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

[Rafael Picanço]

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;
}CCDBS;

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);

Questions

(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
right?

type

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

  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}
    {$DEFINE ELCALLTYPE := cdecl;}
  {$ELSE}
    {$DEFINE ELCALLTYPE := cdecl;}
  {$ENDIF}
{$endif}

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

Best.
R

PS.: You can find a related discussion here
https://stackoverflow.com/questions/76427311/free-pascal-bindings-for-a-typedef-function-inside-a-define-macro-and-a-number-o
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