Paul Eggert wrote on 1998-10-12 19:57 UTC:

...

``non-pathological'' = year range -9999 .. +9999

OK, then here's a non-pathological example of a double-rounding error:

1970-01-01 00:00:01.851401618 UTC - 1970-01-01 00:00:00.000000000 UTC

On the host that I'm talking about (sparc, IEEE, round-to-even) your xtime_diff implementation yields 1.851401618000000137698179969447664916515350341796875 exactly, whereas 1.8514016179999999156535750444163568317890167236328125, which is also exactly representable as an IEEE double, is closer to the correct answer.

That's very interesting. Under Linux/Pentium_II (cpu=686, model=3, vendor_id=GenuineIntel, stepping=4, gcc 2.7.2.3) the effect does not show up immediately. I had to enforce using the software FP-library with gcc -msoft-float to reproduce this effect, because without this option with the hardware FPU I get the correct result (and this is apparently not thanks to Intel's internal 80-bit arithmetic, because I did use gcc -ffloat-store to enforce 64-bit rounding). There seem to be interesting differences between IEEE and IEEE. ----------------------------------------------------------------------- #include <stdio.h> typedef long long int_fast64_t; typedef long int_fast32_t; struct xtime { int_fast64_t sec; int_fast32_t nsec; }; double xtime_diff(struct xtime t1, struct xtime t2) { if (t1.nsec > 1000000000) t1.nsec = 1000000000; if (t2.nsec > 1000000000) t2.nsec = 1000000000; return (t1.sec - t2.sec) + (t1.nsec - t2.nsec) / 1.0e9; } int main() { struct xtime t1 = { 1, 851401618 }, t2 = { 0, 0 }; double d; d = xtime_diff(t1, t2); printf("%.60f\n", d); return 0; } ----------------------------------------------------------------------- Suggestions for a better portable implementation? A somewhat related question: Given a floating-point value X, is there in C9X a portable way to get the next smaller and the next larger floating point value? Something like the Double'Succ(X) and Double'Pred(X) built-in function attributes available in Ada <http://wuarchive.wustl.edu/languages/ada/userdocs/docadalt/rm95/0305.htm>. This might be very useful to write portable C code that is guaranteed to deliver the closest value. Markus -- Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK email: mkuhn at acm.org, home page: <http://www.cl.cam.ac.uk/~mgk25/>