函数 av_xiphlacing 

pub unsafe extern "C" fn av_xiphlacing(
    s: *mut c_uchar,
    v: c_uint,
) -> c_uint

Encode extradata length to a buffer. Used by xiph codecs.

@param s buffer to write to; must be at least (v/255+1) bytes long @param v size of extradata in bytes @return number of bytes written to the buffer.