Date: Sun, 10 Oct 1999 12:41:55 -0400 (EDT) From: Robert Watson <robert@cyrus.watson.org> To: hackers@freebsd.org Subject: [Fwd: clock(3) runs backwards! (fwd)] (fwd) Message-ID: <Pine.BSF.3.96.991010124008.5692A-100000@fledge.watson.org>
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This was a BSDI mailing list post, but I gave it a try on my 3.3-RELEASE FreeBSD machine and got the following: fledge:/tmp> ./clocktest 1.000000 - 0.000000 = 1.000000 2.000000 - 1.000000 = 1.000000 1.000000 - 2.000000 = 4294967295.000000 3.000000 - 1.000000 = 2.000000 2.000000 - 3.000000 = 4294967295.000000 4.000000 - 2.000000 = 2.000000 3.000000 - 4.000000 = 4294967295.000000 5.000000 - 3.000000 = 2.000000 4.000000 - 5.000000 = 4294967295.000000 6.000000 - 4.000000 = 2.000000 5.000000 - 6.000000 = 4294967295.000000 7.000000 - 5.000000 = 2.000000 6.000000 - 7.000000 = 4294967295.000000 8.000000 - 6.000000 = 2.000000 7.000000 - 8.000000 = 4294967295.000000 9.000000 - 7.000000 = 2.000000 10.000000 - 9.000000 = 1.000000 9.000000 - 10.000000 = 4294967295.000000 10.000000 - 11.000000 = 4294967295.000000 12.000000 - 10.000000 = 2.000000 Seems like I'm seeing the behavior they describe--whether that's bad is another question, but seemed interesting. Any suggestions? Robert N M Watson robert@fledge.watson.org http://www.watson.org/~robert/ PGP key fingerprint: AF B5 5F FF A6 4A 79 37 ED 5F 55 E9 58 04 6A B1 TIS Labs at Network Associates, Safeport Network Services ---------- Forwarded message ---------- Date: Sun, 10 Oct 1999 12:33:16 -0400 From: support@safeport.com To: Robert Watson <robert@cyrus.watson.org> Subject: [Fwd: clock(3) runs backwards! (fwd)] Did you see this? If its not on FreeBSD, and you are not archiving the BSDI list, I can send you the thread. It turns our this is not a silly programming error, this guy wrote the current C ANSI standard. -------- Original Message -------- Subject: clock(3) runs backwards! (fwd) Date: Thu, 7 Oct 1999 14:53:48 -0400 (EDT) From: larry.jones@sdrc.com (Larry Jones) To: bsdi-users@mailinglists.org FYI. Also sent to problems@bsdi.com. > Description: > Occasionally, clock(3) will return a value which is less than > the value returned by a previous call. > > Release: > 4.0.1 > > Repeat-By: > Run the following test program; a single run is usually sufficient > to observe the problem. > > #include <stdio.h> > #include <stdlib.h> > #include <time.h> > > int main() > { > int i; > clock_t t0, t1; > > for (i = 0; i < 20; i++) { > t0 = clock(); > do { > t1 = clock(); > } while(t0 == t1); > printf( " %f - %f = %f\n", (double)t1, (double)t0, > (double)(t1-t0)); > } > return 0; > } > > Fix: > Converting the user and system time to clock ticks before adding > them together causes the problem due to roundoff error and the > system time not being monotonically increasing. Assuming that > the actuall sum of user and system time *is* monotonically > increasing (which it seems to be), adding them together before > converting will solve the problem: > > --- /cdrom/lib/libc/gen/clock.c Thu Feb 2 18:20:13 1995 > +++ ./clock.c Wed Oct 6 13:14:20 1999 > @@ -39,12 +39,6 @@ > #include <sys/time.h> > #include <sys/resource.h> > > -/* > - * Convert usec to clock ticks; could do (usec * CLK_TCK) / 1000000, > - * but this would overflow if we switch to nanosec. > - */ > -#define CONVTCK(r) (r.tv_sec * CLK_TCK + r.tv_usec / (1000000 / CLK_TCK)) > - > clock_t > clock() > { > @@ -52,5 +46,7 @@ > > if (getrusage(RUSAGE_SELF, &ru)) > return ((clock_t) -1); > - return((clock_t)((CONVTCK(ru.ru_utime) + CONVTCK(ru.ru_stime)))); > + return((clock_t)((ru.ru_utime.tv_sec + ru.ru_stime.tv_sec) * CLK_TCK + > + (ru.ru_utime.tv_usec + ru.ru_stime.tv_usec) / > + (1000000 / CLK_TCK))); > } -Larry Jones Whatever it is, it's driving me crazy! -- Calvin --------------------------------------------------------------------- To unsubscribe, e-mail: bsdi-users-unsubscribe@mailinglists.org For additional commands, e-mail: bsdi-users-help@mailinglists.org To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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