I've been playing around a bit with GLSL as of late, and have encountered a situation which is hard for me to ascribe to anything but a linker bug. It seems that varying variables from the vertex and fragment shaders aren't matched correctly.

I've been trying to write a module shader architecture that generates a main function to call other shaders in separately compiled shader objects. Just trying to get everything working correctly, I was trying to use the texture coordinates of a model as the output color for the fragments. Currently, I'm compiling and linking the following shaders:

 -- Vertex shader source:
varying vec4 texc;
void null() {texc = gl_MultiTexCoord0;}

 -- Generated vertex shader main function:
void null();
varying vec4 b;

void main()
{
    vec4 fcol = gl_Color;
    vec4 bcol = gl_Color;
    vec4 objv = gl_Vertex;
    vec3 objn = gl_Normal;
    null();
    vec4 eyev = gl_ModelViewMatrix * objv;
    vec3 eyen = gl_NormalMatrix * objn;
    gl_FrontColor = fcol;
    gl_BackColor = bcol;
    b = vec4(objn, 0);
    gl_Position = gl_ProjectionMatrix * eyev;
}

 -- Fragment shader source:
varying vec4 texc;
void null(inout vec4 res) {res = vec4(texc.xy, 0.0, 1.0);}

 -- Generated fragment shader main function:

void null(inout vec4 res);
varying vec4 b;
void main()
{
    vec4 res = gl_Color;
    null(res);
    gl_FragColor = b;
}


I started out without the "b" variable, setting gl_FragColor in the main function to the "res" variable instead, but that made all the fragments yellow (as if texc contained only 1.0), so I added the "b" variable for debugging. As you can see in the main vertex shader function, I set it to each vertex's normal. However, in the actual output, I'm seeing the texture coordinates rendered as the fragments' final color (my originally intended result).

Is there any way to interpret this other than "b" in the fragment program being linked to "texc" from the vertex program?

I've got an Intel Arrandale chipset, which uses the i965 DRI driver, which is why I filed the bug with that, but I guess that choice of component may be completely wrong.

I'm running stock Debian Squeeze, which means Mesa version 7.7.1, xserver-xorg-video-intel 2.13.0 and Linux 2.6.32.