在网络上传输数据时，由于数据传输的两端可能对应不同的硬件平台，采用的存储字节顺序也可能不一致，因此 TCP/IP 协议规定了在网络上必须采用网络字节顺序(大端模式) 。
通过对大小端的存储原理分析可发现，对于 char 型数据，由于其只占一个字节，所以不存在这个问题，这也是一般情况下把数据缓冲区定义成 char 类型 的原因之一。对于 IP 地址、端口号等非 char 型数据，必须在数据发送到网络上之前将其转换成大端模式，在接收到数据之后再将其转换成符合接收端主机的存储模式。
在redis3.0源代码中endianconv.c中存在将大端模式转换为小端模式的方法，该方法不会被直接调用，但是会被endianconv.h的宏定义使用，如果存储文件是小端，不需要定义其他操作。redis 尽量把所有的多字节类型按照小端编码（但是有些场景要向后兼容，仍然是大端），是因为大多数生产环境是小端；在内存中ziplist、intset、zipmap在内存中是endin-中性的场景，需要很多转换操作，因为在写磁盘持久化的时候直接系统调动write(2)，而没有其他步骤。
* This functions are never called directly, but always using the macros
* defined into endianconv.h, this way we define everything is a non-operation
* if the arch is already little endian.
*
* Redis tries to encode everything as little endian (but a few things that need
* to be backward compatible are still in big endian) because most of the
* production environments are little endian, and we have a lot of conversions
* in a few places because ziplists, intsets, zipmaps, need to be endian-neutral
* even in memory, since they are serialied on RDB files directly with a single
* write(2) without other additional steps.只有在宿主机是大端的情况下，定义的宏才真正有效，小端情况无需操作。
实现部分，记住两点即可（1）逆序（2）无符号数
/* Toggle the 16 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev16(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[1];
x[1] = t;
}
/* Toggle the 32 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev32(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[3];
x[3] = t;
t = x[1];
x[1] = x[2];
x[2] = t;
}
/* Toggle the 64 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev64(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[7];
x[7] = t;
t = x[1];
x[1] = x[6];
x[6] = t;
t = x[2];
x[2] = x[5];
x[5] = t;
t = x[3];
x[3] = x[4];
x[4] = t;
}
一个完整的例子：
/* endinconv.c -- Endian conversions utilities.
*
* This functions are never called directly, but always using the macros
* defined into endianconv.h, this way we define everything is a non-operation
* if the arch is already little endian.
*
* Redis tries to encode everything as little endian (but a few things that need
* to be backward compatible are still in big endian) because most of the
* production environments are little endian, and we have a lot of conversions
* in a few places because ziplists, intsets, zipmaps, need to be endian-neutral
* even in memory, since they are serialied on RDB files directly with a single
* write(2) without other additional steps.
*
* ----------------------------------------------------------------------------
*
* Copyright (c) 2011-2012, Salvatore Sanfilippo <antirez at gmail dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
#include <stdint.h>
/* Toggle the 16 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev16(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[1];
x[1] = t;
}
/* Toggle the 32 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev32(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[3];
x[3] = t;
t = x[1];
x[1] = x[2];
x[2] = t;
}
/* Toggle the 64 bit unsigned integer pointed by *p from little endian to
* big endian */
void memrev64(void *p) {
unsigned char *x = p, t;
t = x[0];
x[0] = x[7];
x[7] = t;
t = x[1];
x[1] = x[6];
x[6] = t;
t = x[2];
x[2] = x[5];
x[5] = t;
t = x[3];
x[3] = x[4];
x[4] = t;
}
uint16_t intrev16(uint16_t v) {
memrev16(&v);
return v;
}
uint32_t intrev32(uint32_t v) {
memrev32(&v);
return v;
}
uint64_t intrev64(uint64_t v) {
memrev64(&v);
return v;
}
#include <stdio.h>
int main(void) {
char buf[32];
sprintf(buf,"ciaoroma");
memrev16(buf);
printf("%s\n", buf);
sprintf(buf,"ciaoroma");
memrev32(buf);
printf("%s\n", buf);
sprintf(buf,"ciaoroma");
memrev64(buf);
printf("%s\n", buf);
/*0x8586,由于运行程序机器为小端模式，因此由低地址到高地址为86 85*/
int16_t val = 21846;
/*使用memrev16函数交换后为 85 86,那么实际代表的值为0x8685*/
/*86对应二进制 01010110*/
/*85对应二进制 01010101*/
/*二进制0101011001010101:十进制22101*/
memrev16(&val);
printf("%d\n",val);
return 0;
}
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