Re: [gentoo-dev] gcc-5 news item wrt C++ ABI

[Mike Frysinger <vapier@gentoo.org>]

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On 05 Oct 2015 20:45, Paweł Hajdan, Jr. wrote:
> On 10/3/15 4:13 AM, Mike Frysinger wrote:
> > Title: GCC 5 Defaults to the New C++11 ABI 
> > Author: Mike Frysinger <vapier@gentoo.org>
> > Content-Type: text/plain
> > Posted: 2015-10-02
> > Revision: 1
> > News-Item-Format: 1.0
> > Display-If-Installed: >=sys-devel/gcc-5
> > 
> > GCC 5 uses the new C++ ABI by default.  When building new code, you might run
> > into link time errors like:
> > ...: undefined reference to '_ZNSt6chrono12steady_clock3nowEv@GLIBCXX_3.4.17'
> > Or you might see linkage failures with "std::__cxx11::string" in the output.
> > 
> > These are signs that you need to rebuild packages using the new C++ ABI.
> > You can quickly do so by using revdep-rebuild like so:
> > # revdep-rebuild --library 'libstdc\+\+\.so\.6'
> > 
> > For more details, feel free to peruse:
> > https://developerblog.redhat.com/2015/02/05/gcc5-and-the-c11-abi/
> > https://blogs.gentoo.org/blueness/2015/03/10/the-c11-abi-incompatibility-problem-in-gentoo/
> 
> Curious, can one reasonably easy downgrade from GCC 5 back to GCC 4?
> 
> Are there any instructions for that? Is it sufficient to do a similar
> revdep-rebuild command?
> 
> In case the downgrade would horribly break systems or be otherwise
> highly nontrivial, I'd recommend some note for that.

it's like any other attempt to downgrade an ABI: you're on your own.
in Gentoo, we guarantee backwards compatibility, not forward -- this
is not specific to GCC.

in practice, as long as you don't unmerge gcc-5, you can switch the
active compiler to gcc-4.9 and use it, but the runtime libs will be
taken from the latest one you have installed (gcc-5).
-mike

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