pack/unpack的摸板字符字符含义，需要的朋友可以参考下。

format 参数的可能值： a – NUL-padded string A – SPACE-padded string h – Hex string, low nibble first H – Hex string, high nibble first c – signed char C – unsigned char s – signed short (always 16 bit, machine byte order) S – unsigned short (always 16 bit,