putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t css, uint32_t cse) { uint32_t sum; if (cse && cse < n) n = cse + 1; if (sloc < n-1) { sum = net_checksum_add(n-css, data+css); stw_be_p(data + sloc, net_checksum_finish(sum)); } }