Re: std::runtime_error on std::locale("")

[Christian Franke <Christian.Franke@t-online.de>]

Brian Inglis via Cygwin wrote:
> On 2023-09-21 10:28, Takashi Yano via Cygwin wrote:
>> On Fri, 22 Sep 2023 01:12:04 +0900
>> Takashi Yano wrote:
>>> I wonder why the following code throws std::runtime_error
>>> even though the LC_ALL is set to valid locale other than "C".
>>> This does not occur only when LC_ALL is set to "C".
>>>
>>> #include <locale>
>>> int main()
>>> {
>>>     std::locale("");
>>>     return 0;
>>> }
>>>
>>> In linux, this occurs only when the LC_ALL is set to invalid
>>> locale (i.e. locale that is not registered in system).
>>
>> Similarly,
>> std::locale("ja_JP.UTF-8")
>> throws std::runtime_error in cygwin.
>
> Looks like the implementation does not like any default "" or explicit 
> "en_US.UTF-8" strings there! See example at link and below; results 
> are always the same:
>
>     https://en.cppreference.com/w/cpp/locale/locale
>
> #include <iostream>
> #include <locale>
>
> int main()
> {
>     std::wcout << "User-preferred locale setting is "
>            << std::locale().name().c_str() << '\n';
>
>     // on startup, the global locale is the "C" locale
>     std::wcout << 1000.01 << '\n';
>
>     // replace the C++ global locale and the "C" locale with the 
> user-preferred locale
>     std::locale::global(std::locale(""));
>     // use the new global locale for future wide character output
>     std::wcout.imbue(std::locale());
>
>     // output the same number again
>     std::wcout << 1000.01 << '\n';
> }
>
> $ g++ -o c++locale{,.cc}
> $ ./c++locale
> User-preferred locale setting is C
> 1000.01
> terminate called after throwing an instance of 'std::runtime_error'
>   what():  locale::facet::_S_create_c_locale name not valid
> Aborted (core dumped)
>

According to libstdc++ source, the internal function 
locale::facet::_S_create_c_locale() calls some __newlocale() which 
apparently does not arrive at newlocale() from cygwin1.dll. But 
cygstdc++-6.dll imports newlocale() from cygwin1.dll.

Only standard locale "C" and its alias "POSIX" work with C++ std::locale().

The cygwin1.dll function newlocale() works as expected - except that it 
does not set errno if the locale name is invalid.

-- 
Regards,
Christian