[Bug libstdc++/108331] [13 Regression] ABI break of std::__c_file and std::fstream for win32 thread model of GCC for windows

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108331

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> --- a/libstdc++-v3/config/io/c_io_stdio.h
> +++ b/libstdc++-v3/config/io/c_io_stdio.h
> @@ -39,7 +39,17 @@ namespace std _GLIBCXX_VISIBILITY(default)
>  {
>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
> +#ifdef __GTHREAD_WIN32
> +  // The layout of __gthread_mutex_t changed in GCC 13, but libstdc++
> doesn't
> +  // actually use the basic_filebuf::_M_lock member, so define it
> consistently
> +  // with the old __gthread_mutex_t to avoid an unnecessary layout change:
> +  struct __c_lock {
> +    long counter;
> +    void *sema;

We should use reserved names here, like __unused1 and __unused2.

> +  };
> +#else
>    typedef __gthread_mutex_t __c_lock;
> +#endif
>  
>    // for basic_file.h
>    typedef FILE __c_file;