Exo is an Xfce library targeted at application development, originally developed by os-cillation. It contains various custom widgets and APIs extending the functionality of GLib and GTK. It also has some helper applications that are used throughout the entire Xfce desktop to manage preferred applications and edit .desktop files. exo is targeted at application development and is considered ready for production use. You no longer need to define the EXO_API_SUBJECT_TO_CHANGE preprocessor symbol.