public static extern string av_get_string(void *obj, string name, AVOption **o_out, string buf, int buf_len);
public static extern AVRational av_get_q(void *obj, string name, AVOption **o_out);
public static extern long av_get_int(void *obj, string name, AVOption **o_out);
public static extern double av_get_double(void *obj, string name, AVOption **o_out);