qt-everywhere-src-5.15.1/qtdoc/doc/src/deployment/deployment-plugins.qdoc

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/*!
    \page deployment-plugins.html
    \title Deploying Plugins
    \brief A guide to the plugin-specific aspects of deploying Qt and Qt Applications.

    This topic explains how to deploy plugin libraries for Qt or your application to 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.

    \section1 The Plugin Directory

    In Qt, when an application starts, the application's executable directory is the base directory
    where Qt searches for plugins.

    For example, on Windows, if the application is in \c{C:\Program Files\MyApp} and it has a style
    plugin, Qt looks in \c{C:\Program Files\MyApp\styles}.

    To find out where your application's executable is located, see
    QCoreApplication::applicationDirPath().

    Qt also looks in the directory specified by QLibraryInfo::location(QLibraryInfo::PluginsPath),
    which typically is located in \c QTDIR/plugins; \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(). If you want to set your own path(s), you can use
    QCoreApplication::setLibraryPaths().

    Alternatively, you can use a \c qt.conf file to override the hard-coded paths that are compiled
    into the Qt library. For more information, see \l {Using qt.conf}.

    Another possibility is to set the \c QT_PLUGIN_PATH environment variable before you run the
    application; multiple paths can be separated with a system path separator. When set, Qt looks
    for plugins in the paths specified in this variable.

    \note Do not export \c QT_PLUGIN_PATH as a system-wide environment variable because it can
    interfere with other Qt installations.

    \section1 Loading and Verifying Plugins Dynamically

    When loading plugins, the Qt library does some sanity checking to determine whether the plugin
    can be loaded and used. This sanity check enables you to have multiple Qt versions and
    configurations installed side by side.

    The following rules apply:

    \list
    \li 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
        \b{Example:} Qt 5.0.0 will \e{not} load a plugin built with Qt 5.0.1.

    \li 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
        \b{Example:} Qt 5.0.1 will \e{not} load a plugin built with Qt 4.8.2.
        \br
        \b{Example:} Qt 5.1.1 will load plugins built with Qt 5.1.0 and Qt 5.0.3.
    \endlist

    When building plugins to extend an application, it's 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. Except for Unix operating systems,
    where the plugin system will not load plugins built in a different mode from the application.

    If you configure Qt to be built in both debug and release modes, but only build your
    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 (\c{.pro}) file:

    \code
        CONFIG += release
    \endcode

    This ensures that the plugin is compatible with the version of the library used in the
    application.

    \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.

    To obtain diagnostic information from Qt, about each plugin it tries to load, use the
    \c QT_DEBUG_PLUGINS environment variable. Set this variable to a non-zero value in the
    environment where your application is launched.

    The following table describes the common causes of problems developers experience when creating
    plugins and possible solutions.

    \table
      \header
        \li Problem \li Cause \li Solution
      \row
        \li Plugins sliently fail to load even when opened directly by the application.
            \QD shows the plugin libraries in its \uicontrol{Help|About Plugins} dialog, but no
            plugins are listed under each of them.
        \li The application and its plugins are built in different modes.
        \li 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.
    \endtable

*/