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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page atomic-operations.html
\title Implementing Atomic Operations
\brief A guide to implementing atomic operations on new architectures.
\ingroup best-practices
\ingroup qt-embedded-linux
Qt uses an optimization called \l {Implicitly Shared
Classes}{implicit sharing} for many of its value classes.
Starting with Qt 4, all of Qt's implicitly shared classes can
safely be copied across threads like any other value classes,
i.e., they are fully \l {Reentrancy and Thread-Safety}{reentrant}.
This is accomplished by implementing reference counting
operations using atomic hardware instructions on all the
different platforms supported by Qt.
To support a new architecture, it is important to ensure that
these platform-specific atomic operations are implemented in a
corresponding header file (\c qatomic_ARCH.h), and that this file
is located in Qt's \c src/corelib/arch directory. For example, the
Intel 80386 implementation is located in \c
src/corelib/arch/qatomic_i386.h.
Currently, Qt provides two classes providing several atomic
operations, QAtomicInt and QAtomicPointer. These classes inherit
from QBasicAtomicInt and QBasicAtomicPointer, respectively.
When porting Qt to a new architecture, the QBasicAtomicInt and
QBasicAtomicPointer classes must be implemented, \e not QAtomicInt
and QAtomicPointer. The former classes do not have constructors,
which makes them POD (plain-old-data). Both classes only have a
single member variable called \c _q_value, which stores the
value. This is the value that all of the atomic operations read
and modify.
All of the member functions mentioned in the QAtomicInt and
QAtomicPointer API documentation must be implemented. Note that
these the implementations of the atomic operations in these
classes must be atomic with respect to both interrupts and
multiple processors.
\warning The QBasicAtomicInt and QBasicAtomicPointer classes
mentioned in this document are used internally by Qt and are not
part of the public API. They may change in future versions of
Qt. The purpose of this document is to aid people interested in
porting Qt to a new architecture.
*/
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