[fpc-pascal] Free Pascal bindings for EyeLink C code (with a god-cast)

Rafael Picanço cpicanco42 at gmail.com
Tue Jun 13 05:19:10 CEST 2023

Hi everyone,

First of all, I am sorry if this is not the best place for this
interoperability issue.

The problem I want to solve is how to make a Free Pascal binding for to the
following piece of C code:

typedef struct _CCDBS {
  void *userdata;
  char buttons[256];
  void *internal;

typedef int (ELCALLBACK *getExButtonStates)(CCDBS *);

#define EXTERNAL_DEV_NONE ((getExButtonStates)0)
#define EXTERNAL_DEV_CEDRUS ((getExButtonStates)1)
#define EXTERNAL_DEV_SYS_KEYBOARD ((getExButtonStates)2)

int ELCALLTYPE enable_external_calibration_device(getExButtonStates
buttonStatesfcn, const char * config, void *userData);


(1) I have no idea on how to translate these god-casts, eg.,
((getExButtonStates)0), to free pascal. Right now, I think that even if I
don't really need them, I just want to understand and learn in the process.

(2) So, the whole point here is binding the
"enable_external_calibration_device" function. It expects a
"getExButtonStates buttonStatesfcn". Can you check if my current binding is


  TCCDBS = record
    userdata: Pointer;
    buttons: array[0..255] of Char;
    internal: Pointer;

  TGetExButtonStatesFunction = function (accdbs: PCCDBS): Int32; ELCALLTYPE

  function enable_external_calibration_device(
    buttonStatesfcn: TGetExButtonStatesFunction;
    config: PChar; userData: Pointer): Int32; ELCALLTYPE ELDLL

(3) I am using the ELCALLTYPE macro just because they were using them
too, I am assuming backward compatibility. But I am not sure if I really
need them. EyeLink code depends on custom SDL2 headers and I am running
SDL2 Free Pascal related stuff just fine (and it has cdecl in everything).
What do you think about that? For reference, this is the macro:

{$ifndef ELCALLTYPE}
  {$IFDEF Windows}
    {$DEFINE ELCALLTYPE := stdcall;}
  {$ELSEIF Linux}
    {$DEFINE ELCALLTYPE := cdecl;}
    {$DEFINE ELCALLTYPE := cdecl;}

(4) I can send the code and soon will release it on my GitHub (cpicanco).


PS.: You can find a related discussion here
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freepascal.org/pipermail/fpc-pascal/attachments/20230613/736b36de/attachment-0001.htm>

More information about the fpc-pascal mailing list