The MPFR multiple-precision floating-point library
http://mpfrlib.org/

The MPFR library is a C library for multiple-precision floating-point
computations with exact rounding (also called correct rounding). It is
based on the GMP multiple-precision library.

The main goal of MPFR is to provide a library for multiple-precision
floating-point computation which is both efficient and has a
well-defined semantics. It copies the good ideas from the ANSI/IEEE-754
standard for double-precision floating-point arithmetic (53-bit
mantissa).