Attaching multiple shaders of the same type in a single OpenGL program?

Question

In reading the OpenGL specs, I have noticed throughout that it mentions that you can include multiple shaders of the same kind in a single program (i.e. more than one GL_VER

Answer 1

I found that including more than one void main() caused linking errors

For each shader stage there must be only a main entry function.

a practical example where this is used (pre-GL4)?

You can declare a function in a shader source and not define it, and at linking time you can provide a definition from another shader source (very similar to c/c++ linking).

Example:

generate_txcoord.glsl:

#version 330
precision highp float;

const vec2 madd = vec2(0.5, 0.5);

vec2 generate_txcoord(vec2 v)
{ 
  return v * madd + madd; 
}

vertex.glsl:

#version 330
precision highp float;

in vec2 vxpos;

out vec2 out_txcoord;

vec2 generate_txcoord(vec2 vxpos); // << declared, but not defined

void main()
{
  // generate 0..+1 domain txcoords and interpolate them
  out_txcoord = generate_txcoord(vxpos);

  // interpolate -1..+1 domain vxpos
  gl_Position = vec4(vxpos, 0.0, 1.0);
}