[Valgrind-users] precision problem

[jody <jod...@gm...>]

Hi
 I'm using valgrind-3.2.1 on fedora 6.

I have encounter a situation where computations with doubles
give different result when the application is run normally or under valgrind.



Here is a very simple application showing the problem,  vgt.c:

#include <stdio.h>
#include <math.h>


int main(int argc, char *argv) {


        double a;
        double b;
        double c;
        FILE *f = fopen("num.dat", "rb");
        fread(&a, sizeof(double), 1, f);
        fread(&b, sizeof(double), 1, f);
        fread(&c, sizeof(double), 1, f);
        fclose(f);

        printf("a:%.20f, b:%.20f, c:%.20f\n", a,b,c);
        printf("a-b:%.20f, (a-b)/c:%.20f\n", a-b,(a-b)/c);
        double d = (a-b)/c;
        printf("%.20f\n", floor(d));
        return 0;
}

Here is the hexdump of the data file num.dat:
[jody]:~/progs/neander/import:$hexdump -C num.dat
00000000  a9 9a fe ff ff ff 48 40  00 00 00 00 00 00 2e c0  |......H@........|
00000010  6b 55 10 11 11 11 a1 3f                           |kU.....?|
00000018

The normal output:
[jody]:~/progs/neander/import:$./vgt
a:49.99999999935000261075, b:-15.00000000000000000000, c:0.03333333333300000229
a-b:64.99999999934999550533, (a-b)/c:1950.00000000000000000000
1950.00000000000000000000

Output under valgrind:
[jody]:~/progs/neander/import:$valgrind --tool=memcheck ./vgt
==23277== Memcheck, a memory error detector.
==23277== Copyright (C) 2002-2006, and GNU GPL'd, by Julian Seward et al.
==23277== Using LibVEX rev 1658, a library for dynamic binary translation.
==23277== Copyright (C) 2004-2006, and GNU GPL'd, by OpenWorks LLP.
==23277== Using valgrind-3.2.1, a dynamic binary instrumentation framework.
==23277== Copyright (C) 2000-2006, and GNU GPL'd, by Julian Seward et al.
==23277== For more details, rerun with: -v
==23277==
a:49.99999999935000261075, b:-15.00000000000000000000, c:0.03333333333300000229
a-b:64.99999999934999550533, (a-b)/c:1949.99999999999977262632
1949.00000000000000000000
==23277==
==23277== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 17 from 1)
==23277== malloc/free: in use at exit: 0 bytes in 0 blocks.
==23277== malloc/free: 1 allocs, 1 frees, 352 bytes allocated.
==23277== For counts of detected errors, rerun with: -v
==23277== All heap blocks were freed -- no leaks are possible.


It is important that both calculations (normal & valgrind) are the same -
with the faulty calculation my real program never gets to the point i am
interested in.

Is this a bug or did i overlook something?

Thanks
  jody