Function gc_register_finalizer 

pub unsafe extern "C" fn gc_register_finalizer(
    obj: *mut c_void,
    cd: *mut c_void,
    finalizer: Option<unsafe extern "C" fn(obj: *mut c_void, cd: *mut c_void)>,
)

@brief Register a callback that gets called when the object is garbage collected. @note Objects can only have a single finalizer. This function overwrites existing values silently. @param[in] obj the object to watch @param[in] cd the data to pass to the finalizer @param[in] finalizer the callback function, called with obj and cd