Gentoo Archives: gentoo-dev

From: Mike Frysinger <vapier@g.o>
To: gentoo-dev@l.g.o
Subject: Re: [gentoo-dev] gcc-5 news item wrt C++ ABI
Date: Tue, 06 Oct 2015 04:46:57
Message-Id: 20151006044647.GI6518@vapier.lan
In Reply to: Re: [gentoo-dev] gcc-5 news item wrt C++ ABI by "Paweł Hajdan
1 On 05 Oct 2015 20:45, Paweł Hajdan, Jr. wrote:
2 > On 10/3/15 4:13 AM, Mike Frysinger wrote:
3 > > Title: GCC 5 Defaults to the New C++11 ABI
4 > > Author: Mike Frysinger <vapier@g.o>
5 > > Content-Type: text/plain
6 > > Posted: 2015-10-02
7 > > Revision: 1
8 > > News-Item-Format: 1.0
9 > > Display-If-Installed: >=sys-devel/gcc-5
10 > >
11 > > GCC 5 uses the new C++ ABI by default. When building new code, you might run
12 > > into link time errors like:
13 > > ...: undefined reference to '_ZNSt6chrono12steady_clock3nowEv@GLIBCXX_3.4.17'
14 > > Or you might see linkage failures with "std::__cxx11::string" in the output.
15 > >
16 > > These are signs that you need to rebuild packages using the new C++ ABI.
17 > > You can quickly do so by using revdep-rebuild like so:
18 > > # revdep-rebuild --library 'libstdc\+\+\.so\.6'
19 > >
20 > > For more details, feel free to peruse:
21 > > https://developerblog.redhat.com/2015/02/05/gcc5-and-the-c11-abi/
22 > > https://blogs.gentoo.org/blueness/2015/03/10/the-c11-abi-incompatibility-problem-in-gentoo/
23 >
24 > Curious, can one reasonably easy downgrade from GCC 5 back to GCC 4?
25 >
26 > Are there any instructions for that? Is it sufficient to do a similar
27 > revdep-rebuild command?
28 >
29 > In case the downgrade would horribly break systems or be otherwise
30 > highly nontrivial, I'd recommend some note for that.
31
32 it's like any other attempt to downgrade an ABI: you're on your own.
33 in Gentoo, we guarantee backwards compatibility, not forward -- this
34 is not specific to GCC.
35
36 in practice, as long as you don't unmerge gcc-5, you can switch the
37 active compiler to gcc-4.9 and use it, but the runtime libs will be
38 taken from the latest one you have installed (gcc-5).
39 -mike

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