source: trunk/doc/src/deployment/deployment-plugins.qdoc@ 846

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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.
*/