unsigned short /* this function generates header checksums */ csum (unsigned short *buf, int nwords) { unsigned long sum; for (sum = 0; nwords > 0; nwords--) // add words(16bits) together sum += *buf++; sum = (sum >> 16) + (sum & 0xffff); //add carry over sum += (sum >> 16); //what does this step do??? add possible left-over //byte? But isn't it already added in the loop (if //any)? return ((unsigned short) ~sum); } I assume nwords in the number of 16bits word, not 8bits byte (if there are odd byte, nword is rounded to next large), is it correct? The line sum = (sum 16) + (sum & 0xffff) is to add carry over to make 16bit complement sum += (sum 16); What's the purpose of this step? Add left-over byte? How? Thanks!