially, the producer - thread is the only one that can do anything; the consumer is - blocked waiting for the \c usedBytes semaphore to be released (its - initial \l{QSemaphore::available()}{available()} count is 0). - Once the producer has put one byte in the buffer, - \c{freeBytes.available()} is \c BufferSize - 1 and - \c{usedBytes.available()} is 1. At that point, two things can - happen: Either the consumer thread takes over and reads that - byte, or the consumer gets to produce a second byte. - - The producer-consumer model presented in this example makes it - possible to write highly concurrent multithreaded applications. - On a multiprocessor machine, the program is potentially up to - twice as fast as the equivalent mutex-based program, since the - two threads can be active at the same time on different parts of - the buffer. - - Be aware though that these benefits aren't always realized. - Acquiring and releasing a QSemaphore has a cost. In practice, it - would probably be worthwhile to divide the buffer into chunks and - to operate on chunks instead of individual bytes. The buffer size - is also a parameter that must be selected carefully, based on - experimentation. -*/ |