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2002-10-14 Benjamin Kosnik

Description of the libstdc++ ABI.

I. What is an ABI? What's covered? What's not?

- scope of document, of use to system integrators.

- What's the deal with C++? Why can't different compiler's object
  files link with each other? Bug? Feature?

- compilation includes and linked library binary must match up..

- shared library only, static is immutable.

- What's an ABI?

- library ABI, compiler ABI different issues, (but related)

- GNU C++ does not have a compiler command line option to switch
  between various different C++ ABIs. For instance, there is no way to
  switch between the gcc-3.0.x ABI, gcc-3.1.x ABI, and the gcc-3.2.x
  ABI during compilation. Other C++ compilers do allow this, and some
  g++ command line options may change the ABI (-fno-exceptions, see
  the complete list), but there is no version switch. Sorry. 

  To use a specific C++ABI, one must use the corresponding GNU C++
  toolchain. 

- How can this complexity be managed? What does C++ versioning mean?
  Because library and compiler changes often make binaries compiled
  with one version of the GNU tools incompatible with binaries
  compiled with other (either newer or older) versions of the same GNU
  tools, specific techniques are used to make managing this complexity
  easier.

  The following techniques are used:

  - Release versioning on the libgcc_s.so binary.

    It is versioned as follows:
    gcc-3.0.0: libgcc_s.so.1
    gcc-3.0.1: libgcc_s.so.1
    gcc-3.0.2: libgcc_s.so.1
    gcc-3.0.3: libgcc_s.so.1
    gcc-3.0.4: libgcc_s.so.1
    gcc-3.1.0: libgcc_s.so.1
    gcc-3.1.1: libgcc_s.so.1
    gcc-3.2.0: libgcc_s.so.1


  - Release versioning on the libstdc++.so binary.

    It is versioned as follows:
    gcc-3.0.0: libstdc++.so.3.0.0
    gcc-3.0.1: libstdc++.so.3.0.1
    gcc-3.0.2: libstdc++.so.3.0.2
    gcc-3.0.3: libstdc++.so.3.0.2 (Error, should be libstdc++.so.3.0.3)
    gcc-3.0.4: libstdc++.so.3.0.4
    gcc-3.1.0: libstdc++.so.4.0.0
    gcc-3.1.1: libstdc++.so.4.0.1
    gcc-3.2.0: libstdc++.so.5.0.0


  - Symbol versioning on the libgcc_s.so binary.
  
    file: gcc/libgcc-std.ver

    It is versioned as follows:
    gcc-3.0.0: GCC_3.0
    gcc-3.0.1: GCC_3.0
    gcc-3.0.2: GCC_3.0
    gcc-3.0.3: GCC_3.0
    gcc-3.0.4: GCC_3.0
    gcc-3.1.0: GCC_3.0
    gcc-3.1.1: GCC_3.0
    gcc-3.2.0: GCC_3.0


  - Symbol versioning on the libstdc++.so binary.
  
    It is versioned as follows:
    gcc-3.0.0: (Error, unversioned)
    gcc-3.0.1: (Error, unversioned)
    gcc-3.0.2: (Error, unversioned)
    gcc-3.0.3: (Error, unversioned)
    gcc-3.0.4: (Error, unversioned)
    gcc-3.1.0: GLIBCPP_3.1, CXXABI_1
    gcc-3.1.1: GLIBCPP_3.1, CXXABI_1
    gcc-3.2.0: GLIBCPP_3.2, CXXABI_1.2
    
    file: libstdc++-v3/config/linker-map.gnu
  

  - Incremental bumping of a compiler pre-defined macro,
    __GXX_ABI_VERSION. This macro is defined as the version of the
    compiler v3 ABI, with g++ 3.0.x being version 100. This macro will
    be automatically defined whenever g++ is used (the curious can
    test this by invoking g++ with the '-v' flag.)
    
    This macro is defined in the file "lang-specs.h" in the gcc/cp directory.
    Later versions define it in "c-common.c" in the gcc directory.

    It is versioned as follows:
    gcc-3.0.x: 100
    gcc-3.1.x: 100 (Error, should be 101)
    gcc-3.2.x: 102


  - Incremental bumping of a library pre-defined macro,
    __GLIBCPP__. This macro is defined as the date the library was
    released, in compressed ISO date format, as an unsigned long.

    This macro is defined in the file "c++config" in the
    "libstdc++-v3/include/bits" directory and is changed every night
    by an automated script.

    It is versioned as follows:
    gcc-3.0.0: 20010615
    gcc-3.0.1: 20010819
    gcc-3.0.2: 20011023
    gcc-3.0.3: 20011220
    gcc-3.0.4: 20020220
    gcc-3.1.0: 20020514
    gcc-3.1.1: 20020725
    gcc-3.2.0: 20020814


  - Incremental bumping of a library pre-defined macro,
    _GLIBCPP_VERSION. This macro is defined as the released version of
    the library, as a string literal. This is only implemented in
    gcc-3.1.0 releases and higher.

    This macro is defined in the file "c++config" in the
    "libstdc++-v3/include/bits" directory and is generated
    automatically by autoconf as part of the configure-time generation
    of config.h.

    It is versioned as follows:
    gcc-3.0.0: "3.0.0"
    gcc-3.0.1: "3.0.0" (Error, should be "3.0.1")
    gcc-3.0.2: "3.0.0" (Error, should be "3.0.2")
    gcc-3.0.3: "3.0.0" (Error, should be "3.0.3")
    gcc-3.0.4: "3.0.0" (Error, should be "3.0.4")
    gcc-3.1.0: "3.1.0"
    gcc-3.1.1: "3.1.1"
    gcc-3.2.0: "3.2"


  - Matching each specific C++ compiler release to a specific set of
    C++ include files. This is only implemented in gcc-3.1.1 releases
    and higher.

    All C++ includes are installed in include/c++, then nest in a
    directory hierarchy corresponding to the C++ compiler's released
    version. This version corresponds to the variable "gcc_version" in
    "libstdc++-v3/acinclude.m4," and more details can be found in that
    file's macro GLIBCPP_CONFIGURE.

    C++ includes are versioned as follows:
    gcc-3.0.0: include/g++-v3
    gcc-3.0.1: include/g++-v3
    gcc-3.0.2: include/g++-v3
    gcc-3.0.3: include/g++-v3
    gcc-3.0.4: include/g++-v3
    gcc-3.1.0: include/g++-v3
    gcc-3.1.1: include/c++/3.1.1
    gcc-3.2.0: include/c++/3.2

  Taken together, these techniques can accurately specify interface
  and implementation changes in the GNU C++ tools themselves. Used
  properly, they allow both the GNU C++ tools implementation, and
  programs using them, an evolving yet controlled development that
  maintains backward compatibility.

- Minimum environment that supports a versioned ABI: what's needed?  A
  supported dynamic linker, a GNU linker of sufficient vintage to
  understand demangled C++ name globbing (ld), a shared executable
  compiled with g++, and shared libraries (libgcc_s, libstdc++-v3)
  compiled by a compiler (g++) with a compatible ABI. Phew.

  On top of all that, an additional constraint: libstdc++ did not
  attempt to version symbols (or age gracefully, really) until version
  3.1.0. 

  Most modern Linux and BSD versions, particularly ones using
  gcc-3.1.x tools, will meet the requirements above.

- What configure options impact symbol versioning?

  It turns out that most of the configure options that change default
  behavior will impact the mangled names of exported symbols, and thus
  impact versioning and compatibility.

  For more information on configure options, including ABI impacts, see:
  http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html

  There is one flag that explicitly deals with symbol versioning:
  --enable-symvers. 

  In particular, libstdc++-v3/acinclude.m4 has a macro called
  GLIBCPP_ENABLE_SYMVERS that defaults to yes (or the argument passed
  in via --enable-symvers=foo). At that point, the macro attempts to
  make sure that all the requirement for symbol versioning are in
  place. For more information, please consult acinclude.m4. 

- How can I tell if symbol versioning is, indeed, active? 

  When the GNU C++ library is being built with symbol versioning on,
  you should see the following at configure time for libstdc++-v3:

  checking versioning on shared library symbols... gnu

  If you don't see this line in the configure output, or if this line
  appears but the last word is 'no', then you are out of luck.

  If the compiler is pre-installed, a quick way to test is to compile
  the following (or any) simple C++ file:

#include <iostream>

int main()
{ std::cout << "hello" << std::endl; return 0; }

%g++ hello.cc -o hello.out
%nm hello.out

If you see symbols in the resulting output with "GLIBCPP_3.x" as part
of the name, then the executable is versioned. Here's an example:

         U _ZNSt8ios_base4InitC1Ev@@GLIBCPP_3.1


II. Library ABI changes

The following will cause the library major version number to
increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.4.0.0".

- (anything) changing in the gcc/g++ compiler ABI

- (anything) changing size of an exported symbol

- (anything) changing alignment of an exported symbol

- (anything) changing the layout of an exported symbol

- (anything) changing mangling on an exported symbol

- (anything) deleting an exported symbol

- (anything) changing the size, alignment, or layout of types
  specified in the C++ standard. These may not necessarily be
  instantiated or otherwise exported in the library binary, and
  include all the required locale facets, as well as things like
  std::basic_streambuf, et al.

Note: adding an exported symbol, if it's in a new and dependent
interface name, is ok.

The following will cause the library revision version number to
increase, say from "libstdc++.so.5.0.0" to "libstdc++.so.5.0.1".

- any release of the gcc toolchain.


III. Versioning

- include files

  - versioning headers with version, why necessary
    (need to control member/non-member functions, add delete files)

- shared library binaries

  - release versions

  - libtool versions

  - when does so version get a bump? what are the options?

  - how is the link map used?  

  - in an non-abi breaking minor release, how are symbols added?
    removed?

  - in an abi-breaking major release, what happens? symbol fall back


IV. Testing ABI changes

Testing for GNU C++ ABI changes is composed of two distinct areas:
testing the C++ compiler (g++) for compiler changes, and testing the
C++ library (libstdc++) for library changes.

Testing the C++ compiler ABI can be done various ways.

One. 
Intel ABI checker. More information can be obtained
<a href="http://developer.intel.com/software/products/opensource/">here.</a>

Two.
The second is yet unreleased, but has been announced on the gcc
mailing list. It is yet unspecified if these tools will be freely
available, and able to be included in a GNU project. Please contact
Mark Mitchell (mark@codesourcery.com) for more details, and current
status.

Three.
Involves using the vlad.consistency test framework. This has also been
discussed on the gcc mailing lists.

Testing the C++ library ABI can also be done various ways.

One. 
(Brendan Kehoe, Jeff Law suggestion to run 'make check-c++' two ways, 
one with a new compiler and an old library, and the other with an old
compiler and a new library, and look for testsuite regressions)

Details on how to set this kind of test up can be found here:
http://gcc.gnu.org/ml/gcc/2002-08/msg00142.html

Two.  
Use the 'make check-abi' rule in the libstdc++-v3 Makefile. 

This is a proactive check the library ABI. Currently, exported symbol
names that are either weak or defined are checked against a last known
good baseline. Currently, this baseline is keyed off of 3.2.0
binaries, as this was the last time the .so number was incremented. In
addition, all exported names are demangled, and the exported objects
are checked to make sure they are the same size as the same object in
the baseline.

This dataset is insufficient, yet a start. Also needed is a
comprehensive check for all user-visible types part of the standard
library for sizeof() and alignof() changes. 

Verifying compatible layouts of objects is not even attempted.  It
should be possible to use sizeof, alignof, and offsetof to compute
offsets for each structure and type in the standard library, saving to
another datafile. Then, compute this in a similar way for new
binaries, and look for differences.

Another approach might be to use the -fdump-class-hierarchy flag to
get information. However, currently this approach gives insufficient
data for use in library testing, as class data members, their offsets,
and other detailed data is not displayed with this flag.
(See g++/7470 on how this was used to find bugs.)

Perhaps there are other C++ ABI checkers. If so, please notify
us. We'd like to know about them!


V. Issues not directly addressed, and possible suggestions

- what to do about multi-ABI systems (nathan scenario)?

  - compatibility libs

  --enable-version-specific-runtime-libs

  - Alexandre Oliva proposal to have extended name attributes, modify ld
 
  - directory-level versioning

- wrapping C++ API's in "C" to use the C ABI.


V. References

ABIcheck, a vague idea of checking ABI compatibility
http://abicheck.sourceforge.net/

C++ ABI reference
http://www.codesourcery.com/cxx-abi/

Intel ABI documentation
"Intel® Compilers for Linux* -Compatibility with the GNU Compilers"
(included in icc 6.0)

Sun Solaris 2.9 docs
Linker and Libraries Guide (document 816-1386)
C++ Migration Guide (document 816-2459)
http://docs.sun.com/db/prod/solaris.9
http://docs.sun.com/?p=/doc/816-1386&a=load

Ulrich Drepper, "ELF Symbol Versioning"
http://people.redhat.com/drepper/symbol-versioning