A special include file called SWI-Prolog.h should be
included with each C-source file that is to be loaded via the foreign
interface. The installation process installs this file in the directory include
in the SWI-Prolog home directory (?- feature(home, Home).).
This C-header file defines various data types, macros and functions that
can be used to communicate with SWI-Prolog. Functions and macros can be
divided into the following categories: