GTK+ has a high level set of functions for doing inter-process communication via the drag-and-drop system. GTK+ can perform drag-and-drop on top of the low level Xdnd and Motif drag-and-drop protocols.
An application capable of GTK+ drag-and-drop first defines and sets up the GTK+ widget(s) for drag-and-drop. Each widget can be a source and/or destination for drag-and-drop. Note that these GTK+ widgets must have an associated X Window, check using GTK_WIDGET_NO_WINDOW(widget)).
Source widgets can send out drag data, thus allowing the user to drag things off of them, while destination widgets can receive drag data. Drag-and-drop destinations can limit who they accept drag data from, e.g. the same application or any application (including itself).
Sending and receiving drop data makes use of GTK+ signals. Dropping an item to a destination widget requires both a data request (for the source widget) and data received signal handler (for the target widget). Additional signal handers can be connected if you want to know when a drag begins (at the very instant it starts), to when a drop is made, and when the entire drag-and-drop procedure has ended (successfully or not).
Your application will need to provide data for source widgets when requested, that involves having a drag data request signal handler. For destination widgets they will need a drop data received signal handler.
So a typical drag-and-drop cycle would look as follows:
Drag data request (when a drop occurs).
Drop data received (may be on same or different application).
Drag data delete (if the drag was a move).
Drag-and-drop procedure done.
There are a few minor steps that go in between here and there, but we will get into detail about that later.