4.4 Foreign callables and foreign functions

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

In Introduction to the FLI we defined a foreign function for calling the Win32 function SetCursorPos . The code for this example is repeated here.

(fli:define-foreign-function (set-cursor-position "SetCursorPos")

    ((x :long)

     (y :long))

  :result-type :boolean)

An FLI foreign function calling some C code. is an illustration of set-cursor-position , represented by a square, calling the C code which constitutes SetCursorPos .

Figure 4.1 An FLI foreign function calling some C code.

The next diagram, C calling a callable function in Lisp., 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 using fli: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 function SetCursorPos .

(fli:define-foreign-callable ("SetCursorPos" 

                              :result-type :boolean)

  ((x :long) (y :long))

  (capi:display-message 

     "The cursor position can no longer be set"))

An FLI foreign function calling a callable function. illustrates what happens when set-cursor-position is called. The foreign function set-cursor-position (represented by the square) calls what it believes to be the Windows function SetCursorPos , but the callable function (represented by the oval), also called SetCursorPos , 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, and define-foreign-callable.