How much libc++ ABI changes FreeBSD can consume?

[Zhihao Yuan]

On Thu, Feb 20, 2020 at 8:17 AM Konstantin Belousov <kib at freebsd.org> wrote:

> >  3. Is MFC required for libc++ updates?  If so, how
> >     does that affect ABI changes?
> It is highly desirable to get libc++ synced between head and all actively
> supported stable versions.
>
> >  4. Is there any desire to make C++ ABI breakage
> >     smoother by ultilzing mechanisms such as
> >     Symbol.map?
> Yes.  More expanded answer below.
>
> Right now any libc++ ABI breakage requires dso version bump. We try hard
> to avoid that because it trivially leads to a situation when multiple
> libc++'s are loaded into same process, unless everything is recompiled
> against same lib. In other words, bumping version for such fundamental
> library is too troublesome.
>
> Symver provides a solution for gradual ABI changes, but by policy
> we never provide symbol versioning for third-party libraries unless
> upstream maintains the versioning. The reason is that we cannot enforce
> upstream ABI policy, which would make versioning broken by updates and
> then pointless.
>
> So for instance libstdc++.so from gcc is versioned, while ncurses are not.
>

To summarize what I heard, even if libc++
stabilizes V2 ABI, we do not want to do an
"ABI break since release 1X" thing.  If we
really upgrade, we break all stable versions.
And we hope/encourage libc++ to
version symbols like what libstdc++ does,
correct?

-- 
Zhihao Yuan, ID lichray
The best way to predict the future is to invent it.
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