Date: Sun, 31 Oct 2004 22:05:56 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Sean Chittenden <sean@chittenden.org> Cc: freebsd-net@FreeBSD.org Subject: Re: Irritation regarding precision of ping(8)... Message-ID: <20041031214441.Q15594@delplex.bde.org> In-Reply-To: <EE1B194A-2B21-11D9-8420-000A95C705DC@chittenden.org> References: <EE1B194A-2B21-11D9-8420-000A95C705DC@chittenden.org>
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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
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