How to create compitable libraries

Question

Hi,
We have some applications dependant on our libraries. All the applications are dynamically linked.(*.so).
Whenever there is a change in class variables,addition of new methods in or libraries all the applications must recompile.
Can we build libraries in such a way that they are always backword compitable?
CC: Sun C++ 5.8 Patch 121017-10 2007/02/21

Answer 1

(I'm assuming that by "change in class variables" you mean adding new variables; obviously, if you change variable's name or type, you simply must recompile or employ some more or less magical trickery)
Library versioning helps to keep your shared libraries backward compatible. More or less detailed description of internal versioning can be found in "Linker and Libraries' Guide" here: http://docs.sun.com/app/docs/doc/817-1984/chapter5-1?a=view
Basically, you define interface of your library (NB: for C++, mangled names must be used) and give this interface a name (version). When you add a symbol, you define another interface by extending first one and give it another name.
When your clients link with the library, they use latest interface; when you provide newer version of the shared library, clients still use the same interface, though it is not latest anymore. When they recompile, they again will use latest interface (this can be changed, it's not necessary to always link with latest version).
This scheme is implemented for C++ run-time support libraries, like libCstd.so and libstlport.so; use pvs to print shared library version information:
$ pvs -d /usr/lib/libCstd.so.1
        libCstd.so.1;
        SUNW_1.1.1;
        SUNW_1.1;
        SUNW_1.2;
        SUNW_1.3;
        SUNW_1.4;
        SUNW_1.4.1;As you can see, this libCstd.so library has 6 versions, latest being SUNW_1.4.1