Tutorial > The XppCbk.exe callback compiler

xppcbk.exe generate a OBJ from a simple syntax script allowing your Xbase++ functions act as true callbacks in any API that require a callback pointer.

In this example we will use the winapi function EnumWindows() that require a pointer to a custom __stdcall function with the following prototype.

BOOL CALLBACK EnumWindowsProc( HWND hwnd, LPARAM lParam);

Because Xbase++ functions cannot be called directly like a C function we will need a intermediate function that fulfill the required prototype and act as a proxy between the caller function and our Xbase++ function.

And this is just the task of our callback compiler so as the first we will create a cbk script file that will produce a .obj with the callback proxy function.

// --------- CALLBACKS.CBK ---------

BEGIN CALLBACK MyEnumWndProc AS BOOL

PARAM HANDLE // hWnd

PARAM DWORD // dwCargo

END CALLBACK

// -------------------------------

Then in our build.bat or from the command line we call the callback compiler to get callbacks.obj from callbacks.cbk ready to be linked or included in our project.

C:\sample>XPPCBK CALLBACKS.CBK

Callback Compiler for Xbase++ Ver 1.0

Compiling CALLBACKS.CBK ...

flat assembler version 1.64

3 passes, 1216 bytes.

C:\sample>

Now we can start writing our PRG

// ------------ TEST.PRG ------------

#include "ot4xb.ch"

// ------------------------------------

function Main()

? EnumWindows( _callback_MyEnumWndProc() , 0 )

The function _callback_MyEnumWndProc() will return a pointer to our proxy function.

inkey(0)

return NIL

// ------------------------------------

DLL USER32 IMPORT BOOL EnumWindows( POINTER32 fpEnum, LPARAM lp)

// ------------------------------------

DLL USER32 IMPORT int GetWindowText( HWND hWnd,LPSTR pStr, int nSize) ;

SYMBOL GetWindowTextA

// ------------------------------------

The next step will be to create our Xbase++ called function.

function MyEnumWndProc( hWnd , nCargo )

local cBuffer := ChrR(0,256)

local nLen := GetWindowText(hWnd,@cBuffer,256)

local cStr := Left(cBuffer,nLen)

if !Empty( cStr )

? ConvToOemCP( cStr)

end

return .T.

// ------------------------------------

The proxy function will call our MyEnumWndProc() providing the 2 parameters and will pass back the return value of MyEnumWndProc() as the result of the proxy callback.

Now our project file will be something like this:

// ------------ TEST.XPJ ------------

[PROJECT]

COMPILE = xpp

COMPILE_FLAGS = /n/m/w

GUI = yes

LINKER = alink

PROJECT.XPJ

[PROJECT.XPJ]

Test.exe

[Test.exe]

Test.prg

callbacks.obj