Why glGetSubroutineIndex returns the same value for different function?

Question

I have got a fragment shader that should operate 2 colors - red and blue.

#version 460 core

layout (location = 0) out vec4 outColor;

subroutine vec4 colorRedBlue();

subroutine (colorRedBlue) 
vec4 redColor() 
{
    return vec4(1.0, 0.0, 0.0, 1.0);
} 

subroutine (colorRedBlue) 
vec4 blueColor() 
{ 
    return vec4(0.0, 0.0, 1.0, 1.0);
}

subroutine uniform colorRedBlue myRedBlueSelection;

void main()
{
    outColor = myRedBlueSelection();
}

when I cout glGetSubroutineIndex(shaderProgram, GL_FRAGMENT_SHADER, "redColor"); and glGetSubroutineIndex(shaderProgram, GL_FRAGMENT_SHADER, "blueColor");

they are the same numbers: 4294967295

Why they are the same numbers? What did I do wrong?

Answer 1

If you want to retrieve information about the subroutine indices the you have to use glGetActiveSubroutineUniformiv:

GLint no_of_subroutines;
glGetActiveSubroutineUniformiv(
    shaderProgram, GL_FRAGMENT_SHADER, 
    0, // Index of the subroutine uniform, but NOT the location of the uniform variable 
    GL_NUM_COMPATIBLE_SUBROUTINES, &no_of_subroutines);

std::vector<GLint> sub_routines( no_of_subroutines ); 
glGetActiveSubroutineUniformiv(
    shaderProgram, GL_FRAGMENT_SHADER, 
    0, // Index of the subroutine uniform, but NOT the location of the uniform variable 
    GL_COMPATIBLE_SUBROUTINES, sub_routines.data());

The name of a subroutine which corresponds to a subroutine index can be get by glGetActiveSubroutineName.

But I recommend to use layout qualifier to specify the subroutine indices in the shader code - see Shader Subroutine - In-shader specification:

subroutine vec4 colorRedBlue();

layout(index = 1) subroutine (colorRedBlue) 
vec4 redColor() 
{
    return vec4(1.0, 0.0, 0.0, 1.0);
} 

layout(index = 2) subroutine (colorRedBlue) 
vec4 blueColor() 
{ 
    return vec4(0.0, 0.0, 1.0, 1.0);
}  

layout(location = 0) subroutine uniform colorRedBlue myRedBlueSelection;

Note, all the subroutines for all the subroutine unifroms of one shader stage have to be set at once, by glUniformSubroutinesuiv. In your case, that's not a big thing, because there is only 1 subroutine uniform. This operation applies to the current shader program (glUsePrgram).
The number of indices wich you have to pass to glUniformSubroutinesuiv has to match the number of GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS but not the number of GL_ACTIVE_SUBROUTINE_UNIFORM - see glGetProgramStageiv:

GLint nLocationCount = 0;
glGetProgramStageiv( 
    program, GL_FRAGMENT_SHADER, GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS, &nLocationCount );

std::vector<GLuint> sub_routines( nLocationCount, 0 );

Glint redBlueSelection_location = 0;
sub_routines[redBlueSelection_location] = 1; // index of 'redColor` or `blueColor`

glUniformSubroutinesuiv(
    GL_FRAGMENT_SHADER, (GLsizei)sub_routines.size(), sub_routines.data() );