CODEWITHSUNDEEP
c++clinuxtimetime-t
user48956
user48956

Reputation: 15788

How to stop time from running backwards on Linux?

Here's a little test I've written to verify that time does indeed only run forwards in Linux.

#include <time.h>
#include <sys/time.h>  

bool timeGoesForwardTest2()
{
   timeval tv1, tv2;   
   double startTime = getTimeSeconds();  // my function

   while ( getTimeSeconds() - startTime < 5 )
   {
      gettimeofday( &tv1, NULL );  
      gettimeofday( &tv2, NULL );  

      if ( tv2.tv_usec == tv1.tv_usec &&
           tv2.tv_sec == tv1.tv_sec )
      {
         continue;  // Equal times are allowed.
      }

      // tv2 should be greater than tv1
      if ( !( tv2.tv_usec>tv1.tv_usec ||
              tv2.tv_sec-1 == tv1.tv_sec ) )
      {
         printf( "tv1: %d %d\n", int( tv1.tv_sec ), int( tv1.tv_usec ) );
         printf( "tv2: %d %d\n", int( tv2.tv_sec ), int( tv2.tv_usec ) );
         return false;
      }         
   }
   return true;
}

Test fails with the result.

 tv1: 1296011067 632550
 tv2: 1296011067 632549

ummm....

Why does this happen?

Here's my setup:

Linux version 2.6.35-22-generic (buildd@rothera) (gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu4) ) #33-Ubuntu SMP Sun Sep 19 20:34:50 UTC 2010 (Ubuntu 2.6.35-22.33-generic 2.6.35.4)
... running inside VirtualBox 3.2.12, in Windows 7.

Upvotes: 24

Views: 5112

Answers (5)

MarkR
MarkR

Reputation: 63558

Time should not run backwards on real hardware; on a VM your mileage may vary.

In any case, your application should probably not assume that time doesn't run backwards by a very small amount (think, maybe 1 second).

Yes, clock_gettime is good but even that could run backwards in the case of faulty hardware (or a VM, as in your example).

I have seen a hardware bug make time run backwards (albeit very occasionally), it was a cause of some very peculiar problems.

In particular, anything which involves comparing file timestamps will go wrong when time goes backwards.

Upvotes: 1

Paul Sasik
Paul Sasik

Reputation: 81489

It's not that it's running backwards. It'd be better to say that it is not reporting the correct time. This is because computers, without the aid of a dedicated timing subsystem, simply are not capable of reporting time very accurately in single millisecond intervals.

The precision will vary with hardware, the OS and even the power supply. Here is an article for starters. A bit old but communicates the idea nicely.

Upvotes: 5

vz0
vz0

Reputation: 32923

There is an open issue at the VirtualBox Bug Tracker. They link to a blog post stating why you shouldn't use gettimeofday() to measure the passage of time:

The most portable way to measure time correctly seems to be clock_gettime(CLOCK_MONOTONIC, ...)

Upvotes: 17

ephemient
ephemient

Reputation: 204808

gettimeofday() is not guaranteed to be monotonic. Use clock_gettime(CLOCK_MONOTONIC) if you need that guarantee.

Upvotes: 30

phooji
phooji

Reputation: 10395

Machine timers on most machines only have about 15 usec precision (even to native code). Time going 'backward' is odd, but you really can't rely on that level (1 usec) anyway. (Also note: there is a difference between precision and accuracy; the accuracy of most timers is worse than its precision). The use of a virtual machine may aggravate this as well.

Update: Typo

Upvotes: 10

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