rem: unsigned long checksum(char *filename)
add: checksum(const char *filename)
context: unsigned long checksum(char *filename){	int fh;	unsigned long chk= 0, buf = 0;	fh = open(filename,O_RDONLY);	if (fh == -1)		return 0;	while (read (fh,&buf,sizeof(buf)) > 0 )		chk = ((chk << 1) | (chk >> 31)) ^ buf;	close(fh);	return (chk);}