[Bug libstdc++/94987] New: Missing vtable for std::__future_base::_State_base on libstdc++.a

[Bug libstdc++/94987] New: Missing vtable for std::__future_base::_State_base on libstdc++.a

[kndevl at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94987

            Bug ID: 94987
           Summary: Missing vtable for std::__future_base::_State_base on
                    libstdc++.a
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kndevl at outlook dot com
  Target Milestone: ---

# Ubuntu 18.04 has gcc 5.5.0

$ nm -C `g++ --print-file-name libstdc++.a` | grep 'vtable for
std::__future_base::_State_base'

< empty output >

------------------------------------------------------------
$ nm -CD `g++ --print-file-name libstdc++.so` | grep 'vtable for
std::__future_base::_State_base'

00000000003d3e58 V vtable for std::__future_base::_State_base
------------------------------------------------------------

# Ubuntu 14.04 has gcc 4.8.4

$ nm -C `g++ --print-file-name libstdc++.a` | grep 'vtable for
std::__future_base::_State_base'

nm: compatibility-debug_list-2.o: no symbols
0000000000000000 V vtable for std::__future_base::_State_base

------------------------------------------------------------

$ nm -CD `g++ --print-file-name libstdc++.so` | grep 'vtable for
std::__future_base::_State_base'

00000000002eb880 V vtable for std::__future_base::_State_base

------------------------------------------------------------

I built a static library on 4.8.4 and tried using it on 5.5.0 with static
libstdc++. This library depends on the missing symbol and does not compile on
newer libstdc++.a. I tried this on gcc 9.3.0 too and the libstdc++.a does not
have this symbol. Dynamic linking with libstdc++.so works fine.

I keep track of breaking ABI changes from
https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html and I could not find
anything related to this. Is this a breaking change? If so, where do I keep
myself updated on these?

[Bug libstdc++/94987] Missing vtable for std::__future_base::_State_base on libstdc++.a

[kndevl at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94987

--- Comment #1 from Karthik Nishanth <kndevl at outlook dot com> ---
gcc 5.5.0

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.5.0-12ubuntu1~16.04' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.5.0 20171010 (Ubuntu 5.5.0-12ubuntu1~16.04)

------------------------------------------------------------------------------

gcc 4.8.4

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
4.8.4-2ubuntu1~14.04.4' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.4)

------------------------------------------------------------------------------

gcc 9.3.0

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/9.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-pkgversion='Arch Linux 9.3.0-1'
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-shared
--enable-threads=posix --with-system-zlib --with-isl --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--disable-libssp --enable-gnu-unique-object --enable-linker-build-id
--enable-lto --enable-plugin --enable-install-libiberty
--with-linker-hash-style=gnu --enable-gnu-indirect-function --enable-multilib
--disable-werror --enable-checking=release --enable-default-pie
--enable-default-ssp --enable-cet=auto gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
gcc version 9.3.0 (Arch Linux 9.3.0-1)

[Bug libstdc++/94987] Missing vtable for std::__future_base::_State_base on libstdc++.a

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94987

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C++11 support in GCC 4.8 is experimental and there is no guarantee of
compatibility with other versions. In particular, you cannot mix code that uses
C++11 built with 4.8 and 5 (or anything later than 5). See
https://stackoverflow.com/questions/46746878/is-it-safe-to-link-c17-c14-and-c11-objects/49119902#49119902
for more details.

The reason there is no _State_base in libstdc++.a is that the type was replaced
with _State_baseV2. The old type is still exported from the shared library,
because we can't remove symbols from that, but it's not in the static library
because _State_base is not declared in the headers for any GCC version with
non-experimental C++11 support.

[Bug libstdc++/94987] Missing vtable for std::__future_base::_State_base on libstdc++.a

[kndevl at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94987

--- Comment #3 from Karthik Nishanth <kndevl at outlook dot com> ---
Thank you, I understand now.

Is the reason why gcc preserves the symbol in the SO because of forward
compatibility with existing binaries dynamically depending on the symbol?

[Bug libstdc++/94987] Missing vtable for std::__future_base::_State_base on libstdc++.a

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94987

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, removing any symbols from the DSO is forbidden.