Irritation regarding precision of ping(8)...

[Bruce Evans]

On Sun, 31 Oct 2004, Sean Chittenden wrote:

> This has long bugged me and tonight I finally snapped and had to do
> something about it.  Example output:
>
> 64 bytes from a.b.c.d: icmp_seq=935 ttl=126 time=33.824 ms
> 64 bytes from a.b.c.d: icmp_seq=936 ttl=126 time=29.138 ms
> 64 bytes from a.b.c.d: icmp_seq=937 ttl=126 time=28.262 ms
> 64 bytes from a.b.c.d: icmp_seq=938 ttl=126 time=29.67 ms
> 64 bytes from a.b.c.d: icmp_seq=939 ttl=126 time=30.963 ms
> 64 bytes from a.b.c.d: icmp_seq=940 ttl=126 time=30.283 ms
> 64 bytes from a.b.c.d: icmp_seq=941 ttl=126 time=29.455 ms
>
> The source of irritation being line seq 930.  The time should be 29.670
> since we are accurately measuring the precision to the thousands place.

I think you mean line seq 938.

This seems to be a local bug in printf.  The correct format for printing
3 digits after the decimal point (%.3f) is already used, and printf of
29.67 with format %.3f gives 29.67 here.

% Index: ping.c
% ===================================================================
% RCS file: /home/ncvs/src/sbin/ping/ping.c,v
% retrieving revision 1.106
% diff -u -r1.106 ping.c
% --- ping.c	30 Sep 2004 07:35:56 -0000	1.106
% +++ ping.c	31 Oct 2004 09:40:35 -0000
% @@ -998,7 +998,7 @@
%  			   seq);
%  			(void)printf(" ttl=%d", ip->ip_ttl);
%  			if (timing)
% -				(void)printf(" time=%.3f ms", triptime);
% +				(void)printf(" time=%.30f ms", triptime);
%  			if (dupflag)
%  				(void)printf(" (DUP!)");
%  			if (options & F_AUDIBLE)

%.30f is a horribly wrong format.  It gives 30 digits after the decimal
point, and inexact representation of 0.001 gives garbage nonzero digits
for about half of the extra 27.

% @@ -1293,7 +1293,7 @@
%  		double avg = tsum / n;
%  		double vari = tsumsq / n - avg * avg;
%  		(void)printf(
% -		    "round-trip min/avg/max/stddev = %.3f/%.3f/%.3f/%.3f ms\n",
% +		    "round-trip min/avg/max/stddev = %.30f/%.30f/%.30f/%.30f ms\n",
%  		    tmin, avg, tmax, sqrt(vari));
%  	}
%

This part also has some style bugs.

Bruce