How to calculate the TCP checksum

I am writing a kernel module that actually modifies the TCP header(both at
sending and receiving) and I need to re-calculate the header because of
that.
I am capturing the packets using netfilter, so I have access to the whole
SKB, I am using Linux 2.6.35-22

Looking into the source code of the TCP/IP, I found this function
void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb);
The comment above it says this routing computes the IPv4 TCP checksum

Trying to use this function, I called it passing to it skb->sk and skb
With no success so far.

So my question is, how do I correctly calculate the TCP checksum? I know
there's a psudoheader that must be created, and added to the calculation
but there must be a function in the kernel I can call that already does
that.


Thanks in advance,
Adel
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