4 Advanced Uses of the FLI

4.3 Foreign callables and foreign functions

The two main macros for interfacing LispWorks with a foreign language arefli:define-foreign-callable which defines Lisp functions that can be called from the foreign language, andfli:define-foreign-function which defines a short linking function that can call functions in a foreign language.

In Chapter 1, "Introduction to the FLI" we defined a foreign function for calling the Win32 functionSetCursorPos. The code for this example is repeated here.

(fli:define-foreign-function (set-cursor-position "SetCursorPos")
    ((x :long)
     (y :long))
  :result-type :boolean)

Figure 4.1 is an illustration ofset-cursor-position, represented by a square, calling the C code which constitutesSetCursorPos.

Figure 4.1 An FLI foreign function calling some C code.

The next diagram, Figure 4.2, illustrates a callable function. Whereas a foreign function consists of a Lisp function name calling some code in C, a callable function consists of Lisp code, represented by an oval in the diagram, which can be called from C.

Figure 4.2 C calling a callable function in Lisp.

Callable functions are defined usingfli:define-foreign-callable, which takes as its arguments, amongst other things, the name of the C function that will call Lisp, the arguments for the callable function, and a body of code which makes up the callable function.

The following Lisp code defines a foreign callable function that takes the place of the Windows functionSetCursorPos.

(fli:define-foreign-callable ("SetCursorPos" 
                              :result-type :boolean)
  ((x :long) (y :long))
  (capi:display-message 
     "The cursor position can no longer be set"))

Figure 4.3 illustrates what happens whenset-cursor-position is called. The foreign functionset-cursor-position (represented by the square) calls what it believes to be the Windows functionSetCursorPos, but the callable function (represented by the oval), also calledSetCursorPos, is called instead. It pops up a CAPI pane displaying the message "The cursor position can no longer be set".

Figure 4.3 An FLI foreign function calling a callable function.

For more information on calling foreign code and defining foreign callable functions see define-foreign-function, page 50, and define-foreign-callable, page 48.