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    41 

    42 /*!

    43     \page atomic-operations.html

    44     \title Implementing Atomic Operations

    45     \brief A guide to implementing atomic operations on new architectures.

    46 

    47     \ingroup best-practices

    48     \ingroup qt-embedded-linux

    49 

    50     Qt uses an optimization called \l {Implicitly Shared

    51     Classes}{implicit sharing} for many of its value classes.

    52 

    53     Starting with Qt 4, all of Qt's implicitly shared classes can

    54     safely be copied across threads like any other value classes,

    55     i.e., they are fully \l {Reentrancy and Thread-Safety}{reentrant}.

    56     This is accomplished by implementing reference counting

    57     operations using atomic hardware instructions on all the

    58     different platforms supported by Qt.

    59 

    60     To support a new architecture, it is important to ensure that

    61     these platform-specific atomic operations are implemented in a

    62     corresponding header file (\c qatomic_ARCH.h), and that this file

    63     is located in Qt's \c src/corelib/arch directory. For example, the

    64     Intel 80386 implementation is located in \c

    65     src/corelib/arch/qatomic_i386.h.

    66 

    67     Currently, Qt provides two classes providing several atomic

    68     operations, QAtomicInt and QAtomicPointer. These classes inherit

    69     from QBasicAtomicInt and QBasicAtomicPointer, respectively.

    70 

    71     When porting Qt to a new architecture, the QBasicAtomicInt and

    72     QBasicAtomicPointer classes must be implemented, \e not QAtomicInt

    73     and QAtomicPointer. The former classes do not have constructors,

    74     which makes them POD (plain-old-data). Both classes only have a

    75     single member variable called \c _q_value, which stores the

    76     value. This is the value that all of the atomic operations read

    77     and modify.

    78 

    79     All of the member functions mentioned in the QAtomicInt and

    80     QAtomicPointer API documentation must be implemented. Note that

    81     these the implementations of the atomic operations in these

    82     classes must be atomic with respect to both interrupts and

    83     multiple processors.

    84 

    85     \warning The QBasicAtomicInt and QBasicAtomicPointer classes

    86     mentioned in this document are used internally by Qt and are not

    87     part of the public API. They may change in future versions of

    88     Qt. The purpose of this document is to aid people interested in

    89     porting Qt to a new architecture.

    90 */