我有一个结构,我已经写了,它应该代表整个UDP数据包,以及以太网头和所有。在这里它是:结构中的字段跳过字节
#pragma pack(1)
struct UDPPacket {
// an array to hold the destination mac address of the packet
unsigned char dstmac[6];
// an array to hold the source mac address of the packet
unsigned char srcmac[6];
// two bytes to hold the packet type, this is almost always IP (08 00)
WORD ethtype;
// each of the subfields of this take up 4 bits. ver, the first half,
// is the ip version, which should usually be 4 for ipv4, and the second
// is the length of the header divided by 4, which is almost always 5
struct {
unsigned ver : 4;
unsigned len : 4;
} verlen;
// this isn't used in ipv4 and is 0
BYTE tos;
// the total length of the header + data
WORD iplen;
// the id of this datagram for reassembling fragmented packets
WORD id;
// the first subfield occupies 3 bits and is the flags of this packet, which is usually 0
// the second subfield is the fragmentation offset for large datagrams that have been split up for sending, usually 0
struct {
unsigned flags : 3;
unsigned fragmentation : 13;
} flagfrag;
// time to live; usually 35 or 128
BYTE ttl;
// the protocol with which this packet is being transported
// 1 = ICMP, 2 = IGMP, 6 = TCP, 17 = UDP
BYTE protocol;
// the ip checksum of this packet
WORD ipchecksum;
// the source ip of this packet
DWORD src;
// the destination ip of this packet
DWORD dest;
// the port from which this packet is coming
WORD srcport;
// the port this packet is headed to
WORD destport;
// the length of the udp header + data, not including the ip header
// so it's usually basically iplen - 20
WORD udplen;
// the udp checksum of this packet
WORD udpchecksum;
// a char pointer to the data of the packet
unsigned char data[10000];
};
#pragma pack()
当然,这是一个真正的UDP包的表示,该字节必须在相同的偏移,因为它们将在一个信息包,和指针于这种类型的结构的将是投到unsigned char*
s发送。
我的问题是,当我尝试在UDPPacket.verlen
之后分配任何内容时,它会跳过大约5个字节并从那里开始。例如,当我指定iplen
字段时,不是将字节设置为偏移量16和17,而是将它们指定为类似于23和24的字符(我不能完全说明,因为我的手机中没有可用的程序)。
有没有一个明显的原因,我错过了,或者我只是做错了什么?
顺便说一下,我在x64 Windows 7上使用Visual Studio 2008,但是为x86编译。 – Hock 2010-08-18 14:39:30