Home/ Questions/Q 58987
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-10T17:56:04+00:00

I have a C++ program representing a TCP header as a struct: #include stdafx.h

  • 0

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; }

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?

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

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. See this question: Why isn't sizeof for a struct equal to the sum of sizeof of each member? .

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

    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.

    • 0