總結(jié)一下，長(zhǎng)點(diǎn)記性：
Bit Field總是從低地址往高地址計(jì)算。
DNS的網(wǎng)絡(luò)數(shù)據(jù)中有兩個(gè)字節(jié)的標(biāo)志位（下面來(lái)自真實(shí)數(shù)據(jù)）：
0x81 0x80
這個(gè)標(biāo)志位分別表示：
1... .... .... .... = Response: Message is a response
.000 0... .... .... = Opcode: Standard query (0)
.... .0.. .... .... = Authoritative: Server is not an authority for domain
.... ..0. .... .... = Truncated: Message is not truncated
.... ...1 .... .... = Recursion desired: Do query recursively
.... .... 1... .... = Recursion available: Server can do recursive queries
.... .... .0.. .... = Z: reserved (0)
.... .... ..0. .... = Answer authenticated: Answer/authority portion was not authenticated by the server
.... .... .... 0000 = Reply code: No error (0)
想都沒(méi)有再想，下意識(shí)地寫(xiě)了下面的結(jié)構(gòu)體（惡夢(mèng)開(kāi)始了）：
struct NetworkOrder
{
union{
unsigned short Flags;
struct{
unsigned short Response:1;
unsigned short Opcode:4;
unsigned short Authoritative:1;
unsigned short Truncated:1;
unsigned short RecursionDesired:1;
unsigned short RecursionAvailable:1;
unsigned short Zero:1;
unsigned short AnswerAuthenticated:1;
unsigned short Pad:1;
unsigned short ReplyCode:4;
};
};
};
解析結(jié)果當(dāng)然不對(duì)：
{
Flags = 0x8081,
{
Response = 0x1,
Opcode = 0x0,
Authoritative = 0x0,
Truncated = 0x0,
RecursionDesired = 0x1,
RecursionAvailable = 0x0,
Zero = 0x0,
AnswerAuthenticated = 0x0,
Pad = 0x0,
ReplyCode = 0x8
}
}
然后又下意識(shí)地想（其實(shí)根本沒(méi)有經(jīng)過(guò)想的過(guò)程，要不然也不會(huì)這么慘了），可能是字節(jié)序的問(wèn)題，換成主機(jī)序：
struct NetworkOrder
{
union{
unsigned short Flags;
struct{
unsigned short Response:1;
unsigned short Opcode:4;
unsigned short Authoritative:1;
unsigned short Truncated:1;
unsigned short RecursionDesired:1;
unsigned short RecursionAvailable:1;
unsigned short Zero:1;
unsigned short AnswerAuthenticated:1;
unsigned short Pad:1;
unsigned short ReplyCode:4;
};
};
};
我靠，結(jié)果還不對(duì)：
{
Flags = 0x8081,
{
Response = 0x1,
Opcode = 0x0,
Authoritative = 0x0,
Truncated = 0x0,
RecursionDesired = 0x1,
RecursionAvailable = 0x0,
Zero = 0x0,
AnswerAuthenticated = 0x0,
Pad = 0x0,
ReplyCode = 0x8
}
}
這個(gè)時(shí)候背上直冒冷汗，這么簡(jiǎn)單的問(wèn)題自己都沒(méi)搞定，快速?gòu)哪X海中掃了一遍char、short、int在處理數(shù)據(jù)上的差異......想了半天，
只好用最后的笨辦法了，看一下到底是怎么回事，再次改變一下結(jié)構(gòu)：
struct NetworkOrder3
{
union{
unsigned short Flags;
struct{
unsigned short A0:1;
unsigned short A1:1;
unsigned short A2:1;
unsigned short A3:1;
unsigned short B0:1;
unsigned short B1:1;
unsigned short B2:1;
unsigned short B3:1;
unsigned short C0:1;
unsigned short C1:1;
unsigned short C2:1;
unsigned short C3:1;
unsigned short D0:1;
unsigned short D1:1;
unsigned short D2:1;
unsigned short D3:1;
};
};
};
結(jié)果出來(lái)一看，恍然大悟：
{
Flags = 0x8081,
{
A0 = 0x1,
A1 = 0x0,
A2 = 0x0,
A3 = 0x0,
B0 = 0x0,
B1 = 0x0,
B2 = 0x0,
B3 = 0x1,
C0 = 0x0,
C1 = 0x0,
C2 = 0x0,
C3 = 0x0,
D0 = 0x0,
D1 = 0x0,
D2 = 0x0,
D3 = 0x1
}
}
我靠，平時(shí)Bit Field用得少，老是一個(gè)字節(jié)，兩個(gè)字節(jié)地考慮，忘了它從低地址開(kāi)始一位一位往上計(jì)算的，修改最后的結(jié)構(gòu)，得到正確的結(jié)果：
struct NetworkOrder4
{
union{
unsigned short Flags;
struct{
unsigned short RecursionDesired:1;
unsigned short Truncated:1;
unsigned short Authoritative:1;
unsigned short Opcode:4;
unsigned short Response:1;
unsigned short ReplyCode:4;
unsigned short Pad:1;
unsigned short AnswerAuthenticated:1;
unsigned short Zero:1;
unsigned short RecursionAvailable:1;
};
};
};
結(jié)果：
{
Flags = 0x8081,
{
RecursionDesired = 0x1,
Truncated = 0x0,
Authoritative = 0x0,
Opcode = 0x0,
Response = 0x1,
ReplyCode = 0x0,
Pad = 0x0,
AnswerAuthenticated = 0x0,
Zero = 0x0,
RecursionAvailable = 0x1
}
}
真是慚愧啊，好記性不如爛筆頭，記在這里引以為簽。