glutGetProcAddress man page
glutGetProcAddress — Determine if an OpenGLUT feature or extension is available.
OpenGLUT - openglut
glutGetProcAddress(const char *procName);
Given a function name, searches for the function (or "procedure", hence "Proc") in internal tables. If the function is found, a pointer to the function is returned. If the function is not found,
In addition to an internal OpenGLUT table, this function will also consult glX (on X systems) or wgl (on WIN32 and WINCE), if the OpenGLUT tables do not have the requested function. It should return any OpenGL, glX, or wgl function if those functions are available.
The code makes no effort towards efficiency. It is advised that you cache the pointer retrieved from this function if you need a certain function repeatedly.