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Date: Fri, 16 Jan 2026 23:38:06 +0900
To: cygwin AT cygwin DOT com
Subject: Re: Request to update libc++ related packages for current
 Clang/LLVM toolchain
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Fri, 16 Jan 2026 00:05:31 +0200
Florin wrote:
> On Thu, Jan 15, 2026 at 5:00 PM Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
> wrote:
> 
> > On Thu, 15 Jan 2026 21:53:21 +0900
> > Takashi Yano wrote:
> > > On Sun, 11 Jan 2026 23:11:00 +0200
> > > Florin wrote:
> > > > Hello Cygwin list,
> > > >
> > > > I’ve noticed that the Clang/LLVM stable packages in Cygwin are at
> > version
> > > > 20.1.x, while the libc++-devel, libc++1, libc++abi-devel, libc++abi1,
> > > > libunwind-devel and libunwind1 packages are still at 8.0.1. According
> > to
> > > > the package summaries, they were last updated in 2019.
> > > >
> > > > This version mismatch can present issues using modern C++ features and
> > > > makes it hard to use a complete LLVM toolchain with libc++ on Cygwin.
> > > >
> > > > I searched the Cygwin mailing list archives for prior posts about
> > updating
> > > > libc++, but I didn’t find a clear request or discussion about
> > coordinating
> > > > libc++ versions with newer Clang.
> > > >
> > > > Could you clarify if there is a plan to update libc++ to match more
> > recent
> > > > LLVM releases, or if the recommended C++ standard library for newer and
> > > > future Clang versions on Cygwin should be libstdc++.
> > > >
> > > > Thank you for your time and I'm grateful for the effort you all put
> > into
> > > > this project.
> > >
> > > I have just uploaded libcxx, compiler-rt, and libunwind packages.
> > > However, you cannot use them with -stdlib=libc++, -rtlib=compiler-rt,
> > > and -unwindlib=libunwind. This is because clang package has incomplete
> > > cygwin driver.
> > >
> > > You can link them to, for example hello.cc, with:
> > > clang++ -stdlib=libc++ -isystem /usr/include/c++/v1 hello.cc -lc++
> > $(clang --print-resource-dir)/lib/cygwin/libclang_rt.builtins-x86_64.a
> > -lunwind
> > >
> > > The resulted a.exe will be free from libgcc_s. Please try.
> >
> > I also uploaded 21.1.4-1 (Test) which works with clang/llvm 21.1.4-1
> > (Test).
> > This test version does not have the problem above.
> >
> > --
> > Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
> >
> > --
> >
> Problem reports:      https://cygwin.com/problems.html
> > FAQ:                  https://cygwin.com/faq/
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> 
> Hi Takashi,
> 
> I appreciate the short response time and the details you provided.
> 
> The test with a simple Hello World source on clang-20 based on your compile
> command succeeded and the result does not depend, indeed, on
> *cyggcc_s-seh-1.dll*. I didn't test the *-fuse-ld=lld* flag.
> 
> For the test versions (21.1.4-1) I kept getting link errors. The
> command *clang++
> -v -stdlib=libc++ -fuse-ld=lld hello.cc -lc++ -lunwind* produces a few
> errors like this:
> 
> > ld.lld: error: undefined symbol: std::__1::basic_ostream<char,
> > std::__1::char_traits<char>>::sentry::sentry(std::__1::basic_ostream<char,
> > std::__1::char_traits<char>>&)
> > >>> referenced by /tmp/hello-a3d7ec.o:(std::__1::basic_ostream<char,
> > std::__1::char_traits<char>>& std::__1::__put_character_sequence
> > *[abi:ne210104]*<char,
> > std::__1::char_traits<char>>(std::__1::basic_ostream<char,
> > std::__1::char_traits<char>>&, char const*, unsigned long))
> 
> 
> The default *ld* linker (*clang++ -v -stdlib=libc++ hello.cc -lc++ -lunwind* )
> behaves similarly:
> 
> > /usr/bin/ld:
> > /tmp/hello-e03638.o:hello.cc:(.text$_ZNSt3__124__put_character_sequenceB8ne210104IcNS_11char_traitsIcEEEERNS_13basic_ostreamIT_T0_EES7_PKS4_m[_ZNSt3__124__put_character_sequenceB8ne210104IcNS_11char_traitsIcEEEERNS_13basic_ostreamIT_T0_EES7_PKS4_m]+0x1ad):
> > undefined reference to `std::__1::basic_ostream<char,
> > std::__1::char_traits<char> >::sentry::~sentry()'
> >
> 
> I've tried different workarounds but couldn't make it work for clang-21
> (-std=c++14, -D_LIBCPP_ABI_VERSION=1, -fno-exceptions, -fno-rtti).
> 
> Do you have a working compile command I could try?
> 
> The symbols demanded by compiler require *[abi:ne210104]* while the libc++
> contains symbols with different ABI.
> The command *nm /usr/lib/libc++.dll.a | grep "basic_ostream" *produces
> lines like "*0000000000000000 T
> _ZTv0_n24_NSt3__113basic_ostreamIcNS_11char_traitsIcEEED1Ev*", which lacks
> the expected ABI tag.
> 
> Is libc++ compiled with the correct flags?
> Has the compiler some defaults not matching what current libc++ provides?
> 
> Thank you for your time and we love your work,

Thanks for testing and the report. That issue will be fixed in 21.1.4-2 (Test).
Please test 21.1.4-2 (Test) as well that will be uploaded shortly.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

-- 
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