This topic explains how to deploy plugin libraries for Qt or your application to load at runtime. If you use static plugins, then the plugin code is already part of your application executable and no separate deployment steps are required.

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:\Program Files\MyApp and it has a style plugin, Qt looks in 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::path(QLibraryInfo::PluginsPath), which typically is located in QTDIR/plugins; 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 qt.conf file to override the hard-coded paths that are compiled into the Qt library. For more information, see Using qt.conf.

Another possibility is to set the 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 QT_PLUGIN_PATH as a system-wide environment variable because it can interfere with other Qt installations.

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:

• Plugins linked with a Qt library that has a higher version number will not be loaded by a library with a lower version number.

  
  Example: Qt 5.0.0 will not load a plugin built with Qt 5.0.1.

• 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.

  
  Example: Qt 5.0.1 will not load a plugin built with Qt 4.8.2.
  Example: Qt 5.1.1 will load plugins built with Qt 5.1.0 and Qt 5.0.3.

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 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 (.pro) file:

CONFIG += release

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

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 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.