On Sat, 12 Apr 2025, Jeremy Drake via Cygwin-apps wrote:
> On Sun, 13 Apr 2025, Takashi Yano via Cygwin-apps wrote:
>
> > > Yes. However, see my previous mail for the strange issue I ran into
> > > trying to do this (strange because it works fine on MSYS2 but fails on
> > > Cygwin)
> >
> > The same happens for me as well...
>
> Maybe you'll have some luck debugging it. I really don't use Cygwin (the
> software distribution), but rather MSYS2 (and indirectly Cygwin the DLL as
> msys2-runtime). The fact that this doesn't happen on MSYS2 is very
> strange and probably a good start to trying to narrow it down.
OK, here's my theory:
#12 0x00007ffda3510f9a in free (
p=0x100557010 <std::basic_string<char, std::char_traits<char>,
std::allocator<char> >::_Rep::_S_empty_rep_storage>)
p is in the address space of the exe (0x00000001004e210e) , not
cygstdc++-6 (0x00000003fb09c960). So it seems that it inlined the
_S_empty_rep_storage[] when it shouldn't have. Looking at
/usr/lib/gcc/13.3.0/include/c++/bits/basic_string.tcc (on MSYS2):
// Inhibit implicit instantiations for required instantiations,
// which are defined via explicit instantiations elsewhere.
#if _GLIBCXX_EXTERN_TEMPLATE
// The explicit instantiation definitions in src/c++11/string-inst.cc and
// src/c++17/string-inst.cc only instantiate the members required for C++17
// and earlier standards (so not C++20's starts_with and ends_with).
// Suppress the explicit instantiation declarations for C++20, so C++20
// code will implicitly instantiate std::string and std::wstring as needed.
# if __cplusplus <= 201703L && _GLIBCXX_EXTERN_TEMPLATE > 0
extern template class basic_string<char>;
# elif ! _GLIBCXX_USE_CXX11_ABI
// Still need to prevent implicit instantiation of the COW empty rep,
// to ensure the definition in libstdc++.so is unique (PR 86138).
extern template basic_string<char>::size_type
basic_string<char>::_Rep::_S_empty_rep_storage[];
# elif _GLIBCXX_EXTERN_TEMPLATE > 0
// Export _M_replace_cold even for C++20.
extern template void
basic_string<char>::_M_replace_cold(char *, size_type, const char*,
const size_type, const size_type);
# endif
You could check what some of these defines are on Cygwin, but I'm guessing
https://cygwin.com/cgit/cygwin-packages/gcc/tree/gcc.cygport?h=main&id=3b18f0b66c716fd70492f55ebebd5680a611739c#n171
(--with-default-libstdcxx-abi=gcc4-compatible). MSYS2 does not do this,
it's small enough that we can plausibly rebuild every MSYS2-packages
package that links against msys-stdc++-6.dll for an ABI break. Cygwin
doesn't have that luxury, since it could be that 3rd parties are shipping
Cygwin binaries.
