source: trunk/athena/etc/track/cksum.c @ 1471

#include "mit-copyright.h"
#include <sys/file.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <netinet/in.h>

/*                      I N _ C K S U M
 *
 * Checksum routine for Internet Protocol family headers (C Version).
 * NOTE: this code is taken from 4.3bsd ping, and modified to read a file.
 *
 */
unsigned
in_cksum( filename, statp) char *filename; struct stat *statp;
{
        int fd;
        struct stat statbuf;
        static char buf[ 8192];
        register int nleft;
        register u_short *w;
        register unsigned int sum = 0;
        unsigned int carry = 0;

        fd = open( filename, O_RDONLY);

        /* get the file's optimal blocksize:
         * we want to call read() as little as possible,
         * because each call spills the register variables.
         */
        if ( ! statp) {
                statp = &statbuf;
                fstat( fd, statp);
        }
        /*
         * Our algorithm is simple, using a 48 bit accumulator (sum),
         * we add sequential 16 bit words to it, and at the end, fold
         * back all the carry bits from the top 32 bits into the lower
         * 16 bits.
         * this means we can handle files containing 2**32 16-bit words,
         * that is, 2**33 bytes, which is a little more than 8 gigabytes.
         */
        while ( 0 < ( nleft = read( fd, buf, (*statp).st_blksize))) {
                w = (u_short *) buf;
                while( nleft > 1 )  {
                        sum += *w++;
                        nleft -= 2;
                }

                /* mop up an odd byte, if necessary */
                if( nleft == 1 ) {
                        ((u_char *)w)[ 1] = '\0';
                        sum += *w;
                }
                carry += sum >> 16;
                sum &= 0xffff;
        }
        close( fd);

        /*
         * add back carry's top 16 bits and low 16 bits.
         * this makes this checksum byte-order independent.
         */
        sum = sum + (carry >> 16) + (carry & 0xffff);
        sum += (sum >> 16);                     /* add resulting carry */
        return( htons( 0xffff & ~sum));         /* discard last carry. */
        /* the ~ (one's complement) isn't really necessary here;
         * it makes a nonzero checksum for a file of zeroes,
         * which is useful for network applications of this algorithm.
         */
}