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/****************************************************************************
**
** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\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.pro 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.
*/
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