Re: How to get GWorld's pixel address

[bill@cambridge.apple.com (Bill St. Clair)]

At 3:05 AM 5/3/95, Minagawa,Kazushi wrote:
>I'm having a problem with getting GWorld's pixel address using the
>GetPixBaseAddr trap.  I must be doing something wrong.  
>
>Here's the code. 
>
>(defun access-to-gworld ()
>        (rlet ((gw-p :ointer)
>                  (gw-r :rect
>                            :topleft #@(0 0)
>                            :bottomright #@(100 100)))
>    (#_NewGWorld gw-p 32 gw-r (%null-ptr) (%null-ptr) 0)
>    (with-macptrs ((pix-addr (#_GetPixBaseAddr (#_GetGWorldPxMap gw-p))))
>      (print (%get-byte pix-addr)))))
>
>With macsbug I show that the error is "bus error". This means the address I
>got using the GetPixBaseAddress trap is invalid.
>
>Does anybody have any ideas about what is wrong here?

gw-p is a pointer to a pointer to a cgrafport. #_GetGWorldPixmap
expects a pointer to a cgrafport, so you need an additional
%get-ptr call (CAPITALIZED BELOW):

(defun access-to-gworld ()
        (rlet ((gw-p :pointer)
               (gw-r :rect
                     :topleft #@(0 0)
                     :bottomright #@(100 100)))
    (#_NewGWorld gw-p 32 gw-r (%null-ptr) (%null-ptr) 0)
    (with-macptrs ((pix-addr (#_GetPixBaseAddr (#_GetGWorldPixMap
                                                (%GET-PTR GW-P)))))
      (print (%get-byte pix-addr)))))