Here is an example for a simple program that uses CHICKEN's foreign function interface. It is the Gtk 2.0 Hello, world program as presented in the GTK tutorial, which is available here.

This example is intended to show various features of the FFI and to demonstrate that one can go relatively far without writing tons of glue code. The comments are only to show the CHICKEN specific things, see the original tutorial for more information.

 ;;;; gtkhello.scm

 (require-extension easyffi)

 ;; Include this code directly into generated .c file:

 (foreign-declare "#include <gtk/gtk.h>")

 ;; Parse declarations and generate Scheme bindings:

 (foreign-parse "
 const int GTK_WINDOW_TOPLEVEL;

 void gtk_main_quit();
 void gtk_init(int *, char ***);
 GtkWidget *gtk_window_new(int);
 int g_signal_connect(void *, char *, void *, void *);
 int g_signal_connect_swapped(void *, char *, void *, void *);
 void gtk_container_set_border_width(GtkContainer *, int);
 GtkWidget *gtk_button_new_with_label(char *);
 void gtk_widget_destroy(GtkWidget *);
 void gtk_container_add(GtkContainer *, GtkWidget *);
 void gtk_widget_show(GtkWidget *);

 /* the ___safe marker is needed, because gtk_main() may
   (and will) call Scheme callbacks. */
 ___safe void gtk_main();
 ")


 ;; Define a few callback function:

 (define-external (hello ((pointer "GtkWidget") widget) 
			 (c-pointer data)) 
   void
   (print "Hello, world") )

 (define-external (delete_event ((pointer "GtkWidget") widget)
				((pointer "GdkEvent") event)
				(c-pointer data) )
   bool
   (print "Delete event occured")
   #t)

 (define-external (destroy ((pointer "GtkWidget") widget)
			   (c-pointer data) )
   void
   (gtk_main_quit) )

 ;; This is a bit ugly, but there is no elegant way to get
 ;; at argc/argv at the Scheme level and pass pointers to it
 ;; to foreign code:

 (foreign-code "gtk_init(&C_main_argc, &C_main_argv);")

 (define window (gtk_window_new GTK_WINDOW_TOPLEVEL))

 ;; #$... with a callback-name returns the function pointer:

 (g_signal_connect window "delete_event" #$delete_event #f)
 (g_signal_connect window "destroy" #$destroy #f)

 (gtk_container_set_border_width window 10)

 (define button (gtk_button_new_with_label "Hello World"))

 (g_signal_connect button "clicked" #$hello #f)

 ;; Here we use foreign-value to get the function pointer.
 ;; #$gtk_widget_destroy wouldn't work: the function is not
 ;; a callback:

 (g_signal_connect_swapped button "clicked" 
   (foreign-value gtk_widget_destroy c-pointer) window)

 (gtk_container_add window button)
 (gtk_widget_show button)
 (gtk_widget_show window)
 (gtk_main)

To compile it, we have to ensure the proper C compiler and linker flags are passed:

$ csc -vk gtkhello.scm -C "`pkg-config --cflags gtk+-2.0`" -L "`pkg-config --libs gtk+-2.0`" -X easyffi

(the easyffi egg is required)