[libcxx] Duplicate symbol with libcpmt.lib for static libc++ builds

[FatihBAKIR]

I understand parts of libc++ is implemented by linking with the MSVC's standard C++ library implementation. As far as I can tell, however, for static builds this requires linking against libcpmt, which causes duplicate symbols when linking executables.

lld-link: error: duplicate symbol: int __cdecl std::uncaught_exceptions(void)
>>> defined at D:\a\_work\1\s\src\vctools\crt\github\stl\src\uncaught_exceptions.cpp:9
>>>            libcpmt.lib(uncaught_exceptions.obj)
>>> defined at libc++.lib(exception.cpp.obj)

Is this a known/expected issue?

[mstorsjo]

This is not a known/expected issue.

The libcxx CI has got one test configuration, clang-cl-static, https://github.com/llvm/llvm-project/blob/llvmorg-21.1.1/.github/workflows/libcxx-build-and-test.yaml#L246 and https://github.com/llvm/llvm-project/blob/llvmorg-21.1.1/libcxx/utils/ci/run-buildbot#L670-L675, which gets tested on every commit, and in this configuration, all tests are passing as expected.

So you'll need to provide instructions on how exactly to reproduce this issue for it to be actionable.

[FatihBAKIR]

Some more details: I'm building libcxx with the following configuration with clang-cl, and linking everything with lld-link:

• LIBCXX_USE_COMPILER_RT=ON
• LIBCXX_ENABLE_SHARED=OFF

Which builds as expected. Then, when building/linking a binary, I'm supplying libc++ include directory manually via -Xclang -isystem/path/ and passing -stdlib=libc++, which links in the c++.lib as expected.

However, if I don't also explicitly link in libcpmt.lib, I get missing symbols for new_handler and exception_ptr related functionality:

lld-link: error: undefined symbol: void (__cdecl * __cdecl std::set_new_handler(void (__cdecl *)(void)))(void)
>>> referenced by CMakeFiles/smoke-test.dir/test.cpp.obj:(main)

lld-link: error: undefined symbol: void __cdecl __ExceptionPtrCreate(void *)
>>> referenced by libc++.lib(exception.cpp.obj):(public: __cdecl std::exception_ptr::exception_ptr(void))
>>> referenced by libc++.lib(exception.cpp.obj):(public: __cdecl std::exception_ptr::exception_ptr(std::nullptr_t))
>>> referenced by libc++.lib(exception.cpp.obj):(public: class std::exception_ptr & __cdecl std::exception_ptr::operator=(std::nullptr_t))
>>> referenced 4 more times

If I do link libcpmt.lib, then I get duplicate symbols as in the initial post.

Is this how the build should work or is there some flag/option I'm missing?

I'll try to provide a full reproducer.

The test program is just this:

#include <new>
#include <iostream>

int main() {
  std::cout << "hi\n";
  std::set_new_handler(nullptr);
}

[mstorsjo]

Your post still doesn't specify which symbols that supposedly are duplicated.

[FatihBAKIR]

Oops I pasted it in the same line as the triple-backticks and markdown didn't render it, I updated the original post.

[FatihBAKIR]

I can reproduce the duplicate symbol issue by building the fmtlib test suite, I'm trying to figure out a shorter repro, but just calling std::uncaught_exceptions() from my small reproducer does not immediately result in the duplicate symbol.

[FatihBAKIR]

OK I may have found a lead, normally I do not explicitly link libc++.lib and let the linker pick it up from the /defaultlib directive in the object files.

Explicitly linking libc++.lib before libcpmt.lib does resolve the duplicate symbol issue.

Though linking my small reproducer without the explicit libc++.lib still works, so something more might be there.

[mstorsjo]

OK I may have found a lead, normally I do not explicitly link libc++.lib and let the linker pick it up from the /defaultlib directive in the object files.

Explicitly linking libc++.lib before libcpmt.lib does resolve the duplicate symbol issue.

Right, this probably explains it; the tests in the CI all explicitly link against libc++.lib I think. (Those tests mostly link through the clang driver with -l options, so it looks slightly different from how it normally would be used through clang-cl though.)

I remember that someone has discussed that we should make the object files in libc++.lib similarly pull in the right library (libcpmt.lib) through defaultlib directives so the caller doesn't have to specify it manually - that would also take care of this issue, I presume.

Though linking my small reproducer without the explicit libc++.lib still works, so something more might be there.

A duplicate definition error like this may depend a lot on the exact symbols that are included, which ones end up pulling in which object files, so that's not surprising here. But your observation above sounds like the right root cause.