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  1. 1. redis源码阅读-endianconv(大小端字节序)
    1. 1.1. endianconv主要功能
      1. 1.1.1. 什么是字节序?
    2. 1.2. endianconv.h
    3. 1.3. endianconv.c
    4. 1.4. 文献整理
redis源码阅读-endianconv(大小端字节序)
|源码阅读Credis工具类
字数总计: 1.5k|阅读时长: 7 分钟|阅读量:

redis源码阅读-endianconv(大小端字节序)

endianconv主要功能

endianconv主要功能就是用来将转换字节序的。

什么是字节序?

计算机硬件有两种存储数据的方式：大端字节序和小端字节序。大端字节序就是高位字节在前，低位字节在后;而小端字节序就是低位字节在前，高位字节在后。
我们的计算机电路是先处理低位字节，然后再处理高位字节。如果采用小端字节序，其字节排序与计算机处理顺序一样，这样处理效率就很高。而采用大端字节序，其字节排序与计算机相反的，这样处理效率就低的多。
大端字节序是人类读写数值的方法，主要是人来看，面向的是使用者;而小端字节序主要是计算机使用，面向的是计算机。
这两种存储方法的使用场景不一样，在追求效率的情况下，就要迁就计算机，计算机内部处理都是使用小端字节序。而面向人的网络传输和文件存储就是使用大端字节序。

endianconv.h

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/* See endianconv.c top comments for more information
 *
 * ----------------------------------------------------------------------------
 *
 * 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.
 */

#ifndef __ENDIANCONV_H
#define __ENDIANCONV_H

#include "config.h"
#include <stdint.h>

void memrev16(void *p);
void memrev32(void *p);
void memrev64(void *p);
uint16_t intrev16(uint16_t v);
uint32_t intrev32(uint32_t v);
uint64_t intrev64(uint64_t v);

/* variants of the function doing the actual conversion only if the target
 * host is big endian */
// 如果字节序是小端字节序，就不需要重新排序
#if (BYTE_ORDER == LITTLE_ENDIAN)
#define memrev16ifbe(p) ((void)(0))
#define memrev32ifbe(p) ((void)(0))
#define memrev64ifbe(p) ((void)(0))
#define intrev16ifbe(v) (v)
#define intrev32ifbe(v) (v)
#define intrev64ifbe(v) (v)
#else // 否则就是大端字节序，就需要重新排序
#define memrev16ifbe(p) memrev16(p)
#define memrev32ifbe(p) memrev32(p)
#define memrev64ifbe(p) memrev64(p)
#define intrev16ifbe(v) intrev16(v)
#define intrev32ifbe(v) intrev32(v)
#define intrev64ifbe(v) intrev64(v)
#endif

/* The functions htonu64() and ntohu64() convert the specified value to
 * network byte ordering and back. In big endian systems they are no-ops. */
// 如果字节序是大端字节序，就将值转换为网络字节序(网络字节序就是大端的)
#if (BYTE_ORDER == BIG_ENDIAN) 
#define htonu64(v) (v)
#define ntohu64(v) (v)
#else
#define htonu64(v) intrev64(v)
#define ntohu64(v) intrev64(v)
#endif

#ifdef REDIS_TEST
int endianconvTest(int argc, char *argv[]);
#endif

#endif

endianconv.c

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/* 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 */
// 将16位转换为小端字节序 0000 1111 --> 1111 0000
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 */
// 将32位转换为小端字节序 0000 0101 1100 1111 -> 1111 1100 0101 0000
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 */
// 将64位转换小端字节序 0000 0100 0110 0111 1000 1001 1011 1111 -> 1111 1011 1001 1000 0111 0110 0100 0000
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;
}

// 将16位地址转换为小端字节序，返回内容
uint16_t intrev16(uint16_t v) {
    memrev16(&v);
    return v;
}
// 将32位地址转换为小端字节序，返回内容
uint32_t intrev32(uint32_t v) {
    memrev32(&v);
    return v;
}
// 将64位地址转换为小端字节序，返回内容
uint64_t intrev64(uint64_t v) {
    memrev64(&v);
    return v;
}

#ifdef REDIS_TEST
#include <stdio.h>

#define UNUSED(x) (void)(x)
int endianconvTest(int argc, char *argv[]) {
    char buf[32];

    UNUSED(argc);
    UNUSED(argv);

    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);

    return 0;
}
#endif

文献整理

  1. 理解大小端字节序
文章作者: rack-leen
文章链接: http://yoursite.com/2019/12/22/%E6%BA%90%E7%A0%81%E9%98%85%E8%AF%BB/C/redis/redis%E6%BA%90%E7%A0%81%E9%98%85%E8%AF%BB-endianconv-%E5%A4%A7%E5%B0%8F%E7%AB%AF%E5%AD%97%E8%8A%82%E5%BA%8F/
版权声明: 本博客所有文章除特别声明外，均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来自 rack-leen's blog！
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