Why is difftime Only Returning Whole Numbers?

Question

For some reason difftime is only returning whole numbers. My code is pretty simple.

#include <time.h>

int main()
{
    time_t test = time(NULL);
    while (1)
    {
        std::cout << difftime(time(NULL), test) << std::endl;
    }
}

My output looks like

0...
1...
2...
3...

Isn't difftime supposed to return doubles?

For sub-second timing on POSIX systems, consider [`clock_gettime()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_gettime.html) (nanosecond resolution, modern, preferred) or [`gettimeofday()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html) (microsecond resolution, less modern, strictly deprecated, but available on Mac OS X where `clock_gettime()` is not). There used to be [`ftime()`](http://pubs.opengroup.org/onlinepubs/009695399/functions/ftime.html) (millisecond resolution, ancient, deprecated) too — that's from the POSIX 2001 (2004) manual. — Jonathan Leffler, Jul 04 '15 at 02:42

score 4 · Accepted Answer · edited Jul 04 '15 at 02:37

4

The function time() returns to the nearest second and difftime() just returns the difference of those. Any whole number minus a whole number is a whole number basically (but it is returned as a double).

By the way, for a more accurate timer:

time_t test = clock();
while (1)
{
    std::cout << float(clock() - test) / CLOCKS_PER_SEC << std::endl;
}

edited Jul 04 '15 at 02:37

Jonathan Leffler

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answered Jul 04 '15 at 01:56

SunsetQuest

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[time](http://www.cplusplus.com/reference/ctime/time/) and [clock](http://www.cplusplus.com/reference/ctime/clock/) – Rodolfo Jul 04 '15 at 02:24

score 1 · Answer 2 · edited May 23 '17 at 10:28

1

This has nothing to do with time() or difftime(). It is purely cout in c++.

See here: How do I print a double value with full precision using cout?

edited May 23 '17 at 10:28

Community

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answered Nov 04 '15 at 16:20

user1602017

1,109
8
8

score -1 · Answer 3 · answered Jul 04 '15 at 02:06

-1

int main()
{
    time_t o_test, f_test;
    time(&o_test);
    while (1)
    {
        time(&f_test);
        std::cout << difftime(f_test, o_test) << std::endl;
    }
}

answered Jul 04 '15 at 02:06

jdl

6,151
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1

You've not explained what you've changed or why. The `time()` function takes a pointer to `time_t` argument and will write the value it returns in the variable pointed at if the pointer is not null. The original code is perfectly correct. Your revision is also OK. (Personally, I very seldom use `time(&var)` and almost always use `time(0)` or `time(NULL)` instead). But you've done nothing about explaining why the output from `difftime()` is always an integral value. – Jonathan Leffler Jul 04 '15 at 02:46

Why is difftime Only Returning Whole Numbers?

3 Answers3