I've used a #define to set a value, I've set a declared variable to the same text value and I've set a second declared variable to the #define value. The declared values are the same, the #define value differs.

If this were a simple round off error, all three values should be the same. As they are not the same, the #define has a different value. I think the #define might have access to all of the guard bits in the FPU whereas the declared values are truncated.

Also, please note, this problem does not happen with gcc 2.9.2 and I'm running on Red Hat Linux

==============================================================================
cat prec.c
==============================================================================
#include <stdio.h>

#define VALUE 1e-4

int
main()
{
   double v = 1.0;
   double a = 1e-4;
   double b = VALUE;

   printf("a = %e\n", a);
   printf("def/var - 1 = %e, var/def - 1 = %e\n", (VALUE*VALUE)/(a*a) - v,
      (a*a)/(VALUE*VALUE) - v);
   printf("def/var2 - 1 = %e, var2/def - 1 = %e\n", (VALUE*VALUE)/(b*b) - v,
      (b*b)/(VALUE*VALUE) - v);
   printf("var/var - 1 = %e, def/def - 1 = %e\n", (a*a)/(a*a) - v,
      (VALUE*VALUE)/(VALUE*VALUE) - v);
   printf("var2/var1 - 1 = %e, var2/var2 - 1 = %e\n", (b*b)/(a*a) - v,
      (b*b)/(b*b) - v);
}
========================================================================================
$ gcc prec.c
$ ./a.out
a = 1.000000e-04
def/var - 1 = -7.486416e-17, var/def - 1 = 7.491837e-17
def/var2 - 1 = -7.486416e-17, var2/def - 1 = 7.491837e-17
var/var - 1 = 0.000000e+00, def/def - 1 = 0.000000e+00
var2/var1 - 1 = 0.000000e+00, var2/var2 - 1 = 0.000000e+00
$