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diff --git a/doc/dev/cxx.rst b/doc/dev/cxx.rst new file mode 100644 index 000000000..e8ab9d4de --- /dev/null +++ b/doc/dev/cxx.rst @@ -0,0 +1,27 @@ +C++17 and libstdc++ ABI +======================= + +Ceph has switched over to C++17 in mimic. To build Ceph on old distros without +GCC-7, it is required to install GCC-7 from additionary repos. On RHEL/CentOS, +we are using devtoolset-7_ from SCLs_ for building Ceph. But devltoolset-7 is +always using the old ABI_ even if ``_GLIBCXX_USE_CXX11_ABI=1`` is defined. So, +on RHEL/CentOS, the old implementations of ``std::string`` and ``std::list`` +are still used. In other words, ``std::string`` is still copy-on-write, and +``std::list::size()`` is still O(n) on these distros. But on Ubuntu Xenial, +Ceph is built using the new ABI. So, because we are still using libstdc++ and +devtoolset for building packages on RHEL/CentOS, please do not rely on the +behavior of the new ABI or the old one. + +For those who argue that "GCC supports dual ABI!", here comes the long story. +The problem is in the system shared library and ``libstdc++_nonshared.a`` model. +If some symbol is exported from the system shared library, we must use that, and +cannot override it. Also, the dual ABI support requires several of the system +shared library symbols to behave differently (e.g. for locale facets, need +to register twice as many, one set for old ABI, another for new ABI). So, this +leaves us with no options but to stick with the old ABI, if we want to enable +the built binaries to run on old distros where only the libstdc++ with the old +ABI is available. + +.. _ABI: https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html +.. _devtoolset-7: https://www.softwarecollections.org/en/scls/rhscl/devtoolset-7/ +.. _SCLs: https://www.softwarecollections.org/ |