RE: about TCP header

[wanghg <wanghg@cwc.nus.edu.sg>]

Thanks Benjamini.

Haiguang


At 08:20 AM 10/11/01 +0200, Gilad Benjamini wrote:

I am hacking the code for the TCP. For the definition of the TCP header in the kernel,
I have following questions.


struct tcphdr {
        __u16   source;
        __u16   dest;
        __u32   seq;
        __u32   ack_seq;
#if defined(__LITTLE_ENDIAN_BITFIELD)
        __u16   res1:4,

                  what is meaning of 4 here, 4 bits?
};

Yes. It's 4 bits.
It's a standard (ANSI) C definition to allow easy access to variables that are located
on non-standard boundaries.
This type of access is interpeted differently on big/little endian machines, hence the "ifdef".
For more details find a good C book (Kerningham & Ritchie) and some TCP header documentation.