Using Clang with built libstdc++ produces undefined symbol _ZSt15__once_callable - clang

Question

I have built libstdc++ with no modifications yet:
cd gccsrcdir/libstdc++-v3/build
../configure --prefix=$PWD/../install
make && make install
I am using Ubuntu 21.10 and I set the following environment variables:
export LIBRARY_PATH=gccsrcdir/libstdc++-v3/install/lib
export LD_LIBRARY_PATH=gccsrcdir/libstdc++-v3/install/lib
export CPLUS_INCLUDE_PATH=gccsrcdir/libstdc++-v3/install/include/c++/13.0.0
When I then use the system's GCC, I get no problems. When I use the system's Clang, it produces a symbol lookup error - even with no parameters:
clang++
clang++: symbol lookup error: /lib/x86_64-linux-gnu/libicuuc.so.67: undefined symbol: _ZSt15__once_callable, version GLIBCXX_3.4.11
In fact I only need to update LD_LIBRARY_PATH to arrive here. What am I doing wrong?

Answer 1

The symbol -- std::__once_callable is defined in your system libstdc++.so.6 (it has version GLIBCXX_3.4.11 in my build, which means it was added in GCC-4.4.0).
Your build of libstdc++.so.6 should define this symbol as well, but for some reason does not. That is a problem -- any binary which uses this symbol will fail at runtime when using your build of libstdc++.so.6 (which is happening because you've pointed LD_LIBRARY_PATH to it).
Note: in your case it's the clang++ binary that is failing to run -- any flags you add to it (such as -femulated-tls) are irrelevant -- they only affect the binary that would have been generated IF clang++ itself didn't fail.
I just repeated your configure && make steps, and the library built this way also doesn't define this symbol.
I then repeated the configure && make, but starting from top-level GCC directory, and libstdc++.so.6 built that way does define the symbol.
Conclusion: libstdc++ is configured differently during "normal" GCC build.
The definition comes from mutex.o, which is built from ./libstdc++-v3/src/c++11/mutex.cc, and which has this chunk of code:
#ifdef _GLIBCXX_HAS_GTHREADS
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
#ifdef _GLIBCXX_HAVE_TLS
__thread void* __once_callable;
__thread void (*__once_call)();
...
So it sounds like either _GLIBCXX_HAS_GTHREADS or _GLIBCXX_HAVE_TLS is not defined when doing configure && make in the libstdc++-v3 directly.
Digging further, I see that libstdc++-v3 determines _GLIBCXX_HAS_GTHREADS by trying to compile #include "gthr.h", and that file is available in libgcc/gthr.h, but not in "standard" installed GCC.
../libstdc++-v3/configure && grep _GLIBCXX_HAS_GTHREADS config.h
/* #undef _GLIBCXX_HAS_GTHREADS */
TL;DR: correctly configuring libstdc++.so is complicated, and you will be better off building complete GCC.
Once you have a complete build, you will have a libstdc++-v3 directory properly configured, and can just rebuilt in that directory:
grep _GLIBCXX_HAS_GTHREADS ./x86_64-pc-linux-gnu/libstdc++-v3/config.h
#define _GLIBCXX_HAS_GTHREADS 1