大小端

1. 定义

1.1 大端字节序

大端字节序是指一个整数的高位字节(23-31 bit)存储在内存的低地址处， 低字节(0-7 bit)存储在内存的高地址处。

1.2 小端字节序

小端字节序则是指整数的高位字节存储在内存的高地址处， 而低位字节则存储在内存的低地址处。

2. 判断机器字节序

int main(){
    union{
        short value;
        char union_bytes[sizeof(short)];
    }test;
    test.value = 0x0102;
    if((test.union_bytes[0]==1)&&(test.union_bytes[1]==2)){
        std::cout << "big endian" << std::endl;
    }else if((test.union_bytes[0]==2)&&(test.union_bytes[1]==1)){
        std::cout << "small endian" << std::endl;
    }else{
        std::cout << "unknow..." << std::endl;
    }
    return 0;
}

3. 转换

Linux提供了如下4个函数来完成主机字节序和网络字节序之间的转换

#include <netinet/in.h>
unsigned long int htonl(unsigned long int hostlong);
unsigned short int htons(unsigned short int hostshort)
unsigned long int ntohl(unsigned long int netlong);
unsigned short int ntohs(unsigned short int netshort)