Re: [Chicken-users] Check for NULL struct

[Felix]

From: Cosme Enmanuel Zamudio Salazar <address@hidden>
Subject: [Chicken-users] Check for NULL struct
Date: Tue, 29 Mar 2011 09:46:53 -0700

> Is there a way to check for a NULL structure returned from a C function?
> 
> this is the structure
> 
> (define-record sdl-surface pointer)
> 
> (define-record-printer (sdl-surface s out)
>   (for-each (lambda (x) (display x out))
>     (list "#<sdl-surface "(sdl-surface-pointer s)">")))
> 
> (define-foreign-type SDL_Surface (c-pointer "SDL_Surface")
>   sdl-surface-pointer
>   make-sdl-surface)
> 
> 
> the NULL can be returned when i try to load an image that doesnt exists:
> 
> #;12> (img-load "test.png")
> #<sdl-surface #f>
> 
> but how do i check if its NULL ?

"c-pointer" usually maps to "#f" (as Kon pointed out in his reply).
But since you are using automatic argument/result conversion by wrapping
the pointer into a struct, you will get an "#f" in the pointer field.

You ccould use a different result wrapper in your foreign type definition:

(define (make-sdl-surface-wrapper ptr)
  (and ptr (make-sdl-surface ptr)))

Or check explicitly with "(not (sdl-surface-pointer SURFACE))".

Raw pointer objects that contain a NULL pointer are usually mapped to
"#f" by the FFI, but in some cases are not, so you can check a pointer
object for a NULL-address with "(null-pointer? PTR)" (I think this is
not necessary here, I just mention it for completeness).


cheers,
felix