floating point bug -- or misunderstanding?

[Derrick Bass]

I may have found a bug in egcs, but more likely I just don't
understand floating point math.

Can someone tell me enough about floating point (or point me to an
explanation) to explain the output of the following program?

#include <stdio.h>
main() {
  float a,b,c,d;
  
  a = 1;
  b = 3;
  c = a/b;
  d = c * b;

  printf("%g\n", a - c * b);	
  printf("%g\n", a - d);
}

I compiled the program with egcs-19980803 on a pentium pro linux
machine with no optimization and the output was
-2.98023e-08
0

Shouldn't these numbers be equal?

Derrick Bass