source: trunk/doc/src/examples/semaphores.qdoc@ 109

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/*!
    \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.
*/