装配二进制字符串（pack函数）

最近看一点python的代码，发现struct这个包在处理网络编程上非常强大，pack和unpack函数能很轻松地将数据组包拆包，如下所示：

#! /usr/bin/env python

import socket, struct, time, re

if __name__ == '__main__':
    num1 = 5
    num2 = 10
    str1 = 0x00001000
    str2 = 0x00002000
    cha1 = 3
    cha2 = 65
    pack_res = struct.pack('!HHIIBB', num1, num2, str1, str2, cha1, cha2)
    print "after pack "
    print "pack_res", pack_res

    n1,n2,s1,s2,c1,c2 = struct.unpack("!HHIIBB", pack_res)
    print "after unpack"
    print "n1 = ", n1
    print "n2 = ", n2
    print "s1 = ", s1
    print "s2 = ", s2
    print "c1 = ", c1
    print "c2 = ", c2

    '''
    after pack 
    pack_res 
	??????????...
	after unpack
    n1 =  5
    n2 =  10
    s1 =  4096
    s2 =  8192
    c1 =  3
    c2 =  65
	'''

官方手册有格式参数的说明，摘录如下，其中感叹号表示网络序字节流，H代表无符号短整型，B代表无符号字符，I代表无符号整形。

Character	Byte order	Size	Alignment
@	native	native	native
=	native	standard	none
<	little-endian	standard	none
>	big-endian	standard	none
!	network (= big-endian)	standard	none

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)

python语法支持函数返回多个结果值，而php没有这种性质，所以php的unpack函数使用上稍有区别，比如同样是实现上面的功能，php的写法如下：

<?php

$num1 = 5;
$num2 = 10;
$str1 = 0x00001000;
$str2 = 0x00002000;
$cha1 = 3;
$cha2 = 65;

$pack_res = pack("nnNNCC",$num1,$num2,$str1,$str2,$cha1,$cha2);
echo "after pack\n";
echo "pack_res=".$pack_res;

$unpack_res = unpack("n2num/N2str/C*cha",$pack_res);

var_dump($unpack_res);

/*
after pack
?????????
pack_res=array(6) {
  ["num1"]=>
  int(5)
  ["num2"]=>
  int(10)
  ["str1"]=>
  int(4096)
  ["str2"]=>
  int(8192)
  ["cha1"]=>
  int(3)
  ["cha2"]=>
  int(65)
}
*/
?>

可以看到我们unpack的时候，是打包到一个关联数组中，关联数组的内容在format格式化时说明，还有一点需要注意的是，php与python的格式化参数不一致的，需要参照各自的定义。比如python的HIB分别对应php的nNC。php的格式化参数如下：

Code	Description
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, machine byte order)
n	unsigned short (always 16 bit, big endian byte order)
v	unsigned short (always 16 bit, little endian byte order)
i	signed integer (machine dependent size and byte order)
I	unsigned integer (machine dependent size and byte order)
l	signed long (always 32 bit, machine byte order)
L	unsigned long (always 32 bit, machine byte order)
N	unsigned long (always 32 bit, big endian byte order)
V	unsigned long (always 32 bit, little endian byte order)
f	float (machine dependent size and representation)
d	double (machine dependent size and representation)
x	NUL byte
X	Back up one byte
@	NUL-fill to absolute position