系统大小端的头文件定义

　　系统大小端的判断，虽说可以写函数来判断，但有时需要在预处理时就知道大小端，可使用C库头文件<endian.h>来判断，写法入下：

1.#include <endian.h>
2.typedef struct {
3.    u_short id;     /* query identification number */
4.
5.#if BYTE_ORDER == BIG_ENDIAN
6.            /* fields in third byte */
7.    u_char  qr:1;       /* response flag */
8.    u_char  opcode:4;   /* purpose of message */
9.    u_char  aa:1;       /* authoritive answer */
10.    u_char  tc:1;       /* truncated message */
11.    u_char  rd:1;       /* recursion desired */
12.            /* fields in fourth byte */
13.    u_char  ra:1;       /* recursion available */
14.    u_char  pr:1;       /* primary server required (non standard) */
15.    u_char  unused:2;   /* unused bits */
16.    u_char  rcode:4;    /* response code */
17.#else
18.            /* fields in third byte */
19.    u_char  rd:1;       /* recursion desired */
20.    u_char  tc:1;       /* truncated message */
21.    u_char  aa:1;       /* authoritive answer */
22.    u_char  opcode:4;   /* purpose of message */
23.    u_char  qr:1;       /* response flag */
24.            /* fields in fourth byte */
25.    u_char  rcode:4;    /* response code */
26.    u_char  unused:2;   /* unused bits */
27.    u_char  pr:1;       /* primary server required (non standard) */
28.    u_char  ra:1;       /* recursion available */
29.#endif
30.
31.
32.            /* remaining bytes */
33.    u_short qdcount;    /* number of question entries */
34.    u_short ancount;    /* number of answer entries */
35.    u_short nscount;    /* number of authority entries */
36.    u_short arcount;    /* number of resource entries */
37.} HEADER;

这是一个结构体定义，因为使用了位域，所以大小端需要区分对待。