Adding upstream version 18.2.2.upstream/18.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 27 insertions, 0 deletions

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/