`-print-search-dirs` does not take `-stdlib` into account when printing library search directories

[boris-kolpackov]

On my Debian with Clang 18 snapshot:

$ clang-18 -print-search-dirs
programs: =/usr/bin:/usr/lib/llvm-18/bin:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/bin
libraries: =/usr/lib/llvm-18/lib/clang/18:/usr/bin/../lib/gcc/x86_64-linux-gnu/13:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../lib64:/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:/usr/lib/../lib64:/lib:/usr/lib

$ clang-18 -stdlib=libc++ -print-search-dirs
programs: =/usr/bin:/usr/lib/llvm-18/bin:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/bin
libraries: =/usr/lib/llvm-18/lib/clang/18:/usr/bin/../lib/gcc/x86_64-linux-gnu/13:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../lib64:/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:/usr/lib/../lib64:/lib:/usr/lib

Notice how in the second invocation the library search directories still point to GCC's libstdc++ rather than libc++.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Boris Kolpackov (boris-kolpackov)

On my Debian with Clang 18 snapshot:

$ clang-18 -print-search-dirs
programs: =/usr/bin:/usr/lib/llvm-18/bin:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/bin
libraries: =/usr/lib/llvm-18/lib/clang/18:/usr/bin/../lib/gcc/x86_64-linux-gnu/13:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../lib64:/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:/usr/lib/../lib64:/lib:/usr/lib

$ clang-18 -stdlib=libc++ -print-search-dirs
programs: =/usr/bin:/usr/lib/llvm-18/bin:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/bin
libraries: =/usr/lib/llvm-18/lib/clang/18:/usr/bin/../lib/gcc/x86_64-linux-gnu/13:/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../lib64:/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:/usr/lib/../lib64:/lib:/usr/lib

Notice how in the second invocation the library search directories still point to GCC's libstdc++ rather than libc++.

[MaskRay]

Clang's behavior is expected. -stdlib=libc++ does not change library paths. Many ELF OSes detect GCC installations by default, and even if libstdc++ is unused, crtbeginS.o, libgomp.a, libatomic.a may be used. --gcc-triple= disables GCC installation detection. For Clang<18, specify a dummy --gcc-toolchain=donotexist (the option is deprecated).

% /tmp/Rel/bin/clang -print-search-dirs | sed -n -e '/^libraries: =/ {s/.*: =//; s/:/\n/g; p}' | xargs realpath -q | sort -u
/tmp/Rel/lib/clang/19
/tmp/Rel/lib/clang/19/lib/x86_64-unknown-linux-gnu
/tmp/Rel/lib/x86_64-unknown-linux-gnu
/usr/lib
/usr/lib64
/usr/lib/gcc/x86_64-linux-gnu/13
/usr/lib/x86_64-linux-gnu
% /tmp/Rel/bin/clang --gcc-triple= -print-search-dirs | sed -n -e '/^libraries: =/ {s/.*: =//; s/:/\n/g; p}' | xargs realpath -q | sort -u
/tmp/Rel/lib/clang/19
/tmp/Rel/lib/clang/19/lib/x86_64-unknown-linux-gnu
/tmp/Rel/lib/x86_64-unknown-linux-gnu
/usr/lib
/usr/lib64
/usr/lib/x86_64-linux-gnu

[boris-kolpackov]

@MaskRay Thanks for the clarification.

I guess what is still unclear to me is in which directories Clang searches for a library that is specified as, say, -lfoo on the command line when -stdlib=libc++ is specified. Is the list the same as printed with just -print-search-dirs or as --gcc-triple= -print-search-dirs? I think the answer is the former (so that libatomic.a, etc., can be found) but if you could confirm my understand, that would be great. Thanks!

[MaskRay]

I have an alias fclang to /tmp/Rel/bin/clang.

% COMPILER_PATH=/tmp/c fclang -B/tmp --gcc-toolchain=donotexist -print-search-dirs
programs: =/tmp:/tmp/c:/tmp/Rel/bin
libraries: =/tmp/Rel/lib/clang/19:/tmp/Rel/bin/../lib/x86_64-unknown-linux-gnu:/tmp/Rel/lib/clang/19/lib/x86_64-unknown-linux-gnu:/lib/x86_64-linux-gnu:/lib/../lib64:/usr/lib/x86_64-linux-gnu:/usr/lib/../lib64:/lib:/usr/lib

programs includes -B, COMPILER_PATH, and the installation directory.
libraries includes the resource directory (fa9771f, to support --print-file-name=include), the libc++/libc++abi/libunwind lib directory, sanitizer/builtins/crt/etc lib directory, GCC lib dir, system lib dirs