Greatest Common Factor Function

I am attempting to write a C++ function that evalutes two integers and calculates their greatest common factor(Once using a post test loop and once using a pretest loop, so two examples!).  I have been generally succesful, but in some instances my function has failed.  How would you write such a function and implement error checking for two int's with no common factor?
matlurzAsked:
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mlevCommented:
ANY two integers ALWAYS have a greatest common divisor.
(Sometimes, it equals one.)

int gcd(int a, int b)
{
   if (a < 0)
      a = -a;
   if (b < 0)
      b = -b;

   while (a != 0 && b != 0)
      if (a < b)
         b %= a;
      else
         a %= b;

   return a + b;
}

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