How much libc++ ABI changes FreeBSD can consume?
Konstantin Belousov
kostikbel at gmail.com
Fri Feb 21 12:36:46 UTC 2020
On Fri, Feb 21, 2020 at 01:03:25PM +0100, Joerg Sonnenberger wrote:
> On Fri, Feb 21, 2020 at 03:24:32AM -0600, Zhihao Yuan wrote:
> > 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?
>
> Symbol versioning helps really little for this kind of ABI breaks. It
> still ends up effectively being a flag day as libraries build before and
> after don't interact that well with each other.
It helps some, and from my undertanding, symver + properly used inline
namespaces cover much, if not everything.
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