Using <filesystem> on macOS 10.13 with clang++-mp-13

[Ken Cunningham]

This is an issue that will need solving, otherwise as filesystem is used
more (and it seems popular) all the systems prior to 10.15 will be left
behind.

Filesystem uses features in libc++.dylib that are only present in 10.15 and
newer. You are blocked from trying to use them on 10.14 and below by a
blocker in the clang "__config" header, that is easy to override, but when
you try to use filesystem, it will not find the libc++.dylib features and
will fail.

You can (sometimes) use macports-libcxx to get past this. This however
introduces two versions of libc++.dylib into the softare mixture, and this
sometimes causes errors due to incompatibilities between libc++ versions
(ODR errors). If you are taking this route, linking in the newer libc++.a
statically has a higher chance of working properly (that is what Chrome
does on older MacOS systems).

You can use gcc/libgcc, and newer versions of that have filesystem support,
but that causes it's own issues.

There was an early version of filesystem available as
<experimental/filesystem> with a linking static library on older versions
of clang. I forget when that disappeared, but maybe clang-8.0. It is
possible that this might be brought forward to newer clangs and made
available to older systems with modest effort. This is likely the most
robust approach going forward, as it is the same code (although older).

There are a few versions of filesystem available from other places, some
header-only, that might or might not work in a general sense. These have
the disadvantage of having nothing to do with clang/llvm, therefore I
presume the chances of unfavourable interactions would be somewhat higher.

Hope this is helpful,

Ken

On Sun, Jul 3, 2022 at 4:12 AM Mojca Miklavec <mojca at macports.org> wrote:

> Hi,
>
> I'm trying to build some sources that require <filesystem> on 10.13
> using clang 13 (I can try with clang 14, but I doubt that it helps).
>
> What's the correct way to compile the following minimal example? Do I
> need to link against a newer libc++ somehow?
>
> If it's not feasible to do that, I'll target a newer OS, I was just
> silently hoping to have some success ;)
>
> Thank you,
>     Mojca
>
> #include <cstdlib>
> #include <filesystem>
>
> int main() {
>     auto cwd = std::filesystem::current_path();
>     printf("%s", cwd.c_str());
>     return EXIT_SUCCESS;
> }
>
> > clang++-mp-13 test.cpp -o test
> test.cpp:5:16: error: no member named 'filesystem' in namespace 'std';
> did you mean 'std::__fs::filesystem'?
>     auto cwd = std::filesystem::current_path();
>                ^~~~~~~~~~~~~~~
>                std::__fs::filesystem
> /opt/local/libexec/llvm-13/bin/../include/c++/v1/filesystem:268:1:
> note: 'std::__fs::filesystem' declared here
> _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
> ^
> /opt/local/libexec/llvm-13/bin/../include/c++/v1/__config:816:58:
> note: expanded from macro '_LIBCPP_BEGIN_NAMESPACE_FILESYSTEM'
>   _LIBCPP_BEGIN_NAMESPACE_STD namespace __fs { namespace filesystem {
>                                                          ^
> 1 error generated.
>
> > clang++-mp-13 -std=c++17 test.cpp -o test
> Undefined symbols for architecture x86_64:
>   "std::__1::__fs::filesystem::__current_path(std::__1::error_code*)",
> referenced from:
>       std::__1::__fs::filesystem::current_path() in test-1b2bbf.o
> ld: symbol(s) not found for architecture x86_64
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.macports.org/pipermail/macports-dev/attachments/20220703/2b653f9f/attachment.htm>