c++ - 结构中的字段跳过字节

Question

我有一个我写的结构，它应该代表一个完整的 UDP 数据包，带有以太网头和所有。在这里:

#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 数据包的表示，字节必须与它们在数据包中的偏移量相同，并且指向这种类型的结构的指针将被转换为 unsigned char* 用于发送。
我的问题是，当我尝试在 UDPPacket.verlen 之后分配任何内容时，它会向前跳过大约 5 个字节并从那里开始。例如，当我分配 iplen 字段时，不是将字节设置在偏移量 16 和 17 处，而是将它们分配在 23 和 24 之类的位置(我不能准确地说，因为我没有我的程序在我的手机上可用)。
是否有明显的原因导致我遗漏了这一点，或者我只是做错了什么？

Answer 1

您的#pragmas 看起来是正确的。位域不会“自动打包”为适合明确指定的位数的最小类型。我怀疑 verlen 正在采用您给定的类型“unsigned”并假设它是一个大小为 unsigned int 的 bitfiend，这在您的编译器中听起来像是 32 位。尝试将 verlen 的字段改为“unsigned char”。

更多在这里。此处指定“unsigned char”的能力是 MSFT 扩展(对 ANSI C)，但应该有效:http://msdn.microsoft.com/en-us/library/yszfawxh(VS.80).aspx

注意flagfrag 也一样，应该是“unsigned short”。