#### checksum.c1.3 KB History Raw

 1234567891011121314151617181920212223242526272829303132333435363738394041424344454647 ``````/* * Checksum routine for Internet Protocol family headers (C Version). * * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989, * pp. 86-101, for additional details on computing this checksum. */ #include #include "checksum.h" int /* return checksum in low-order 16 bits */ in_cksum(void *parg, int nbytes) { u_short *ptr = parg; register long sum; /* assumes long == 32 bits */ u_short oddbyte; register u_short answer; /* assumes u_short == 16 bits */ /* * Our algorithm is simple, using a 32-bit accumulator (sum), * we add sequential 16-bit words to it, and at the end, fold back * all the carry bits from the top 16 bits into the lower 16 bits. */ sum = 0; while (nbytes > 1) { sum += *ptr++; nbytes -= 2; } /* mop up an odd byte, if necessary */ if (nbytes == 1) { oddbyte = 0; /* make sure top half is zero */ *((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */ sum += oddbyte; } /* * Add back carry outs from top 16 bits to low 16 bits. */ sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */ sum += (sum >> 16); /* add carry */ answer = ~sum; /* ones-complement, then truncate to 16 bits */ return(answer); } ``````