Time from gettimeofday not updating as it should

Question

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

int main()
{
   float time;
   struct timeval tv;
   gettimeofday( &tv, NULL );
   time = tv.tv_sec + ( tv.tv_usec / 1000000.0 );
   printf( "time: %f\n", time );
   return 0;
}

Running binary generated by this code repeatedly, I tend to get the same time value:

$ ./a.out
time: 1348059520.000000
$ ./a.out
time: 1348059520.000000
$ ./a.out
time: 1348059520.000000

This happens until several seconds later at which I get an updated time value.

`int main()` is undefined behavior. This is C, where `()` is not the same as `(void)`. — Jens, Sep 20 '12 at 07:20

score 4 · Accepted Answer · answered Sep 19 '12 at 13:04

4

It appears that float is too small to contain the result of tv.tv_sec + ( tv.tv_usec / 1000000.0 ). Use double instead:

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

int main()
{
   double time;
   struct timeval tv;
   gettimeofday( &tv, NULL );
   time = tv.tv_sec + ( tv.tv_usec / 1000000.0 );
   printf( "time: %f\n", time );
   return 0;
}

answered Sep 19 '12 at 13:04

tshepang

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Not only 'appears' — that is precisely the trouble. `float` can store 6-7 decimal digits; the time stamp at second level needs 10 decimal digits in the integer portion of the time, so to get 6 decimal places too, you need 16 decimal digits, which is just in range of `double` (assuming IEEE 754 floating point arithmetic). – Jonathan Leffler Sep 19 '12 at 13:40
+1 _Never_ put time in a `float`. Always use a `double` or `time_t`, as I've learnt the hard way. P.s. With gcc's (and other compiler's) shadow warnings, one shouldn't use `time` as a variable name. – Joseph Quinsey Feb 14 '14 at 23:52

Jens · Answer 2 · 2012-09-19T15:07:31.530

3

Why do you use floating point at all?

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

int main (void)
{
   struct timeval tv;
   gettimeofday (&tv, NULL);
   printf ("time: %d.%06d\n", (int)tv.tv_sec, (int)tv.tv_usec);
   return 0;
}

./a.out
time: 1348067289.069908

It looks floaty, but it's inty :-) Since the microseconds value is in the range 0..999999 all you need to do is zero-pad it to 6 digits. No FP arithmetic required.

edited Sep 19 '12 at 15:07

answered Sep 19 '12 at 15:00

Jens

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By the way, I like this answer MUCH better as a way to accomplish what OP wants, but Tshepang's answer better explains the mechanism of the specific problem OP encountered. +1 to both, anyway. – R.. GitHub STOP HELPING ICE Sep 19 '12 at 18:03

Time from gettimeofday not updating as it should

2 Answers2