[libcxx-dev] [llvm-dev] Shipping custom libc++ on MacOS

Ken Cunningham via libcxx-dev libcxx-dev at lists.llvm.org
Fri Aug 6 06:34:57 PDT 2021

On Thu, Jul 22, 2021 at 2:08 PM Louis Dionne via libcxx-dev <
libcxx-dev at lists.llvm.org> wrote:

> On Jul 10, 2021, at 09:33, Isuru Fernando via libcxx-dev <
> libcxx-dev at lists.llvm.org> wrote:
> Hi Ken,
> I'm in the same situation as you in conda package manager.
> Note that with libcxx 12, https://reviews.llvm.org/D91517 was merged where
> codecvt with char8_t is added in the middle of a structure instead of at
> the end.
> This means that you have to be careful that all applications link to the
> custom libc++.
> For libcxx developers,
> While we are discussing how to avoid ODR violations, is it possible to
> move the two lines at
> https://github.com/llvm/llvm-project/blob/llvmorg-12.0.1/
> libcxx/src/locale.cpp#L210-L211
> to the bottom to avoid segfaults resulting from this?
> Generally speaking, we make the assumption that there's a single copy of
> libc++ in use inside a final linked image. Or if you want to use libc++ as
> an implementation detail and link it statically, ensure it doesn't export
> any symbols and has absolutely no ABI surface outside of the executable
> where it's being used. Chrome does that.
> Anything else is very brittle, and I'm reluctant to add workarounds that
> make it look like it works, because you may run into other issues that we
> won't be able to patch so easily.
> Louis
> _______________________________________________
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> libcxx-dev at lists.llvm.org
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I had thought libc++ was more immune to this kind of problem given Apple's
usual support to deploy on older systems.

If the libc++ structures have changed and are now incompatible with
previous versions of libc++, would that now mean that an Apple build system
using the new libc++ structures can no longer target a deployment target
with an older libc++ (any system prior to the change)?

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