Big Endian和Little Endian的区别

1．故事的起源

“endian”这个词出自《格列佛游记》。小人国的内战就源于吃鸡蛋时是究竟从大头(Big-Endian)敲开还是从小头(Little-Endian)敲开，由此曾发生过六次叛乱，其中一个皇帝送了命，另一个丢了王位。

我们一般将endian翻译成“字节序”，将big endian和little endian称作“大尾”和“小尾”。

2．什么是Big Endian和Little Endian？

在设计计算机系统的时候，有两种处理内存中数据的方法。一种叫为little-endian，存放在内存中最低位的数值是来自数据的最右边部分（也就是数据的最低位部分）。

比如某些文件需要在不同平台处理，或者通过Socket通信。这方面我们可以借助ntohl(), ntohs(), htonl(), and htons()函数进行格式转换，
个人补充：一个操作数作htonl或ntohl结果不一定相同，当机器字节序跟网络字节序刚好是仅仅big endian和little endian的区别时是相同的。

3. 如何理解Big Endian和Little Endian

举个例子：
int a = 1;
a这个数本身的16进制表示是0x00 00 00 01
在内存中怎么存储呢?
如果你的CPU是intel x86架构的(基本上就是通常我们说的奔腾cpu),那么就是<wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x01</wbr><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">, 这也就是所谓的little-endian, 低字节存放在内存的低位.<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 如果你的CPU是老式AMD系列的(很老很老的那种，因为最新的AMD系列已经是x86架构了), 它的字节序就是big-endian, 其内存存储就是<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">在内存中从高字节开始存放。<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 现在世界上绝大多数的CPU都是little-endian。<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 4. 了解big-endian和little-endian有什么作用？<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 一个重要的作用就是了解在网络上不同的机器间的数据如何传输。<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 假设我们在网络上有两台机器A和B, 其中A为little-endian,B为big-endian<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 机器A要传输上面的整数a给机器B，如何传输呢？<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 过程是这样的：<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 机器A先把a在内存中的四个字节<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x 01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">转化为网络字节序<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">，然后一个字节一个字节(从0x00到0x01)喂到网络上去<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 然后机器B从网络上一个字节一个字节地取出四个字节<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">后又会转化为本地字节序<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">后放入内存。因而B正确地得到了来自A的数据a<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 如果数据缺少在网络上的字节序转换的话，情况会怎样呢？<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 机器A先把a由在内存的四个字节<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x 01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">一个字节一个字节地喂到网络上，然后机器B从网络上一个字节一个字节地收到<u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x 01</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><u style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">0x00</wbr></u><wbr style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px">并放入到内存中, B认为他收到了0x01000000, 也就是十进制数1677216,这显然是错误的.<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 5．如何判断系统是Big Endian还是Little Endian？<br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"><br style="padding-bottom:0px; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 在/usr/include/中（包括子目录）查找字符串BYTE_ORDER(或_BYTE_ORDER, __BYTE_ORDER)，确定其值。这个值一般在endian.h或machine/endian.h文件中可以找到,有时在feature.h中，不同的操作系统可能有所不同。一般来说，Little Endian系统BYTE_ORDER(或_BYTE_ORDER,__BYTE_ORDER)为1234，Big Endian系统为4321。大部分用户的操作系统（如windows, FreeBsd,Linux）是Little Endian的。少部分，如MAC OS ,是Big Endian 的。本质上说，Little Endian还是Big Endian与操作系统和芯片类型都有关系。</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>