On 31 Oct 2024, at 06:40, Brian Inglis via Cygwin <cygwin@cygwin.com> wrote:
>
> On 2024-10-30 10:30, Dimitry Andric via Cygwin wrote:
>> On 30 Oct 2024, at 17:14, Csaba Ráduly via Cygwin wrote:
>>> On 30/10/2024 17:00, Dimitry Andric via Cygwin wrote:
>>>> #include<string>
>>>> #include<cassert>
>>>>
>>>> constexpr bool foo()
>>>> {
>>>> std::string str2{"abcwe"};
>>>> return str2.size()==5;
>>>> }
>>>>
>>>> static_assert(foo());
>>>>
>>>> int main()
>>>> {
>>>> assert(foo());
>>>> }
>>>
>>> Seems like _GLIBCXX_USE_CXX11_ABI is not defined by default.
>> Indeed,
>> /usr/lib/gcc/x86_64-pc-cygwin/12/include/c++/x86_64-pc-cygwin/bits/c++config.h
>> shows:
>> 317 # define _GLIBCXX_USE_DUAL_ABI 1
>> 318
>> 319 #if ! _GLIBCXX_USE_DUAL_ABI
>> 320 // Ignore any pre-defined value of _GLIBCXX_USE_CXX11_ABI
>> 321 # undef _GLIBCXX_USE_CXX11_ABI
>> 322 #endif
>> 323
>> 324 #ifndef _GLIBCXX_USE_CXX11_ABI
>> 325 # define _GLIBCXX_USE_CXX11_ABI 0
>> 326 #endif
>> Also, g++ -v shows --with-default-libstdcxx-abi=gcc4-compatible, so I guess
>> this was deliberate on the part of the packagers. Probably for backwards
>> compatibility reasons?
>
> Okay, sorry, I typoed an extra _ before _GLIBCXX_NOEXCEPT!
>
> Cygwin gcc/g++ 13.3 is available as a test package; only with .4/.5 are gcc
> version features and fixes considered stable.
>
> The build is requesting -std=gnu++20, so why is 11 ABI relevant?
>
> As I want to use the latest language, why is gcc 4 compatible ABI *default*
> relevant?
>
> What can I do in the package build to get g++ to not use the default but
> whatever is required to support the latest language?
>
> Where can I find docs on these symbols and their meanings, uses, and settings?
It is a historical artifact that has been dragged along for years. See
https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html for more
information.
During the gcc 5.1 release, changes were made to libstdc++ that made the ABI
incompatible with previous releases, specifically for std::string. You can
define _GLIBCXX_USE_CXX11_ABI to 1 to use the 'new' ABI, but then it becomes
tricky to link your code to libraries compiled with older versions of
libstdc++, or newer versions that define _GLIBCXX_USE_CXX11_ABI to 0.
I think most Linux distributions have switched fully to the new ABI by now, and
dropped support for the old ABI, so they configure their gcc's with
_GLIBCXX_USE_CXX11_ABI=1 by default.
-Dimitry
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