Subtraction of long in C loses precision?

Question

I'm sure the answer is simple, but I don't quite get it. I'm trying to calculate the delta between two struct timespec using this code:

struct timespec start, finish, diff;
int ndiff;

/* Structs are filled somewhere else */

diff.tv_sec = finish.tv_sec - start.tv_sec;
ndiff = finish.tv_nsec - start.tv_nsec;
if (ndiff < 0) {
    diff.tv_sec--;
    ndiff = 1L - ndiff;
}
diff.tv_nsec = ndiff;

printf("Elapsed time: %ld.%ld seconds.\n", diff.tv_sec, diff.tv_nsec);

However, the output is always something like Elapsed time: 0.300876000 seconds. which seems to indicate that I'm losing the last three digits of the nanoseconds (since those shouldn't always be zero). Can someone point out what's causing that?

There is no guarantee that a timespec is going to be nanosecond accurate. — rici, Nov 06 '17 at 22:13
... or perhaps it would be more precise to say that the system interfaces by which you might *populate* a `struct timespec` are not guaranteed to provide nanosecond resolution. — John Bollinger, Nov 06 '17 at 22:14
That kind of precision is pointless anyway, since the accuracy is hugely less. — Martin James, Nov 06 '17 at 22:19
And your point is what, @mxgx? `struct timespec` supports nanosecond resolution if `clock_gettime()` happens to provide that on your system, for the particular clock you are using, but clock resolution is system-dependent. You can get yours from `clock_getres()`. — John Bollinger, Nov 06 '17 at 22:22
@JohnBollinger Ok, I didn't know that, `clock_getres` does indeed show 1000 ns resolution. At least the code is correct then. Thanks for your help. — gmolau, Nov 06 '17 at 22:28

chux - Reinstate Monica · Accepted Answer · 2017-11-06T22:27:53.790

Elapsed time: 0.300876000 seconds. which seems to indicate that I'm losing the last three digits of the nanoseconds (since those shouldn't always be zero). Can someone point out what's causing that?

The code's clock reported precision is 1000 ns. @John Bollinger @rici

and/or

diff.tv_sec is not necessarily a long. Use a matching specifier.

// printf("Elapsed time: %ld.%ld seconds.\n", diff.tv_sec, diff.tv_nsec);
// Also insure fraction is printed with 9 digits
printf("Elapsed time: %lld.%09ld seconds.\n", (long long) diff.tv_sec, diff.tv_nsec);

Also, incorrect "borrow" math when updating the ndiff.

ndiff = finish.tv_nsec - start.tv_nsec;
if (ndiff < 0) {
  diff.tv_sec--;
  // ndiff = 1L - ndiff;
  ndiff += 1000000000;
}

Even better, drop the int diff variable.

diff.tv_sec = finish.tv_sec - start.tv_sec;
diff.tv_nsec = finish.tv_nsec - start.tv_nsec;
if (diff.tv_nsec < 0) {
    diff.tv_sec--;
    diff.tv_nsec += 1000000000;
}

Should finish occur before start, then other code may be desired to keep the 2 members of diff with the same sign.

Subtraction of long in C loses precision?

1 Answers1