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

The information in this article has been deprecated. It may or may not be relevant for contemporary usage. Handle with care!

This article deals with instructions for upgrading systems from GCC 4.x to GCC 5.x. It has been deprecated in favor of the more modern Upgrading GCC article.

Critical news[edit | edit source]

The following (GLEP 42) message has been sent to inform users about the important changes[1]. 

 Title: GCC 5 Defaults to the New C++11 ABI
 Author: Mike Frysinger <vapier@gentoo.org>
 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:
 # revdep-rebuild --library 'libstdc++.so.6' -- --exclude gcc
 
 For previous versions of gentoolkit:
 # revdep-rebuild --library 'libstdc\+\+\.so\.6' -- --exclude gcc
 
 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/

For Gentoo users[edit | edit source]

C++ changes[edit | edit source]

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[edit | edit source]

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<char, std::char_traits<char>, std::allocator<char> > 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.

For Gentoo developers[edit | edit source]

Instructions by the GNU project[edit | edit source]

C changes[edit | edit source]

A significant change of GCC 5 was the move from -std=gnu89 to -std=gnu11 as default C standard. Many packages in the tree have implicitly relied on the C standard being -std=gnu89 and now fail with different types of errors:

Missing symbol (-std=gnu89 inline emits an externally visible definition, -std=gnu11 inline 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 -std=gnu89 inline): 

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 flag-o-matic and adding -std=gnu89 to the CFLAGS variable:

CODE 
inherit flag-o-matic
 
src_prepare() {
	# fix bug 987654 by restoring pre-GCC5 inline semantics
	append-cflags -std=gnu89
	default
 }

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

Bugzilla entries about gcc-5.3[edit | edit source]

See also[edit | edit source]

References[edit | edit source]

  1. ↑ News Item GCC 5 Defaults to the New C++11 ABI
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