[racket] Help debugging a ffi crash

Looking at 

  http://clang.llvm.org/doxygen/CXString_8h_source.html

it seems that CXString as returned by clang_getCursorSpelling() is not
a pointer:

 typedef struct {
   const void *data;
   unsigned private_flags;
 } CXString;

If that's right, I'm a little surprised that `cursor-spelling` works
--- but when you get representation wrong, strange things can happen,
including something working when it shouldn't.

Am I looking at the right library/definitions?

At Sun, 28 Sep 2014 10:48:06 -0700, Eric Dobson wrote:
> I'm trying to debug an FFI crash that I'm seeing, and because it is
> dealing with C code the error just presents as a segfault. I believe I
> have tracked down what is causing the problem, but don't understand
> how it could be doing so.
> 
> I have two racket functions which take a "cursor" (the foreign
> libraries object) and return a string representation of it, which I'm
> trying to use for debugging.
> 
> (define raw-clang-get-cstring
>   (get-ffi-obj "clang_getCString" lib-clang
>     (_fun _pointer -> _string)))
> 
> (define raw-cursor-spelling
>  (get-ffi-obj "clang_getCursorSpelling" lib-clang
>    (_fun _CXCursor -> _pointer)))
> 
> (define (cursor-spelling c)
>   (raw-clang-get-cstring (raw-cursor-spelling c)))
> 
> (define cursor-spelling2
>  (get-ffi-obj "clang_getCursorSpelling" lib-clang
>    (_fun _CXCursor -> (make-ctype _pointer values (λ (v)
> (raw-clang-get-cstring v))))))
> 
> If I use cursor-spelling, I have not been able to trigger a crash. But
> if I use cursor-spelling2 I can reliably trigger a crash.
> 
> Is there anything obvious on how these functions are different?
> Because they look to me like they should be doing the same thing. If
> it would be helpful I can try to get my code in a portable enough
> shape so that it will work/crash on another machine.
> 
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users