http://www.rpgchina.net/simple/?t29996.html
GMP(GNU Multiple Precision)库
http://www.mpfr.org/mpfr-2.2.1/mpfr.html摘录：
MPFR is a portable library written in C for arbitrary precision arithmetic on floating-point numbers. It aims to extend the class of floating-point numbers provided by the GNU MP library by a precise semantics. The main differences with the mpf class from GNU MP are