It should be noted that in order for a vertex shader input variable to be considered active, it must be used within the vertex shader. In other words, a variable is considered active if it is determined by the GLSL linker that it may be accessed during program execution. If a variable is declared within a shader, but not used, the previous code will not display the variable because it is not considered active and is effectively ignored by OpenGL.

The previous code is only valid for OpenGL 4.3 and later. Alternatively, you can achieve similar results with the glGetProgramiv, glGetActiveAttrib, and glGetAttribLocation functions.