How to debug a GLSL shader?

Question

I need to debug a GLSL program but I don\'t know how to output intermediate result. Is it possible to make some debug traces (like with printf) with GLSL ?

Answer 1

void main(){
  float bug=0.0;
  vec3 tile=texture2D(colMap, coords.st).xyz;
  vec4 col=vec4(tile, 1.0);

  if(something) bug=1.0;

  col.x+=bug;

  gl_FragColor=col;
}