lemene

说明

big endian和little endian表示如何存放多字节数据。前者低位字节排放在内存的高端，后者相反。将unsigned long数据强制转换成unsigned char*数据，则它们在两种模式下的对应关系如下：

big endian：

ul = (uc[0]<< 24) + (uc[1]<<16) + (uc[2]<<8) + uc[3];

little endian:

ul = (uc[3]<<24) + (uc[2]<<16) + (uc[1]<<8) + uc[0];

实验代码

/**
 * @file little_big_endian.cpp
 * @brief 测试大小端字节序
 * @copyright public domain
 */
#include <iostream>
static bool is_little_endian() {
union {
long l;
char cs[4];
} t;
t.l = 1;
return t.cs[0] == 1;
}
int main() {
unsigned long ul = 0x12345678;
unsigned char* uc = (unsigned char*)&ul;
if (is_little_endian()) {
bool r = (uc[0] + (uc[1]<<8) + (uc[2]<<16) + (uc[3]<<24)) == ul;
std::cout << "little: (uc[0] + (uc[1]<<8) + (uc[2]<<16) + (uc[3]<<24)) == ul is " << (r ? "true" : "false") << std::endl;
} else {
bool r = (uc[3] + (uc[2]<<8) + (uc[1]<<16) + (uc[0]<<24)) == ul;
std::cout << "little: (uc[3] + (uc[2]<<8) + (uc[1]<<16) + (uc[0]<<24)) == ul is " << (r ? "true" : "false") << std::endl;
}
return 0;
}

运行及结果

$ g++ little_big_endian.cpp 
$ ./a.out 
little: (uc[0] + (uc[1]<<8) + (uc[2]<<16) + (uc[3]<<24)) == ul is true

常见问题

字节序的问题容易出现在不同电脑交互数据的时候，因此当数据输出时——保存成文件或在网络上传输——就应该考虑字节序。