通过改变窗口大小让不同头部大小 [英] Getting different header size by changing window size

问题描述

我有一个C ++程序重新presenting TCP报头为结构：

I have a C++ program representing a TCP header as a struct:

#include "stdafx.h"

/*  TCP HEADER

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

*/

typedef struct {    	// RFC793
    WORD		 wSourcePort;
    WORD		 wDestPort;
    DWORD		 dwSequence;
    DWORD		 dwAcknowledgment;
    unsigned int byReserved1:4;
    unsigned int byDataOffset:4;
    unsigned int fFIN:1;
    unsigned int fSYN:1;
    unsigned int fRST:1;
    unsigned int fPSH:1;
    unsigned int fACK:1;
    unsigned int fURG:1;
    unsigned int byReserved2:2;
    unsigned short wWindow;
    WORD		 wChecksum;
    WORD		 wUrgentPointer;
} TCP_HEADER, *PTCP_HEADER;


int _tmain(int argc, _TCHAR* argv[])
{
    printf("TCP header length: %d\n", sizeof(TCP_HEADER));
    return 0;
}

如果我运行这个程序，我得到这个头的大小为24字节，这不是我所期待的大小。如果我改变字段wWindow的类型为无符号整型wWindow：16，它具有相同的比特数为一个无符号短，程序告诉我该结构的大小现在是20个字节，正确的尺寸。这是为什么？

If I run this program I get the size of this header as 24 bytes, which is not the size I was expecting. If I change the type of the field "wWindow" to "unsigned int wWindow:16", which has the same number of bits as an unsigned short, the program tells me the size of the struct is now 20 bytes, the correct size. Why is this?

我使用Microsoft Visual Studio 2005 SP1的一个32位的x86机器上。

I am using Microsoft Visual Studio 2005 with SP1 on a 32-bit x86 machine.

推荐答案

请参阅这个问题：<一href=\"http://stackoverflow.com/questions/119123/why-does-the-sizeof-operator-return-a-size-larger-for-a-structure-than-the-tota\">http://stackoverflow.com/questions/119123/why-does-the-sizeof-operator-return-a-size-larger-for-a-structure-than-the-tota

我相信，编译器会暗示当您使用禁用填充无符号整型wWindow：16。语法

I believe that compiler takes a hint to disable padding when you use the "unsigned int wWindow:16" syntax.

此外，注意，短不保证是16位。担保是：16位＆LT; =尺寸短＆LT的=大小int的

Also, note that a short is not guaranteed to be 16 bits. The guarantee is that: 16 bits <= size of a short <= size of an int.

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