redis源码笔记－endian

对于目标机是大端字节序的机器，进行字节码的转换，提供了16byte、32byte、64byte字节的转换。在intset\ziplist\zipmap三种数据结构中使用，使得不同字节序机器生成的rdb文件格式都是统一的（小端字节序），便于兼容。

代码实在是太简单了，贴上来，不多说了。

endian.h

#ifndef __ENDIAN_H
#define __ENDIAN_H

void memrev16(void *p);
void memrev32(void *p);
void memrev64(void *p);

/* variants of the function doing the actual convertion only if the target
 * host is big endian */
#if (BYTE_ORDER == LITTLE_ENDIAN)
#define memrev16ifbe(p)
#define memrev32ifbe(p)
#define memrev64ifbe(p)
#else
#define memrev16ifbe(p) memrev16(p)
#define memrev32ifbe(p) memrev32(p)
#define memrev64ifbe(p) memrev64(p)
#endif

#endif

endian.c

/* 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;
}

#ifdef TESTMAIN
#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);

    return 0;
}
#endif