1*4d6fc14bSjoerg 2*4d6fc14bSjoerg==================== 3*4d6fc14bSjoergLibc++ ABI stability 4*4d6fc14bSjoerg==================== 5*4d6fc14bSjoerg 6*4d6fc14bSjoergLibc++ aims to preserve stable ABI to avoid subtle bugs when code built to the old ABI 7*4d6fc14bSjoergis linked with the code build to the new ABI. At the same time, libc++ allows ABI-breaking 8*4d6fc14bSjoergimprovements and bugfixes for the scenarios when ABI change is not a issue. 9*4d6fc14bSjoerg 10*4d6fc14bSjoergTo support both cases, libc++ allows specifying the ABI version at the 11*4d6fc14bSjoergbuild time. The version is defined with a cmake option 12*4d6fc14bSjoergLIBCXX_ABI_VERSION. Another option LIBCXX_ABI_UNSTABLE can be used to 13*4d6fc14bSjoerginclude all present ABI breaking features. These options translate 14*4d6fc14bSjoerginto C++ macro definitions _LIBCPP_ABI_VERSION, _LIBCPP_ABI_UNSTABLE. 15*4d6fc14bSjoerg 16*4d6fc14bSjoergAny ABI-changing feature is placed under it's own macro, _LIBCPP_ABI_XXX, which is enabled 17*4d6fc14bSjoergbased on the value of _LIBCPP_ABI_VERSION. _LIBCPP_ABI_UNSTABLE, if set, enables all features at once. 18