将长十六进制字符串转换为十进制字符串

I would need to figure out a way to do the following in standard C. Assume we are given a string representing a hexadecimal number with say n digits. We want to convert this to a string representing the same number in decimal. The strings could have an arbitrary number of digits. What's at least easy to deduce is that the decimal string needs <= 2n letters.I would need to figure out a way to do the foll