Upgrading from gcc-4.x to gcc-5.x

Critical news
The following (GLEP:42) message has been sent to inform users about the important changes.

Title: GCC 5 Defaults to the New C++11 ABI Author: Mike Frysinger  Content-Type: text/plain Posted: 2015-10-22 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 that include lines similar to: ...: 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 (from gentoolkit). For gentoolkit-0.3.1 or higher: For previous versions of gentoolkit: 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/
 * 1) revdep-rebuild --library 'libstdc++.so.6' -- --exclude gcc
 * 1) revdep-rebuild --library 'libstdc\+\+\.so\.6' -- --exclude gcc

C++ changes
In order to be compliant with the C++11 standard, a number of standard template library (STL) types needed to be changed (specifically std::string and std::list in libstdc++). For backwards compatibility the SONAME of libstdc++.so was not bumped, instead, inline namespaces in combination with ABI tags are used now. These necessitate a recompilation of all C++ code, otherwise code crossing file interface boundaries could fail with

undefined references to std::__cxx11
It is not a bug, if packages fail with undefined references to std::__cxx11 or [abi:cxx11] like

cmGlobalGenerator.cxx:(.text+0x12781): undefined reference to `Json::Value::Value(std::__cxx11::basic_string const&)'

It means you need to rebuild the package that provided that symbol first. See https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html for more info.

Instructions by the GNU project

 * https://gcc.gnu.org/gcc-5/porting_to.html Porting to GCC 5

C changes
A significant change of GCC 5 was the move from  to   as default C standard. Many packages in the tree have implicitly relied on the C standard being  and now fail with different types of errors:

Missing symbol ( emits an externally visible definition,   does not): x86_64-pc-linux-gnu-gcc -Wl,-O1 -Wl,--as-needed background.o list.o main.o notes.o options.o savegeom.o -lpangoxft-1.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0 -lfontconfig -lfreetype -lXft -lXrender -lX11 -lXrandr -lXext -o xnots main.o: In function `processConfigureNotify.part.3': main.c:(.text+0x133): undefined reference to `resetNoteWidth' savegeom.o: In function `getGeometryFromList':

Redefinition (permitted by ): argp-eexst.c:(.text+0x0): multiple definition of `argp_usage' ../../lib/libmisc.a(argp-help.o):argp-help.c:(.text+0x3590): first defined here ../../lib/libmisc.a(argp-eexst.o): In function `_option_is_short':

The simplest fix for both cases is restoring pre-GCC 5 inline semantics, i.e., inheriting  and adding   to the CFLAGS variable:

This will lead to the same C language behavior as with GCC 4.9 and below.

Bugzilla entries about gcc-5.3

 * https://bugs.gentoo.org/show_bug.cgi?id=536984 the Tracker bug (gcc-5) GCC 5 porting
 * https://bugs.gentoo.org/buglist.cgi?quicksearch=gcc-5.3&list_id=3005290 Open Bugzilla entries regarding gcc-5.3