Just another thought: When dealing with floating point numbers, we should use 
a floating point representation ("%g"). Not assuming fixed point ("%f"). 
That's where the trouble starts, because big numbers have fever valid digits 
after the comma.
See the program below, followed by its output.

Cheers,
Joachim

/*  /usr/lib/gcc-lib/i586-suse-linux/3.3.1/include/float.h */
#include <float.h>   
#include <stdio.h>
#include <math.h>

int main()
{
   long double a = 0.0001L;           // 0.0001
   long double b = 1000.0001L;        // 100.0001
   long double c = b - 1000.0L - a;   // 0
   
   printf("%2.30Lf\n",a);
   printf("%2.30Lf\n",b);
   printf("%2.30Lf\n",c);
  
   printf("\n");

   printf("%*.*Lg\n", LDBL_DIG+8, LDBL_DIG, a);
   printf("%*.*Lg\n", LDBL_DIG+8, LDBL_DIG, b);
   printf("%*.*Lg\n", LDBL_DIG+8, LDBL_DIG, c);
}

Output on my machine:
0.000100000000000000000001143494
1000.000099999999999988986587595718
-0.000000000000000011013413547776

                    0.0001
                 1000.0001
  -1.10134135477760317e-17
 
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