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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.

*/