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/*!

    \page deployment-plugins.html

    \title Deploying Plugins

    \brief A guide to plugins-specific aspects of deploying Qt and Qt Application

    This document explains how to deploy plugin libraries that Qt or

    your application should load at runtime. If you use

    \l{How to Create Qt Plugins#Static Plugins}{static plugins}, then the

    plugin code is already part of your application executable, and no

    separate deployment steps are required.

    \tableofcontents

    \section1 The Plugin Directory

    When the application is run, Qt will first treat the application's

    executable directory as the \c{pluginsbase}. For example if the

    application is in \c{C:\Program Files\MyApp} and has a style plugin,

    Qt will look in \c{C:\Program Files\MyApp\styles}. (See

    QCoreApplication::applicationDirPath() for how to find out where

    the application's executable is.) Qt will also look in the

    directory specified by

    QLibraryInfo::location(QLibraryInfo::PluginsPath), which typically

    is located in \c QTDIR/plugins (where \c QTDIR is the directory

    where Qt is installed). If you want Qt to look in additional

    places you can add as many paths as you need with calls to

    QCoreApplication::addLibraryPath(). And if you want to set your

    own path or paths you can use QCoreApplication::setLibraryPaths().

    You can also use a \c qt.conf file to override the hard-coded

    paths that are compiled into the Qt library. For more information,

    see the \l {Using qt.conf} documentation. Yet another possibility

    is to set the \c QT_PLUGIN_PATH environment variable before running

    the application. If set, Qt will look for plugins in the

    paths (separated by the system path separator) specified in the variable.

    \section1 Loading and Verifying Plugins Dynamically

    When loading plugins, the Qt library does some sanity checking to

    determine whether or not the plugin can be loaded and used. This

    provides the ability to have multiple versions and configurations of

    the Qt library installed side by side.

    \list

    \o Plugins linked with a Qt library that has a higher version number

       will not be loaded by a library with a lower version number.

      \br

      \bold{Example:} Qt 4.3.0 will \e{not} load a plugin built with Qt 4.3.1.

    \o Plugins linked with a Qt library that has a lower major version

       number will not be loaded by a library with a higher major version

       number.

      \br

      \bold{Example:} Qt 4.3.1 will \e{not} load a plugin built with Qt 3.3.1.

      \br

      \bold{Example:} Qt 4.3.1 will load plugins built with Qt 4.3.0 and Qt 4.2.3.

    \o The Qt library and all plugins are built using a \e {build

       key}. The build key in the Qt library is examined against the build

       key in the plugin, and if they match, the plugin is loaded. If the

       build keys do not match, then the Qt library refuses to load the

       plugin.

      \br \bold{Rationale:} See the \l{#The Build Key}{The Build Key} section below.

    \endlist

    When building plugins to extend an application, it is important to ensure

    that the plugin is configured in the same way as the application. This means

    that if the application was built in release mode, plugins should be built

    in release mode, too.

    If you configure Qt to be built in both debug and release modes,

    but only build applications in release mode, you need to ensure that your

    plugins are also built in release mode. By default, if a debug build of Qt is

    available, plugins will \e only be built in debug mode. To force the

    plugins to be built in release mode, add the following line to the plugin's

    project file:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 3

    This will ensure that the plugin is compatible with the version of the library

    used in the application.

    \section2 The Build Key

    When loading plugins, Qt checks the build key of each plugin against its

    own configuration to ensure that only compatible plugins are loaded; any

    plugins that are configured differently are not loaded.

    The build key contains the following information:

    \list

    \o Architecture, operating system and compiler.

       \e {Rationale:}

       In cases where different versions of the same compiler do not

       produce binary compatible code, the version of the compiler is

       also present in the build key.

    \o Configuration of the Qt library. The configuration is a list

       of the missing features that affect the available API in the

       library.

       \e {Rationale:}

       Two different configurations of the same version of

       the Qt library are not binary compatible. The Qt library that

       loads the plugin uses the list of (missing) features to

       determine if the plugin is binary compatible.

       \e {Note:} There are cases where a plugin can use features that are

       available in two different configurations. However, the

       developer writing plugins would need to know which features are

       in use, both in their plugin and internally by the utility

       classes in Qt. The Qt library would require complex feature

       and dependency queries and verification when loading plugins.

       Requiring this would place an unnecessary burden on the developer, and

       increase the overhead of loading a plugin. To reduce both

       development time and application runtime costs, a simple string

       comparision of the build keys is used.

    \o Optionally, an extra string may be specified on the configure

       script command line.

       \e {Rationale:}

       When distributing binaries of the Qt library with an

       application, this provides a way for developers to write

       plugins that can only be loaded by the library with which the

       plugins were linked.

    \endlist

    For debugging purposes, it is possible to override the run-time build key

    checks by configuring Qt with the \c QT_NO_PLUGIN_CHECK preprocessor macro

    defined.

    \section1 The Plugin Cache

    In order to speed up loading and validation of plugins, some of

    the information that is collected when plugins are loaded is cached

    through QSettings. This includes information about whether or not

    a plugin was successfully loaded, so that subsequent load operations

    don't try to load an invalid plugin. However, if the "last modified"

    timestamp of a plugin has changed, the plugin's cache entry is

    invalidated and the plugin is reloaded regardless of the values in

    the cache entry, and the cache entry itself is updated with the new

    result.

    This also means that the timestamp must be updated each time the

    plugin or any dependent resources (such as a shared library) is

    updated, since the dependent resources might influence the result

    of loading a plugin.

    Sometimes, when developing plugins, it is necessary to remove entries

    from the plugin cache. Since Qt uses QSettings to manage the plugin

    cache, the locations of plugins are platform-dependent; see

    \l{QSettings#Platform-Specific Notes}{the QSettings documentation}

    for more information about each platform.

    For example, on Windows the entries are stored in the registry, and the

    paths for each plugin will typically begin with either of these two strings:

    \snippet doc/src/snippets/code/doc_src_plugins-howto.qdoc 6

    \section1 Debugging Plugins

    There are a number of issues that may prevent correctly-written plugins from

    working with the applications that are designed to use them. Many of these

    are related to differences in the way that plugins and applications have been

    built, often arising from separate build systems and processes.

    The following table contains descriptions of the common causes of problems

    developers experience when creating plugins:

    \table

    \header \o Problem \o Cause \o Solution

    \row \o Plugins sliently fail to load even when opened directly by the

            application. \QD shows the plugin libraries in its

            \gui{Help|About Plugins} dialog, but no plugins are listed under each

            of them.

         \o The application and its plugins are built in different modes.

         \o Either share the same build information or build the plugins in both

            debug and release modes by appending the \c debug_and_release to

            the \l{qmake Variable Reference#CONFIG}{CONFIG} variable in each of

            their project files.

    \row \o A valid plugin that replaces an invalid (or broken) plugin fails to load.

         \o The entry for the plugin in the plugin cache indicates that the original

            plugin could not be loaded, causing Qt to ignore the replacement.

         \o Either ensure that the plugin's timestamp is updated, or delete the

            entry in the \l{#The Plugin Cache}{plugin cache}.

    \endtable

    You can also use the \c QT_DEBUG_PLUGINS environment variable to obtain

    diagnostic information from Qt about each plugin it tries to load. Set this

    variable to a non-zero value in the environment from which your application is

    launched.

*/