[PATCH v2] libstdc++: Search for tzdata on Windows (msys)

[Björn Schäpers]

Am 08.07.2025 um 12:01 schrieb Jonathan Wakely:
> On Mon, 7 Jul 2025 at 23:53, Björn Schäpers <gcc@hazardy.de> wrote:
>>
>> From: Björn Schäpers <bjoern@hazardy.de>
>>
>> Windows does not provide a tzdata.zi, but msys does. Use this, if
>> available, instead of the embedded (and possibly outdated) database.
>>
>> libstdc++-v3/Changelog:
>>
>>          Use msys provided time zone information.
>>
>>          * src/c++20/tzdb.cc (zoneinfo_file): On Windows look relative
>>          from the DLL path for the time zone information.
>>
>> Signed-off-by: Björn Schäpers <bjoern@hazardy.de>
>> ---
>>   libstdc++-v3/src/c++20/tzdb.cc | 34 ++++++++++++++++++++++++++++++++++
>>   1 file changed, 34 insertions(+)
>>
>> diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
>> index 6e244dc656d..9923d14b7a7 100644
>> --- a/libstdc++-v3/src/c++20/tzdb.cc
>> +++ b/libstdc++-v3/src/c++20/tzdb.cc
>> @@ -44,6 +44,12 @@
>>   # include <cstdlib>   // getenv
>>   #endif
>>
>> +#if _GLIBCXX_HAVE_WINDOWS_H
>> +# define WIN32_LEAN_AND_MEAN
>> +# include <windows.h>
>> +# include <psapi.h>
>> +#endif
>> +
>>   #if defined __GTHREADS && ATOMIC_POINTER_LOCK_FREE == 2
>>   # define USE_ATOMIC_LIST_HEAD 1
>>   // TODO benchmark atomic<shared_ptr<>> vs mutex.
>> @@ -1144,6 +1150,34 @@ namespace std::chrono
>>   #ifdef _GLIBCXX_ZONEINFO_DIR
>>         else
>>          path = _GLIBCXX_ZONEINFO_DIR;
>> +#endif
>> +#ifdef _GLIBCXX_HAVE_WINDOWS_H
>> +      if (path.empty())
>> +       {
>> +         HMODULE dll_module;
>> +         if (GetModuleHandleExA(
>> +                 GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS
>> +                     | GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
>> +                 reinterpret_cast<const char *>(&zoneinfo_file), &dll_module))
> 
> Does this assume that libstdc++.dll is installed as part of the msys
> installation?
> 
> What if users build GCC themselves and install it elsewhere?
> 
> Is the correct solution for msys-based systems to build GCC with
> --with-libstdcxx-zoneinfo=/path/to/msys/tzdata.zi ?
> 

Yes the assumption is, that libstdc++-6.dll is installed in 
$msysPath/$msysSubsystem/bin and the time zones are in ../share/zoneinfo.

If supplied with --with-libstdcxx-zoneinfo=/path/to/msys/tzdata.zi path 
shouldn't be empty, but containing "/path/to/msys/tzdata.zi" and nothing changes 
with my patch.

The problem for msys packages is, that $msysPath is unknown, since the user can 
install msys anywhere. I tried using a long string constant and changing that in 
the DLL after installation, so it would remain a constant. But the string 
constant size is used to build the std::string and "/tzdata.zi" is appended to 
that. So I switched to use a runtime approach.

> 
>> +           {
>> +             char dll_path[MAX_PATH];
>> +             if (GetModuleFileNameA(dll_module, dll_path, MAX_PATH) != 0)
>> +               {
>> +                 string_view dll_path_view = dll_path;
>> +                 auto pos = dll_path_view.find_last_of('\\');
>> +                 dll_path_view = dll_path_view.substr(0, pos);
>> +                 if (dll_path_view.ends_with("\\bin"))
>> +                   {
>> +                     constexpr string_view remaining_path = "share\\zoneinfo";
>> +                     dll_path_view.remove_suffix(3); // Remove bin
>> +                     path.reserve(dll_path_view.size()
>> +                                  + remaining_path.size());
>> +                     path = dll_path_view;
>> +                     path += remaining_path;
>> +                   }
>> +               }
>> +           }
>> +       }
>>   #endif
>>         if (!path.empty())
>>          path.append(filename);
>> --
>> 2.50.0
>>
>