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

 1 #ifndef __ENDIAN_H
 2 #define __ENDIAN_H
 3 
 4 void memrev16(void *p);
 5 void memrev32(void *p);
 6 void memrev64(void *p);
 7 
 8 /* variants of the function doing the actual convertion only if the target
 9  * host is big endian */
10 #if (BYTE_ORDER == LITTLE_ENDIAN)
11 #define memrev16ifbe(p)
12 #define memrev32ifbe(p)
13 #define memrev64ifbe(p)
14 #else
15 #define memrev16ifbe(p) memrev16(p)
16 #define memrev32ifbe(p) memrev32(p)
17 #define memrev64ifbe(p) memrev64(p)
18 #endif
19 
20 #endif

endian.c

 1 /* Toggle the 16 bit unsigned integer pointed by *p from little endian to
 2  * big endian */
 3 void memrev16(void *p) {
 4     unsigned char *x = p, t;
 5 
 6     t = x[0];
 7     x[0] = x[1];
 8     x[1] = t;
 9 }
10 
11 /* Toggle the 32 bit unsigned integer pointed by *p from little endian to
12  * big endian */
13 void memrev32(void *p) {
14     unsigned char *x = p, t;
15 
16     t = x[0];
17     x[0] = x[3];
18     x[3] = t;
19     t = x[1];
20     x[1] = x[2];
21     x[2] = t;
22 }
23 
24 /* Toggle the 64 bit unsigned integer pointed by *p from little endian to
25  * big endian */
26 void memrev64(void *p) {
27     unsigned char *x = p, t;
28 
29     t = x[0];
30     x[0] = x[7];
31     x[7] = t;
32     t = x[1];
33     x[1] = x[6];
34     x[6] = t;
35     t = x[2];
36     x[2] = x[5];
37     x[5] = t;
38     t = x[3];
39     x[3] = x[4];
40     x[4] = t;
41 }
42 
43 #ifdef TESTMAIN
44 #include <stdio.h>
45 
46 int main(void) {
47     char buf[32];
48 
49     sprintf(buf,"ciaoroma");
50     memrev16(buf);
51     printf("%s\n", buf);
52 
53     sprintf(buf,"ciaoroma");
54     memrev32(buf);
55     printf("%s\n", buf);
56 
57     sprintf(buf,"ciaoroma");
58     memrev64(buf);
59     printf("%s\n", buf);
60 
61     return 0;
62 }
63 #endif