Function write_callback 

extern "C" fn write_callback(
    _encoder: *const FLAC__StreamEncoder,
    buffer: *const u8,
    bytes: usize,
    _samples: u32,
    _current_frame: u32,
    client_data: *mut c_void,
) -> FLAC__StreamEncoderWriteStatus

Available on crate feature audio-flac only.