transform feedback problem

[cage]

hello!

I am  trying to use transform-feedback  in my program but  i failed to
setup varyings, this is the (i guess) relevant part of the code):

[...]

(let ((shader (gl:create-shader type)))
  (gl:shader-source shader source) ; assuming source is defined elesewhere
  (gl:compile-shader shader)
  (let ((program (gl:create-program)))
    ;; attaching shader to program (not shown)
    (%gl:transform-feedback-varyings-ext program 1
					 (cffi:foreign-alloc :strings
							     :initial-contents (vector "test_output"))
					 :interleaved-attribs)
    (break)
    (gl:link-program program)
    ;; detaching shader(not shown)
    program))

[...]

The  problem here  is that  the call  to "break"  in not  reached, the
program continue to  run apparently ignoring it but the  shader is not
compiled at all resulting in program crash.

i am  quite sure my  driver supports transform-feedback  (checked with
glxinfo) so i think the problem is in the parameters (i am not so good
with FFI).

Any help?

Bye!
C.