This is declaring a variable in the middle of the block. Why don't you perform this initialization at the beginning of the if block (where the declaration was) and only update hc_ssthresh here to ssthresh?

This will make the compiler first write a zero to hc_ssthresh, then a new value. I can actually go without a variable at all, make entire declaration an argument to tcp_hc_update().

Then I would prefer going without a variable...