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++ // Copyright (C) 2016 The Qt Company Ltd.
++ // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
++
++ /*!
++ \example threads/semaphores
++ \examplecategory {Data Processing & I/O}
++ \title Producer and Consumer using Semaphores
++ \ingroup qtconcurrent-mtexamples
++ \ingroup corelib_examples
++
++ \brief The Producer and Consumer using 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{Producer and Consumer using Wait Conditions} example does.
++
++ \section1 Global Variables
++
++ Let's start by reviewing the circular buffer and the associated
++ semaphores:
++
++ \snippet threads/semaphores/semaphores.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
++ 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 threads/semaphores/semaphores.cpp 1
++ \snippet 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 threads/semaphores/semaphores.cpp 3
++ \snippet 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 threads/semaphores/semaphores.cpp 5
++ \snippet 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 producer thread 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.
++ */