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Should std::error_code::message respect the locale set by the user? #4711

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solarispika opened this issue Jun 4, 2024 · 4 comments
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@solarispika
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In our program, we use std::setlocale(LC_ALL, ".UTF8"); to support UTF-8 encoding, as noted in the Microsoft documentation. However, when certain C++ STL functions fail, their message() outputs are not in UTF-8.

After investigating the source code, I found that the error messages are always formatted according to the system's default locale, as shown in this STL implementation:

[[nodiscard]] size_t __CLRCALL_PURE_OR_STDCALL __std_system_error_allocate_message(
const unsigned long _Message_id, char** const _Ptr_str) noexcept {
// convert to name of Windows error, return 0 for failure, otherwise return number of chars in buffer
// __std_system_error_deallocate_message should be called even if 0 is returned
// pre: *_Ptr_str == nullptr
DWORD _Lang_id;
const int _Ret = GetLocaleInfoEx(LOCALE_NAME_SYSTEM_DEFAULT, LOCALE_ILANGUAGE | LOCALE_RETURN_NUMBER,
reinterpret_cast<LPWSTR>(&_Lang_id), sizeof(_Lang_id) / sizeof(wchar_t));
if (_Ret == 0) {
_Lang_id = 0;
}
const unsigned long _Chars =
FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
nullptr, _Message_id, _Lang_id, reinterpret_cast<char*>(_Ptr_str), 0, nullptr);
return _CSTD __std_get_string_size_without_trailing_whitespace(*_Ptr_str, _Chars);
}

While it would be acceptable if the system locale used UTF-8 as the codepage, allowing the messages to be correctly printed to log files, this is not the case. There is a "Beta: Use Unicode UTF-8 for worldwide language support" setting in the Region settings, which does the job correctly, but toggling that setting requires a reboot, which is not acceptable for our requirements.

Question

Should the std::error_code::message function respect the locale set by the user (e.g., via std::setlocale), or should it continue to use the system's default locale? If the former, is there a plan to implement this behavior in the Microsoft STL implementation?

Related Information

  • I learned that the locale is by default set across the whole program, unless specified by configthreadlocale, from the Microsoft documentation.
  • The issue of system_error not honoring the current thread's locale is mentioned in this issue.

Please let me know if you need any additional information or clarification.

@solarispika solarispika added the question Further information is requested label Jun 4, 2024
@jovibor
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jovibor commented Jun 4, 2024

This is basically a duplicate of the #3254.
Also see the #3260.

@miyanyan
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miyanyan commented Jun 5, 2024

I have the same question, when I get filesystem_error, it's message often uses the system locale, and I have to change the message to utf8

@StephanTLavavej StephanTLavavej added bug Something isn't working and removed question Further information is requested labels Jun 12, 2024
@StephanTLavavej
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@jovibor Yep, agreed. Marking as "bug" but not resolving as a duplicate, until we review the pending PR.

@vitaut
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vitaut commented Sep 28, 2024

The standard is unclear what encoding error_category::what should use - I've filed an LWG issue about it: https://cplusplus.github.io/LWG/issue4156. The current proposed resolution there is a bit of a strawman and likely to change. I would recommend waiting until this LWG issue is resolved.

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