[racket] Unknown C Struct definition

Theres a libsdl struct i want to use from racket:  SDL_Window
the problem is that there is nothing in the SDL include files about the
struct fields, just this:

typedef struct SDL_Window SDL_Window,

the weird part is that i been looking at other language bindings to sdl2
and it looks like they have a way to define a pointer to an unknown
structure type, for example:

SchemeSphere:
https://github.com/alvatar/sphere-sdl2/blob/master/src/sdl2.scm#L480
Python:
https://bitbucket.org/marcusva/py-sdl2/src/a62aa0c6cf3c603d98dc0d4566db1d3fe22076dd/sdl2/video.py?at=default#cl-98

is there a way in Racket to do this? right now im just using a _pointer
(define _SDL_Window _pointer)
, but this looks lacking

Thanks!

Cosme.
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