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** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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\page atomic-operations.html
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\title Implementing Atomic Operations
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\brief A guide to implementing atomic operations on new architectures.
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\ingroup best-practices
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\ingroup qt-embedded-linux
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Qt uses an optimization called \l {Implicitly Shared
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Classes}{implicit sharing} for many of its value classes.
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Starting with Qt 4, all of Qt's implicitly shared classes can
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safely be copied across threads like any other value classes,
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i.e., they are fully \l {Reentrancy and Thread-Safety}{reentrant}.
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This is accomplished by implementing reference counting
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operations using atomic hardware instructions on all the
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different platforms supported by Qt.
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To support a new architecture, it is important to ensure that
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these platform-specific atomic operations are implemented in a
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corresponding header file (\c qatomic_ARCH.h), and that this file
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is located in Qt's \c src/corelib/arch directory. For example, the
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Intel 80386 implementation is located in \c
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src/corelib/arch/qatomic_i386.h.
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Currently, Qt provides two classes providing several atomic
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operations, QAtomicInt and QAtomicPointer. These classes inherit
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from QBasicAtomicInt and QBasicAtomicPointer, respectively.
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When porting Qt to a new architecture, the QBasicAtomicInt and
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QBasicAtomicPointer classes must be implemented, \e not QAtomicInt
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and QAtomicPointer. The former classes do not have constructors,
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which makes them POD (plain-old-data). Both classes only have a
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single member variable called \c _q_value, which stores the
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value. This is the value that all of the atomic operations read
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All of the member functions mentioned in the QAtomicInt and
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QAtomicPointer API documentation must be implemented. Note that
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these the implementations of the atomic operations in these
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classes must be atomic with respect to both interrupts and
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\warning The QBasicAtomicInt and QBasicAtomicPointer classes
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mentioned in this document are used internally by Qt and are not
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part of the public API. They may change in future versions of
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Qt. The purpose of this document is to aid people interested in
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porting Qt to a new architecture.