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