Messages in this thread | | | Date | Mon, 24 Jun 2002 20:37:31 -0400 | From | Christian Robert <> | Subject | Re: gettimeofday problem |
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For your eyes,
while reading this thread I wrote a sample program to test if the clock sometimes goes backward/forward.
I started my program while continuing reading threads on the linux-kernel archive. After about 90 minutes of continuous running I went back to the window running the program and surprise I saw this:
$ ./tloop Bump negative -4294967295 ^C Summary: ------- Min = 0 Max = 257845 Avg = 1 (6009092476/5521919279)
So it looks like the time changed somewhere of a value +/- 4,295 seconds.
kernel 2.4.18
$ cat /proc/cpuinfo processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 8 model name : Pentium III (Coppermine) stepping : 1 cpu MHz : 602.566 cache size : 256 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 2 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca cmov pat pse36 mmx fxsr sse bogomips : 1202.58
--------------------- program tloop.c -------------------------
#include <stdio.h> #include <stdlib.h> #include <unistd.h>
#include <sys/time.h> #include <signal.h>
typedef long long LL;
LL GetTime (void) { struct timeval tv; LL retval;
gettimeofday (&tv, NULL); retval = (tv.tv_sec * 1000000) + (tv.tv_usec); return retval; }
volatile int Break = 0;
void Trap (int sig) { Break = 1; }
int main (void) { LL Now, Old; LL Dt, Min=9999999, Max=0, Num=0, Tot=0;
Old = Now = GetTime ();
signal (SIGINT, Trap); signal (SIGQUIT, Trap);
for ( ; Break==0 ; ) { Now = GetTime();
if (Now < Old) { printf ("Bump negative %lld\n", (Now-Old)); } else { Dt = Now-Old;
Min = (Dt < Min) ? Dt : Min; Max = (Dt > Max) ? Dt : Max;
Tot += Dt; Num += 1; }
Old = Now; }
printf ("Summary:\n-------\n Min = %lld\n Max = %lld\n " "Avg = %lld (%lld/%lld)\n", Min, Max, Tot/Num, Tot, Num);
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