glGetProgramStage queries a parameter of 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 the parameter. pname specifies which parameter should be queried. The value or values of the parameter to be queried is returned in the variable whose address is given in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORMS, the number of active subroutine variables in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS, the number of active subroutine variable locations in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINES, the number of active subroutines in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORM_MAX_LENGTH, the length of the longest subroutine uniform for the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_MAX_LENGTH, the length of the longest subroutine name for the stage is returned in values. The returned name length includes space for the null-terminator. If there is no shader present of type shadertype, the returned value will be consistent with a shader containing no subroutines or subroutine uniforms.
glGetProgramStage: man4/glGetProgramStage.xml
glGetProgramStage queries a parameter of 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 the parameter. pname specifies which parameter should be queried. The value or values of the parameter to be queried is returned in the variable whose address is given in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORMS, the number of active subroutine variables in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORM_LOCATIONS, the number of active subroutine variable locations in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINES, the number of active subroutines in the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_UNIFORM_MAX_LENGTH, the length of the longest subroutine uniform for the stage is returned in values. If pname is GL_ACTIVE_SUBROUTINE_MAX_LENGTH, the length of the longest subroutine name for the stage is returned in values. The returned name length includes space for the null-terminator. If there is no shader present of type shadertype, the returned value will be consistent with a shader containing no subroutines or subroutine uniforms.