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preliminary libstdc++ ABI docs


Mostly incomplete. Posted in a plea for help only. At some point, this
information should be posted on the main web page, say right after
configure options.

It needs a lot of work: however, I've been thinking about this for a
long time and have not really gotten too far. People who would like to
write bits of this, or fill in the outline below, are encouraged to
help.

-benjamin


-----

2002-07-01 Benjamin Kosnik

Description of the libstdc++ ABI.

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

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

- scope of document, of use to system integrators.

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

- library ABI, compiler ABI different (but effects)

- 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. The GNU
  Project recommends that

- shared library only, static is immutable.

- 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?
  There is only one: --enable-symvers. For more information see:
  http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html

  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. ABI changes

- (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) adding or deleting an exported symbol


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

- 'make check-abi'??

- other ABI checkers


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 n "C" to use the C ABI.


V. References

ABIcheck, a vauge 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


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