glGetSubroutineIndex

glGetSubroutineIndex: man4/glGetSubroutineIndex.xml

glGetSubroutineIndex returns the index of a subroutine uniform within a shader stage attached to a program object. program contains the name of the program to which the shader is attached. shadertype specifies the stage from which to query shader subroutine index. name contains the null-terminated name of the subroutine uniform whose name to query. If name is not the name of a subroutine uniform in the shader stage, GL_INVALID_INDEX is returned, but no error is generated. If name is the name of a subroutine uniform in the shader stage, a value between zero and the value of GL_ACTIVE_SUBROUTINES minus one will be returned. Subroutine indices are assigned using consecutive integers in the range from zero to the value of GL_ACTIVE_SUBROUTINES minus one for the shader stage.