1.2.1 Defining FLI types

The example uses the FLI to find the position of the cursor using the Windows function GetCursorPos , which has the following C prototype:

The LPPOINT argument is a pointer to the POINT structure, which has the following C definition:

typedef struct tagPOINT {

    LONG x;

    LONG y;

} POINT;

First we use the fli:define-c-typedef macro to define a number of basic types which are needed to pass data to and from the Windows function.

This defines two types, BOOL and LONG , which are used to associate a Lisp boolean value ( t or nil ) with a C boolean of type int , and a Lisp bignum with a C long . These are required because the Windows function GetCursorPos returns a boolean to indicate if it has executed successfully, and the cursor's x and y positions are specified in a long format in the POINT structure.

Next, we need to define a structure for the FLI which is used to get the coordinates of the cursor. These coordinates will consist of an x and a y position. We use the fli:define-c-struct macro for this, and the resulting Lisp FLI code has obvious parallels with the C tagPOINT structure.

(fli:define-c-struct tagpoint

  (x long)

  (y long))

The tagPOINT structure for the FLI, corresponding to the C structure of the same name, has been defined. This now needs to be further defined as a type for the FLI, using fli:define-c-typedef .

Finally, a pointer type to point to the structure is required. It is this FLI pointer which will be passed to the Windows function GetCursorPos , so that GetCursorPos can change the x and y values of the structure pointed to.

All the required FLI types have now been defined. Although it may seem that there is a level of duplicity in the definitions of the structures, pointers and types in this section, this was necessary to match the data structures of the C functions to which the FLI will interface. We can now move on to the definition of FLI functions to perform the interfacing.