Russ Cox of Plan-9 and Go fame posted a blog entry titled Floating Point to Decimal Conversion is Easy. While he is usually right, I believe this time he isn't.
Floating point to decimal conversion is easy if you are okay with ugly results. A good conversion routine will print the shortest decimal representation of the floating-point number, that is, the shortest decimal number whose closest floating-point representation equals the original number. You do not want 0.30000000001, you want 0.3, because the number right above 0.3 is 0.30000000003 and 0.30000000001 does not provide any extra precision.