ceph/doc/dev/cxx.rst

   1 C++17 and libstdc++ ABI
   2 =======================
   3
   4 Ceph has switched over to C++17 in mimic. To build Ceph on old distros without
   5 GCC-7, it is required to install GCC-7 from additionary repos. On RHEL/CentOS,
   6 we are using devtoolset-7_ from SCLs_ for building Ceph. But devltoolset-7 is
   7 always using the old ABI_ even if ``_GLIBCXX_USE_CXX11_ABI=1`` is defined. So,
   8 on RHEL/CentOS, the old implementations of ``std::string`` and ``std::list``
   9 are still used. In other words, ``std::string`` is still copy-on-write, and
  10 ``std::list::size()`` is still O(n) on these distros. But on Ubuntu Xenial,
  11 Ceph is built using the new ABI. So, because we are still using libstdc++ and
  12 devtoolset for building packages on RHEL/CentOS, please do not rely on the
  13 behavior of the new ABI or the old one.
  14
  15 For those who argue that "GCC supports dual ABI!", here comes the long story.
  16 The problem is in the system shared library and ``libstdc++_nonshared.a`` model.
  17 If some symbol is exported from the system shared library, we must use that, and
  18 cannot override it. Also, the dual ABI support requires several of the system
  19 shared library symbols to behave differently (e.g. for locale facets, need
  20 to register twice as many, one set for old ABI, another for new ABI). So, this
  21 leaves us with no options but to stick with the old ABI, if we want to enable
  22 the built binaries to run on old distros where only the libstdc++ with the old
  23 ABI is available.
  24
  25 .. _ABI: https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html
  26 .. _devtoolset-7: https://www.softwarecollections.org/en/scls/rhscl/devtoolset-7/
  27 .. _SCLs: https://www.softwarecollections.org/