Have you tried gtk_window_get_focus ()? Users are frequently interacting with entries, so it may work for you. The documentation says that it "retrieves the current focused widget within the window." You can look it by yourself here. Then, compare if the widget retrieved is one of your entries.
Once you get the focused entry, perhaps you would like to get its text using gtk_entry_get_text () , though, it will get all the text in the entry. If this does not fit your purposes, the solution might be using gtk_editable_copy_clipboard () which copies the contents of the currently selected content in the editable (of course, cast the entry to editable) and puts it on the clipboard. Then if it applies, paste what was copied using gtk_editable_paste_clipboard ().