In OpenGL Shader, Why adding color change vertex position?

Question

I have the following vertex shader:

#version 120

attribute vec4 position;
attribute vec4 acolor;

varying vec4 theColor;

void main(){
    gl_Position = position;
    //theColor = acolor;  //[1]
    theColor = vec4(1.0,0.0,0.0,1.0); //[2]
}

When I use line [1], the color is transmitted rigth to the fragment shader, but when I change the color to the attribute acolor (using the line [2]) the positions of the triangles appear wrong. From the data of the main program, it seems to me like the positions get the color values.

I put two images, 1 with the rigth vertex positions and 2 with the wrong vertex positions.

UPDATE: render code on Github

Answer 1

Check your glBindAttribLocation calls - you'll probably find that one of two things are happening; either (1) you're not doing any such calls and therefore GL is assigning it's own arbitrary attrib locations which are mismatching those used in your glVertexAttribPointer calls, or (2) you have specified a mismatch yourself.

Either way, the recommended approach would be to use "layout(location=" syntax in your vertex shader, which will make this kind of ambiguity go away.