The qt.conf file overrides the hard-coded paths that are compiled into the Qt library. These paths are accessible using the QLibraryInfo class. Without qt.conf, the functions in QLibraryInfo return these hard-coded paths; otherwise they return the paths as specified in qt.conf.

Without qt.conf, the Qt libraries will use the hard-coded paths to look for plugins, translations, and so on. These paths may not exist on the target system, or they may not be accesssible. Because of this, you need qt.conf to make the Qt libraries look elsewhere.

QLibraryInfo will load qt.conf from one of the following locations:

1. :/qt/etc/qt.conf using the resource system
2. on Mac OS X, in the Resource directory inside the application bundle, for example assistant.app/Contents/Resources/qt.conf
3. in the directory containing the application executable, i.e. QCoreApplication::applicationDirPath() + QDir::separator() + "qt.conf"

The qt.conf file is an INI text file, as described in the QSettings documentation. The file should have a Paths group which contains the entries that correspond to each value of the QLibraryInfo::LibraryLocation enum. See the QLibraryInfo documentation for details on the meaning of the various locations.

Absolute paths are used as specified in the qt.conf file. All paths are relative to the Prefix. On Windows and X11, the Prefix is relative to the directory containing the application executable (QCoreApplication::applicationDirPath()). On Mac OS X, the Prefix is relative to the Contents in the application bundle. For example, application.app/Contents/plugins/ is the default location for loading Qt plugins. Note that the plugins need to be placed in specific sub-directories under the plugins directory (see How to Create Qt Plugins for details).

For example, a qt.conf file could contain the following:

[Paths]
Prefix = /some/path
Translations = i18n

Subgroups of the Paths group may be used to specify locations for specific versions of the Qt libraries. Such subgroups are of the form Paths/x.y.z, where x is the major version of the Qt libraries, y the minor, and z the patch level. The subgroup that most closely matches the current Qt version is used. If no subgroup matches, the Paths group is used as the fallback. The minor and patch level values may be omitted, in which case they default to zero.

For example, given the following groups:

Paths
Paths/4
Paths/4.1
Paths/4.2.5
Paths/6

The current version will be matched as shown:

• 4.0.1 matches Paths/4
• 4.1.5 matches Paths/4.1
• 4.6.3 matches Paths/4.2.5 (because 4.2.5 is the latest version with the same major version number)
• 5.0.0 matches Paths
• 6.0.2 matches Paths/6