pub type AEIO_ProgressProc = Option<unsafe extern "C" fn(refcon: *mut c_void, count: A_long, total: A_long) -> A_Err>;
pub enum AEIO_ProgressProc { None, Some(unsafe extern "C" fn(*mut c_void, i32, i32) -> i32), }
No value.
Some value of type T.
T