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Asked: 2026-05-12T23:00:07+00:00

I was trying to write my own put-pixel on (Gdk) pixbuf in Lisp. When

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I was trying to write my own put-pixel on (Gdk) pixbuf in Lisp. When I finally realized how I can operate on C pointers in CL, new obstacle came along – (gdk:pixbuf-get-pixels pb) returns me negative number. My question is: can I convert it somehow to a valid pointer? My attempts to use cffi:convert-from-foreign and cffi:translate-from-foreign (what’s the difference between them anyway?) failed.

Below is my actual (not working) code:

(defun put-pixel (pixbuf x y r g b)
  (let ((p (+ (gdk:pixbuf-get-pixels pixbuf) (* x (gdk:pixbuf-get-n-channels pixbuf)) (* y (gdk:pixbuf-get-rowstride pixbuf)))))
    (setf (cffi:mem-aref p :unsigned-char 0) r)
    (setf (cffi:mem-aref p :unsigned-char 1) g)
    (setf (cffi:mem-aref p :unsigned-char 2) b)))
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1 Answer

  1. Editorial Team

    I think that lambda-gtk incorrectly defined binding for pixbuf-get-pixels.

    The negative value for pointer value might appear because of incorrect interpretation of unsigned integer as a signed integer.

    The simplest way to correct this value is to use mod:

    CL-USER> (mod -1 (expt 2 #+cffi-features:x86 32 #+cffi-features:x86-64 64))
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