Webgl's getAttribLocation oddly returns -1

Question

I have managed to isolate the problem in this code:

var gl;
_main_web = function() {
    gl = document.getElementById("canvas").getContext("experimental-webgl");

    gl = WebGLDebugUtils.makeDebugContext(gl,
    function (err, funcName, args) {
        throw(WebGLDebugUtils.glEnumToString(err) + " was caused by call to " + funcName);
    }
    );

    vert_shader = gl.createShader(gl.VERTEX_SHADER);
    gl.shaderSource(vert_shader,"attribute vec4 vertex;attribute vec2 uv; void main(void) {gl_Position = vertex;}\n");
    gl.compileShader(vert_shader);
    if( !gl.getShaderParameter(vert_shader,gl.COMPILE_STATUS ) ) {
        throw 0;
    }
    frag_shader = gl.createShader(gl.FRAGMENT_SHADER);
    gl.shaderSource(frag_shader,"void main(void) { gl_FragColor = vec4(1.0,1.0,1.0,1.0); } \n");
    gl.compileShader(frag_shader);
    if( !gl.getShaderParameter(frag_shader,gl.COMPILE_STATUS) ) {
        throw 1;
    }
    program = gl.createProgram();
    gl.attachShader(program,vert_shader);
    gl.attachShader(program,frag_shader);
    gl.linkProgram(program);
    if( !gl.getProgramParameter(program,gl.LINK_STATUS) ) {
        throw 2;
    }

    vertexLocation = gl.getAttribLocation(program,"vertex");
    textureLocation = gl.getAttribLocation(program,"uv");
}

vertexLocation is alright, it is 0. But textureLocation is -1, what am I missing?

Answer 1

You're trying to get the location for an attribute that you declare but never use. Your vertex shader code is (expanded for clarity):

attribute vec4 vertex;
attribute vec2 uv;
void main(void) {
    gl_Position = vertex;
}

During the compilation of your shader "uv" will be identified as an unused parameter and be stripped out. Even if you assign it to a varying in this shader but don't ever use it in the fragment shader, it may still be stripped out because it's been identified as not contributing to the final fragment.