pack和unpack在处理二进制流中比较常用的封包、解包格式
# 按照给定的格式(fmt)，把数据封装成字符串(实际上是类似于c结构体的字节流)
pack(fmt, v1, v2, ...)
# 按照给定的格式(fmt)解析字节流string，返回解析出来的tuple
unpack(fmt, string)
# 计算给定的格式(fmt)占用多少字节的内存
calcsize(fmt)
fmt格式为：
| FORMAT | C TYPE | PYTHON TYPE | STANDARD SIZE | NOTES |
|---|
| x | pad byte | no value | | |
| c | char | string of length 1 | 1 | |
| b | signed char | integer | 1 | (3) |
| B | unsigned char | integer | 1 | (3) |
| ? | _Bool | bool | 1 | (1) |
| h | short | integer | 2 | (3) |
| H | unsigned short | integer | 2 | (3) |
| i | int | integer | 4 | (3) |
| I | unsigned int | integer | 4 | (3) |
| l | long | integer | 4 | (3) |
| L | unsigned long | integer | 4 | (3) |
| q | long long | integer | 8 | (2), (3) |
| Q | unsigned long long | integer | 8 | (2), (3) |
| f | float | float | 4 | (4) |
| d | double | float | 8 | (4) |
| s | char[] | string | | |
| p | char[] | string | | |
| P | void * | integer | | (5), (3) |
格式中的第一个字符来改变对齐方式(字节序).定义如下
| CHARACTER | BYTE ORDER | SIZE | ALIGNMENT |
|---|
| @ | native | native | native |
| = | native | standard | none |
| < | little-endian | standard | none |
| > | big-endian | standard | none |
| ! | network (= big-endian) | standard | none |
例如：数据格式为：
unsigned short id;
char[4] tag;
unsigned int version;
unsigned int count;
解包：
import struct
id, tag, version, count = struct.unpack("!H4s2I", s)
封包：
ss = struct.pack("!H4s2I", id, tag, version, count);
例如：格式为：2字节(包长度)+4字节(包id)+包内容 >表示字节序
解包
size, = struct.unpack('>H',raw[0:2])
cmd, = struct.unpack('>H', raw[2:4])
string, = struct.unpack('>{0}s'.format(size - 4), raw[4:size])
封包：
fmt = ">HH{0}s".format(len(result))
args = (len(result), cmd,result)
data = struct.pack(fmt, *args)