C Programming TCP Checksum

Question

I have been having trouble doing the checksum for TCP for several days now. I have looked at many sources on the Internet but none of the examples that I have seen show you

Answer 1

I found a fairly good example on the winpcap-users mailing list which should address Greg's comment about odd length data and give you something to compare your code against.

USHORT CheckSum(USHORT *buffer, int size)
{
    unsigned long cksum=0;
    while(size >1)
    {
        cksum+=*buffer++;
        size -=sizeof(USHORT);
    }
    if(size)
        cksum += *(UCHAR*)buffer;

    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >>16);
    return (USHORT)(~cksum);
}