阿里笔试第二题

描述
Memcached是一款非常流行的开源缓存软件，请根据下面的网络协议说明，实现解析函数。补充题目中的代码片段和结构体定义。程序代码运行于X86 64bit Linux平台。
The format of a packet is a fixed size header followed by three optional variable size components(command - specific extras, the key and the value) :
Byte / 0 | 1 | 2 | 3 |
/ | | | |
| 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 |
+————– - +————– - +————– - +————– - +
0 / HEADER /
/ /
/ /
/ /
+————– - +————– - +————– - +————– - +
24 / Command - specific extras(as needed) /
+/ (note length in the extras length header field) /
+————– - +————– - +————– - +————– - +
m / Key(as needed) /
+/ (note length in key length header field) /
+————– - +————– - +————– - +————– - +
n / Value(as needed) /
+/ (note length is total body length header field, minus /
+/ sum of the extras and key length body fields) /
+————– - +————– - +————– - +————– - +
header:
Byte / 0 | 1 | 2 | 3 |
/ | | | |
| 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 |
+————– - +————– - +————– - +————– - +
0 | Magic | Opcode | Key length |
+————– - +————– - +————– - +————– - +
4 | Extras length | Data type | Reserved |
+————– - +————– - +————– - +————– - +
8 | Total body length |
+————– - +————– - +————– - +————– - +
12 | Opaque |
+————– - +————– - +————– - +————– - +
16 | CAS |
| |
+————– - +————– - +————– - +————– - +
Total 24 bytes
Magic is a magic byte which distinguishes requests from responses.It must be 0x80 for requests and 0x81 for responses.
Opcode specifies the type of command, e.g.GET is 0 and SET is 1.
Key Length specifies the size in bytes of the Key - part of the packet.
Extras length specifies the size in bytes of the Command - specific extras - part of the packet.
Data type is currenly not used and should always be 0.
Bytes 7 - 8 is either Reserved in the request or Status in the response. - The status corresponds to an error code or 0 if successful.
Total body length is the total size of the body in bytes, i.e.Total body length = Key length + Key length + Value length.The value length is not given explicitly in the header, but can be calculated as Total body length - Key length - Extras length.
Opaque is 4 bytes which are passed along in a request and passed back unmodified in the response.
CAS is used for data version checking.When storing a value you can optionally set its CAS value.Future modifications will be rejected if a matching CAS value is not provided.
example:
Byte / 0 | 1 | 2 | 3 |
/ | | | |
| 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 | 0 1 2 3 4 5 6 7 |
+————– - +————– - +————– - +————– - +
0 | 0x80 | 0x00 | 0x00 | 0x05 |
+————– - +————– - +————– - +————– - +
4 | 0x00 | 0x00 | 0x00 | 0x00 |
+————– - +————– - +————– - +————– - +
8 | 0x00 | 0x00 | 0x00 | 0x05 |
+————– - +————– - +————– - +————– - +
12 | 0x00 | 0x00 | 0x00 | 0x00 |
+————– - +————– - +————– - +————– - +
16 | 0x00 | 0x00 | 0x00 | 0x00 |
+————– - +————– - +————– - +————– - +
20 | 0x00 | 0x00 | 0x00 | 0x00 |
+————– - +————– - +————– - +————– - +
24 | 0x48 (‘H’) | 0x65 (‘e’) | 0x6c (‘l’) | 0x6c (‘l’) |
+————– - +————– - +————– - +————– - +
28 | 0x6f (‘o’) |
+————– - +
Total 29 bytes(24 byte header, and 5 bytes key)
Field(offset) (value)
Magic(0) : 0x80
Opcode(1) : 0x00
Key length(2, 3) : 0x0005
Extra length(4) : 0x00
Data type(5) : 0x00
Reserved(6, 7) : 0x0000
Total body(8 - 11) : 0x00000005
Opaque(12 - 15) : 0x00000000
CAS(16 - 23) : 0x0000000000000000
Extras : None
Key(24 - 29) : The textual string : “Hello”
Value : None
问题
struct Packet
{
};
Packet* DeocdeBuffer(char *buffer, int32_t len)
{
}

答案：