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

[Takashi Yano <takashi.yano@nifty.ne.jp>]

On Thu, 21 Sep 2023 21:06:59 +0200
Christian Franke wrote:
> 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.

Thanks for the pointer. I looked into the cygstdc++6.dll source code,
and noticed that the code you mentioned is for glibc. In glibc, 
__newlocale() is defined and newlocale() is a weak alias for that.

For generic libc (i.e. other than glibc), _S_create_c_locale() is
defined as:

  void
  locale::facet::_S_create_c_locale(__c_locale& __cloc, const char* __s,
                    __c_locale)
  {
    // Currently, the generic model only supports the "C" locale.
    // See http://gcc.gnu.org/ml/libstdc++/2003-02/msg00345.html
    __cloc = 0;
    if (strcmp(__s, "C"))
      __throw_runtime_error(__N("locale::facet::_S_create_c_locale "
                "name not valid"));
  }

in /libstdc++-v3/config/locale/generic/c_locale.cc.

Obviously from the code, locale other than "C" causes runtime_error.
The behaviour is as designed but not a bug in cygwin environment.

Thanks for discussion.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>