int get_partial_buffer(ByteIOContext *s, unsigned char *buf, int size)
{
    int len;
    len = s->buf_end - s->buf_ptr;
    if (len == 0) {
        fill_buffer(s);
        len = s->buf_end - s->buf_ptr;
    }
    if (len > size)
        len = size;
    memcpy(buf, s->buf_ptr, len);
    s->buf_ptr += len;
    return len;
}