[Bug libstdc++/70503] New: extern "C" functions in src/c++11/thread.cc are global in libstdc++.a

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70503

            Bug ID: 70503
           Summary: extern "C" functions in src/c++11/thread.cc are global
                    in libstdc++.a
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

In src/c++11/thread.cc we have:

namespace std
{
  namespace
  {
    extern "C" void*
    execute_native_thread_routine(void*)
    {
      // ...
    }

#if _GLIBCXX_THREAD_ABI_COMPAT
    extern "C" void*
    execute_native_thread_routine_compat(void* __p)
    {
      // ...
    }
#endif
  }
  // ...

I was under the mistaken impression that the unnamed namespace meant those
functions got internal linkage, but as PR 70476 points out, that's not true.

This means the functions are global symbols in the static library (they're not
exported from the shared library because the linker script makes everything
internal by default and only selected symbols are exported).

The following program should fail to link, but with -static it finds the
function in libstdc++.a and so crashes:

#include <iostream>

extern "C"
long execute_native_thread_routine(long i, long j);

int main()
{
  std::cout << execute_native_thread_routine(0, 0);
}

Variations can be produced with different link order that will produce a
multiple definition error, which happens with the RHEL devtoolset.

This bug has been present for some time, but is made worse in GCC 6 because I
renamed execute_native_thread_routine to execute_native_thread_routine_compat
and added a different function called execute_native_thread_routine. This means
we now have an additional global function in libstdc++.a, and the behaviour of
the original one changed.