# Flang support for REAL(16) math intrinsics To support most `REAL(16)` (i.e. 128-bit float) math intrinsics Flang relies on third-party libraries providing the implementation. `-DFLANG_RUNTIME_F128_MATH_LIB=libquadmath` CMake option can be used to build `FortranFloat128Math` library that has unresolved references to GCC `libquadmath` library. A Flang driver built with this option will automatically link `FortranFloat128Math` and `libquadmath` libraries to any Fortran program. This implies that `libquadmath` library has to be available in the standard library paths, so that linker can find it. The `libquadmath` library installation into Flang project distribution is not automatic in CMake currently. Testing shows that `libquadmath` versions before GCC-9.3.0 have accuracy issues, so it is recommended to distribute the Flang package with later versions of `libquadmath`. Care must be taken by the distributors of a Flang package built with `REAL(16)` support via `libquadmath` because of its licensing under the GNU Library General Public License. Moreover, static linking of `libquadmath` to the Flang users' programs may imply some restrictions/requirements. This document is not intended to give any legal advice on distributing such a Flang compiler. Flang compiler targeting systems with `LDBL_MANT_DIG == 113` may provide `REAL(16)` math support without a `libquadmath` dependency, using standard `libc` APIs for the `long double` data type. It is not recommended to use the above CMake option for building Flang compilers for such targets.