主机字节序和网络字节序

1.判断主机字节序

#include <iostream>

using namespace std;

union Test

{

int val;

char t[2];

};

int main()

{

union Test tt;

tt.val=0x1234;

if(tt.t[0]==0x34 && tt.t[1]==0x12)

cout<<"Small endian"<<endl;

else

cout<<"big endian"<<endl;

return 0;

}

2.socket API中我们使用过许多转换函数

例如inet_addr, htons, htonl, ntohl, ntohs等等(注意这几个是跨平台的)

htons,htonl 表示将主机字节次序转换为网络字节次序(s表示short, l表示long)

ntohs,ntohl 表示将网络字节次序转换为主机字节次序(s表示short, l表示long)

inet_addr (The inet_addr() function converts the Internet host address cp from IPv4 numbers-and-dots notation into binary data in network byte order.) 将网络主机地址(ipv4)转换为网络表示的地址,但是这种方法有个缺陷(当地址为255.255.255.25时， 在linux下会有问题)

现在我们尝试以上函数的具体实现:

#include <iostream>

#include <ctype.h>

#include <arpa/inet.h> //struct addr structure

#include <stdio.h> //sprintf function useit

using namespace std;

unsigned short htons(unsigned short src)

{

//unsigned char* p = (unsigned char*)&src;

//return (unsigned short)(p[0]<<8 | p[1]);

return (src & 0xff00)>>8 | (src & 0x00ff)<<8;

}

unsigned short ntohs(unsigned short src)

{

unsigned char* p = (unsigned char*)&src;

return (unsigned short)(p[0]<<8 | p[1]);

}

unsigned long htonl(unsigned long src)

{

unsigned char* p = (unsigned char*)&src;

return (unsigned long)(p[0]<<24 | p[1]<<16 | p[2]<<8 | p[3]);

}

unsigned long ntohl(unsigned long src)

{

unsigned char* p = (unsigned char*)&src;

return (unsigned long)(p[0]<<24 | p[1]<<16 | p[2]<<8 | p[3]);

}

char* inet_ntoa(struct in_addr addr)

{

static char buf[16] = {'\0'};

unsigned char* p = (unsigned char*)&addr;

sprintf(buf,"%d.%d.%d.%d",p[0], p[1], p[2], p[3]);

return buf;

}

long net_addr(const char *str)

{

long a, b, c, d;

sscanf(str, "%ld.%ld.%ld.%ld", &a, &b, &c, &d);

long addr = 0;

addr |= d<<24; //d<<24之间千万不要有空格

addr |= c<<16;

addr |= b<<8;

addr |= a;

return addr;

}

int main()

{

unsigned short host_s = 0x1234,net_s;

unsigned long host_l =0x12345678,net_l;

char *addr_dec = "192.168.10.26",*p;

struct in_addr addr;

net_s = htons(host_s);

net_l = htonl(host_l);

cout<<host_s <<" "<<net_s<<endl;

cout<<host_l<<" "<<net_l<<endl;

addr.s_addr = net_addr(addr_dec);

p = inet_ntoa(addr);

cout<<addr.s_addr<<" "<<p<<endl;

return 0;

}

在系统函数下的测试:

#include <iostream>

//#include <arpa/inet.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

using namespace std;

int main()

{

unsigned short host_s = 0x1234,net_s;

unsigned long host_l =0x12345678,net_l;

char *addr_dec = "192.168.10.26",*p;

struct in_addr addr;

net_s = htons(host_s);

net_l = htonl(host_l);

cout<<host_s <<" "<<net_s<<endl;

cout<<host_l<<" "<<net_l<<endl;

addr.s_addr = inet_addr(addr_dec);

p = inet_ntoa(addr);

cout<<addr.s_addr<<" "<<p<<endl;

return 0;

}

get the same result:

4660 13330

305419896 2018915346

436906176 192.168.10.26

参考书籍: WInSock网络编程