OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Selecting Shader Subroutines

Once you have all your subroutine types and functions defined in your shaders, you only need to query a few values from the linked shader program, and then use those values to select the appropriate function. In step 3 described on page 78, a subroutine uniform value was declared, and we will need its location in order to set its value. As compared to other shader uniforms, subroutine uniforms use glGetSubroutineUniformLocation() to retrieve their locations.

GLint glGetSubroutineUniformLocation(GLuint program,                         GLenum shadertype,                         const char* name);

Returns the location of the subroutine uniform named name in program for the shading stage specified by shadertype. name ...