Hi guys, This is a proposal for a way to handle backwards compatibility in the face of continuing C++ compiler ABI breaks. Rationale: - The GCC team have an explicit policy of preferring standards compliance to ABI stability. I think there is no 100% perfect test suite for this standard, and even if there was, it's unlikely that such a complicated document is never open to interpretation. Likewise the C++ spec itself evolves with time. Consequently, it's possible (probable?) that we will be dealing with ABI breaks into the medium term future. These need to be managed. - It is desireable to have binary compatibility and to manage interface breaks. This is what ELF soname versioning is all about, but in the face of compiler breaks as opposed to library breaks, soname versioning is not enough. A library libqt.so.3 compiled with one compiler may not export the same interface as libqt.so.3 compiled with another, yet the user may wish to run binaries compiled with the gcc 3.3 ABI on a 3.4 compiled system. One of my assumptions is that the C++ standard library is parallel installable and has a different soname for each ABI. From what I've read about g++ ABI policy this assumption seems to be correct, please let me know if it is not. The proposal is to modify the dynamic linker so that when a library which depends on a symbol version of CXXABI_X.Y is loaded, each element in the library search path is tested once with $dirname/cxxabi-X.Y and once with $dirname. So, if you have a version of Qt compiled with abi 1.2 (gcc 3.2,3.3), that can be placed in /usr/lib/cxxabi-1.2/libqt.so.3 and then the version compiled with the "native" ABI can be placed in /usr/lib as per normal. A similar technique has been used before with the LD_ASSUME_KERNEL environment variable switching between /lib, /lib/i686, /lib/tls and so on. This mechanism would be similar, except triggered by the CXXABI symbol version rather than an environment variable. This allows C++ binaries built for older versions of the system to operate correctly on newer systems when the right compat-* packages are installed. I propose this here as there are a good set of distro, glibc and gcc developers working for Red Hat, so it seemed like a good place to find people to shoot it down :) What are peoples thoughts on this mechanism? Using CXXABI_ is somewhat inelegant but this has the advantage that it works with the toolchains already deployed. thanks -mike
