How to link to the standard libraries statically in flang-new like GFortran?

Question

I have a C++ project that uses Fortran.

When compiling under GCC, I can link in Fortran statically using target_link_libraries(Project -static-libgfortran gfortran).

However, I am now compiling the same project using Clang, which means instead of gfortran I am using Flang. (flang-new version 15.0.7)

If I don't include any replacement, I get a lot of undefined symbols, like _FortranAioBeginExternalFormattedOutput.

If I try to swap out those two with just "flang", it fails, saying:

lld: error: unable to find library -lflang

LLVM has a single sentence on linking flang, which I don't follow:

"After the LLVM IR is created, the flang driver invokes LLVM’s existing infrastructure to generate object code and invoke a linker to create the executable file."

How do I link in Flang the same way I did GFortran?

Answer 1

-static-libgfortran is listed in https://clang.llvm.org/docs/ClangCommandLineReference.html#fortran-compilation-flags. Try using target_compile_options(Project PUBLIC -static-libgfortran) (or PRIVATE).

For Classic Flang

The compiler option docs don't list a -static-libgfortran flag, but they do list a -static-flang-libs. Try using that instead. But since it's listed as a compiler option, I'd try using target_compile_options.

If you want to support both GFortran and Flang, then wrap the target_link_options / target_link_libraries calls in if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU") or if(CMAKE_CXX_COMPILER_ID STREQUAL "Flang") (see docs for CMAKE_LANG_COMPILER_ID), or use the $<Fortran_COMPILER_ID:...> generator expression.

If this is a linker flag, I'd recommend target_link_options for linker options instead of target_link_libraries just for readability's sake (using target_link_libraries should work too though according to the docs).

Note: I found the info about the -static-flag-libs flag by googling ""flang" "-static-libgfortran"", where my top search result was this SciVision post, which showed the -static-flang-libs flag. Then a google of ""flang" "-static-flang-libs"" led me to the Flang GitHub. Tip for googling flags that start with a dash: enclose in double-quotes to avoid it being used as a exclusion operator.