pub type PFNGLMAPBUFFERARBPROC = Option<unsafe extern "C" fn(target: GLenum, access: GLenum) -> *mut c_void>;
enum PFNGLMAPBUFFERARBPROC { None, Some(unsafe extern "C" fn(_: u32, _: u32) -> *mut c_void), }
No value.
Some value of type T.
T