Code

 Header...
 double fp2_(double base,int exponent){
   ... 

»  if (exponent==1) 
    return base;
 else if (exponent%2==0) 
   return fp2_(base*base, exponent/2);

» return base*fp2_(base*base, exponent/2);
}

 double fp2(double base,int exponent){
   ... 

»  if (exponent<0) {
»    exponent=-exponent;
»    base=1.0/base;
  }
  else if (exponent==0) 
    return 1;
  
»  return fp2_(base, exponent);
}

 double fastexp(double base,int exponent){
   ... 

»  unsigned int ord=1;
»  int bitsdone=0;// Diagnostic
»  double result=1.0;

»  if (exponent<0) {
»    exponent=-exponent;
»    base=1.0/base;
  }
  
»  for ( double e2=base;; e2*=e2) {
»    if (exponent&ord) {
»      ++bitsdone;// Just out of interest
»      result*=e2;
}

»    ord<<=1;
// We do the test here to avoid an extra multiplication
»    if (ord>exponent) 
      break;
  }

»  printf("%d bits used\n", bitsdone);
»  if (!isfinite(result)) 
    printf("Warning: under or overflow\n");

»  return result;
}

 int main(){
   ... 

»  double base, res1, resok, resa;
»  int exponent=-3;

»  while (1) {
»    printf("Please enter the base and exponent\n"
           "make the exponent == 0 to quit\n");
»    scanf("%lg %d", &base, &exponent);

»    if (exponent==0) 
      break;

»    res1=fastexp(base, exponent);
»    resa=fp2(base, exponent);
    // For testing:
»    resok=pow(base, exponent);

»    printf("%.12g %.12g (diff=%.12g)\n", res1, resok, res1-resok);
»    printf("%.12g %.12g (diff=%.12g)\n", resa, resok, resa-resok);
»    printf("%.12g\n", resa-res1);
  }

»  return 0;
}