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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
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****************************************************************************/
/*!
\example threads/waitconditions
\title Wait Conditions Example
The Wait Conditions example shows how to use QWaitCondition and
QMutex 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.
Wait conditions make it possible to have a higher level of
concurrency than what is possible with mutexes alone. If accesses
to the buffer were simply 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 synchronization
tools that protect it are global variables.
An alternative to using QWaitCondition and QMutex to solve the
producer-consumer problem is to use QSemaphore. This is what the
\l{threads/semaphores}{Semaphores} example does.
\section1 Global Variables
Let's start by reviewing the circular buffer and the associated
synchronization tools:
\snippet examples/threads/waitconditions/waitconditions.cpp 0
\c DataSize is the amount 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 wait
conditions and one mutex. The \c bufferNotEmpty condition is
signalled when the producer has generated some data, telling the
consumer that it can start reading it. The \c bufferNotFull
condition is signalled when the consumer has read some data,
telling the producer that it can generate more. The \c numUsedBytes
is the number of bytes in the buffer that contain data.
Together, the wait conditions, the mutex, and the \c numUsedBytes
counter ensure that the producer is never more than \c BufferSize
bytes ahead of the consumer, and that the consumer never reads
data that the consumer hasn't generated yet.
\section1 Producer Class
Let's review the code for the \c Producer class:
\snippet examples/threads/waitconditions/waitconditions.cpp 1
\snippet examples/threads/waitconditions/waitconditions.cpp 2
The producer generates \c DataSize bytes of data. Before it
writes a byte to the circular buffer, it must first check whether
the buffer is full (i.e., \c numUsedBytes equals \c BufferSize).
If the buffer is full, the thread waits on the \c bufferNotFull
condition.
At the end, the producer increments \c numUsedBytes and signalls
that the condition \c bufferNotEmpty is true, since \c
numUsedBytes is necessarily greater than 0.
We guard all accesses to the \c numUsedBytes variable with a
mutex. In addition, the QWaitCondition::wait() function accepts a
mutex as its argument. This mutex is unlocked before the thread
is put to sleep and locked when the thread wakes up. Furthermore,
the transition from the locked state to the wait state is atomic,
to prevent race conditions from occurring.
\section1 Consumer Class
Let's turn to the \c Consumer class:
\snippet examples/threads/waitconditions/waitconditions.cpp 3
\snippet examples/threads/waitconditions/waitconditions.cpp 4
The code is very similar to the producer. Before we read the
byte, we check whether the buffer is empty (\c numUsedBytes is 0)
instead of whether it's full and wait on the \c bufferNotEmpty
condition if it's empty. After we've read the byte, we decrement
\c numUsedBytes (instead of incrementing it), and we signal the
\c bufferNotFull condition (instead of the \c bufferNotEmpty
condition).
\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/waitconditions/waitconditions.cpp 5
\snippet examples/threads/waitconditions/waitconditions.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 bufferNotEmpty condition to be
signalled (\c numUsedBytes is 0). Once the producer has put one
byte in the buffer, \c numUsedBytes is \c BufferSize - 1 and the
\c bufferNotEmpty condition is signalled. 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.
Locking and unlocking a QMutex 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.
*/
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