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		The Debian GNU Compiler Collection Setup
		========================================

Abstract
--------

Debian uses a default version of GCC for most packages; however, some
packages require another version.  So, Debian allows several versions
of GCC to coexist on the same system, and selects the default version
by means of the gcc-defaults package, which creates symbolic links as
appropriate.

FIXME: Needs update for stretch

Versions of GCC present in Debian Stretch
-----------------------------------------

- GCC 5 is the default compiler for all frontends on all architectures
  except for Ada.

- GCC 4.9 is the default compiler for Ada.

- GCC 4.8 was removed for the release of Stretch.

- GCC 4.7 was removed for the release of Jessie.

- GCC 3.3 is not provided anymore; it is used to build libstdc++5 on
  the amd64 and i386 architectures. It is expected that libstdc++5 is
  not available anymore for the release following Lenny.

- GCC 2.95 and GCC 2.7.2.3 were removed for the release of Lenny.

How are the default compilers selected?
---------------------------------------

Starting in Debian 3.0, there is now a gcc-defaults package set. This
creates the actual packages for gcc, gnat, g++, gobjc,
gdc and gpc.  These packages will depend on the corresponding default
compiler for that architecture. For Debian 5.0 for example, "gcc"
depends on "gcc-5", which means that the "gcc-5" package will
install a binary called "gcc-5", which is symlinked to in the "gcc"
package as "gcc".

This may seem confusing, but what it allows you do to is install more
than one version of the GCC compiler collection at the same time,
making sure you are always using the one preferred for that
architecture. To use the other compiler, simply set CC=gcc-5, or
similar.

The default compiler versions for Debian GNU/Linux on amd64 are
(minor version numbers omitted):

	cpp		: cpp-9
	gcc		: gcc-9
	g++		: g++-9
	gfortran	: gfortran-9
	gccgo		: gccgo-10
	gobjc		: gobjc-9
	gobjc++		: gobjc++-9
	gdc		: gdc-10

Documentation for the default compilers can be found in

	/usr/share/doc/<compiler>-<package version>.

Thanks to gcc-defaults, each architecture can choose its own preferred
compiler for each language, and that preference can change without
requiring a complete rebuild of both compiler packages for all
architectures.

Practical implications
----------------------

The most important practical implications are in the merging/linking
of object files built with different compilers; If you use the 4.9
C compiler, you should use the gcc-4.9 compiler driver for all your
work.  When configuring sources, use

    CC=gcc-4.9 ./configure <configure options> 	# bash
    setenv CC gcc-4.9; ./configure <options>	# csh

When calling make, use make CC=gcc-4.9.

libstdc++ ABI
-------------

Starting with Debian 8.0 (stretch), gcc-5 and newer compiler versions
now provides a stable libcxx11 ABI, and stable support for C++11 (GCC
version before 5 called this supported experimental).  This required
some changes in the libstdc++ ABI, and now libstdc++6 provides a dual
ABI, the classic libcxx98 ABI, and the new libcxx11.  Many C++ using
packages were rebuilt; it is likely that third party applications will
need rebuilds too. See https://wiki.debian.org/GCC5.

C Application Binary Interface
------------------------------

Starting with Debian 4.0 (lenny), gcc-4.1 and newer compilers do
support the long double datatype with 128bit on the alpha, powerpc,
s390 and sparc architectures.  Libraries and applications using this
datatype have to be rebuilt using the compiler versions in Lenny
unless these depend on libc6/libc6.1 and libstdc++6, which still have
compatibility with a 64bit long double datatype.

gcc/g++/... are not handled using alternatives
----------------------------------------------

The symlinks in /usr/bin (gcc, g++, ...) are not handled using the Debian
alternative mechanism. There are differences in the architecture specific
ABI on some architectures and the C++ ABI differs as well. Having the
symlinks managed by alternatives doesn't allow reliable builds with the
same major/minor version of the compiler. To use another compiler version,
set the appropriate environment variables as described above in the section
"Practical implications".

C++ libraries
-------------

To use the libstdc++ library for debugging (found in the
libstd++6-<GCC version>-dbg package), add /usr/lib/debug to your
LD_LIBRARY_PATH. For gdb to display the source you need to get the
correspondig gcc-X.Y source package, unpack the source and point gdb
to the location of the source (dir directive).

C++ Application Binary Interface
--------------------------------

Sometimes, the C++ ABI of GCC changes.  It is impossible to link
object files that obey different ABIs into an executable.  When the
ABI changes, Debian provides a new version of libstdc++ with a new
soname.

Version 4 of the ABI was used by GCC 3.0 and 3.1; it is no longer
supported.

Version 5 of the ABI is common to GCC 3.2 and 3.3; GCC 3.3
provides libstdc++5. It is only supported as a runtime library.

Version 6 of the ABI is common to GCC 3.4 and later; GCC 4.3 provides
libstdc++6.

libstdc++6 in GCC 5 provides a dual ABI, see https://wiki.debian.org/GCC5.

Bugs
----

Before submitting a bug, please read README.Bugs in this directory.

Feedback appreciated
--------------------

Feedback about this document is appreciated; preferably as a Severity:
wishlist bug against Package: gcc . For general discussions and
questions, subscribe and/or email the debian-gcc@lists.debian.org mailing
list.

Maintainers of these packages
-----------------------------

Matthias Klose <doko@debian.org>
Ludovic Brenta <ludovic@ludovic-brenta.org>     (gnat)
Iain Buclaw <ibuclaw@ubuntu.com>                (gdc)
Aurelien Jarno <aurel32@debian.org>             (mips*-linux)
Aurelien Jarno <aurel32@debian.org>             (s390X*-linux)

Former and/or inactive maintainers of these packages
----------------------------------------------------

Falk Hueffner <falk@debian.org>                 (alpha-linux)
Ray Dassen <jdassen@debian.org>
Jeff Bailey <jbailey@nisa.net>                  (hurd-i386)
Joel Baker <fenton@debian.org>                  (netbsd-i386)
Randolph Chung <tausq@debian.org>               (ia64-linux)
Philip Blundell <pb@debian.org>                 (arm-linux)
Ben Collins <bcollins@debian.org>               (sparc-linux)
Dan Jacobowitz <dan@debian.org>                 (powerpc-linux)
Thiemo Seufer <ths@networkno.de>                (mips*-linux)
Matt Taggart <taggart@carmen.fc.hp.com>         (hppa-linux)
Gerhard Tonn <GerhardTonn@swol.de>              (s390-linux)
Roman Zippel <zippel@linux-m68k.org>            (m68k-linux)
Arthur Loiret <arthur.loiret@gmail.com>         (gdc)

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