path: root/doc/src/examples/semaphores.qdoc
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authorQt by Nokia <>2011-04-27 12:05:43 +0200
committeraxis <>2011-04-27 12:05:43 +0200
commit38be0d13830efd2d98281c645c3a60afe05ffece (patch)
tree6ea73f3ec77f7d153333779883e8120f82820abe /doc/src/examples/semaphores.qdoc
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+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (
+** This file is part of the documentation of the Qt Toolkit.
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+** GNU Free Documentation License
+** 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.
+** If you have questions regarding the use of this file, please contact
+** Nokia at
+ \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.