I've got this error when compile coroutine against libc++ that I built
"C:\\Program Files\\LLVM\\bin\\lld-link" ... c++.lib c++experimental.lib ...
lld-link: error: undefined symbol: __declspec(dllimport) public: void __cdecl std::experimental::coroutines_v1::suspend_always::await_suspend(class std::experimental::coroutines_v1::coroutine_handle<void>) const
Before that, I've built libc++ using -DLIBCXX_ENABLE_SHARED=YES -DLIBCXX_ENABLE_STATIC=NO -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=YES
set LIB=C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\lib\x64;C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\ucrt\x64;C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\um\x64
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_MAKE_PROGRAM="C:/Program Files (x86)/Ninja/ninja.exe" -DCMAKE_SYSTEM_NAME=Windows -DCMAKE_C_COMPILER="C:/Program Files/LLVM/bin/clang-cl.exe" -DCMAKE_C_FLAGS="-fms-compatibility-version=19.20.27508 --target=x86_64-pc-windows-msvc19.20.27508" -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang-cl.exe" -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.20.27508 --target=x86_64-pc-windows-msvc19.20.27508" -DCMAKE_C_LINK_EXECUTABLE="C:/Program Files/LLVM/bin/lld-link.exe" -DCMAKE_CXX_LINK_EXECUTABLE="C:/Program Files/LLVM/bin/lld-link.exe" -DLLVM_PATH="C:/Program Files/LLVM" -DLIBCXX_ENABLE_SHARED=YES -DLIBCXX_ENABLE_STATIC=NO -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=YES "C:/libcxx-master"
ninja -j 2
So I tried nm c++.lib and nm c++experimental.lib. I found no trace of suspend_always.
It seems suspend_always::await_suspend is missing in the dll. So I delve in some c++ macro.
In C:/libcxx-master/CMakeLists.txt, there is _LIBCPP_BUILDING_LIBRARY defined.
add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
In C:/libcxx-master/include/__config, we have
#if defined(__ELF__)
# define _LIBCPP_OBJECT_FORMAT_ELF 1
#elif defined(__MACH__)
# define _LIBCPP_OBJECT_FORMAT_MACHO 1
#elif defined(_WIN32)
# define _LIBCPP_OBJECT_FORMAT_COFF 1
#elif defined(__wasm__)
# define _LIBCPP_OBJECT_FORMAT_WASM 1
#else
# error Unknown object file format
#endif
I'm in _WIN32 so we have _LIBCPP_OBJECT_FORMAT_COFF well defined.
Further downward, we have
#if defined(_LIBCPP_OBJECT_FORMAT_COFF)
#ifdef _DLL
# define _LIBCPP_CRT_FUNC __declspec(dllimport)
#else
# define _LIBCPP_CRT_FUNC
#endif
#if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
# define _LIBCPP_DLL_VIS
# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
# define _LIBCPP_OVERRIDABLE_FUNC_VIS
# define _LIBCPP_EXPORTED_FROM_ABI
#elif defined(_LIBCPP_BUILDING_LIBRARY)
# define _LIBCPP_DLL_VIS __declspec(dllexport)
# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
# define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
# define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
#else
# define _LIBCPP_DLL_VIS __declspec(dllimport)
# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
# define _LIBCPP_OVERRIDABLE_FUNC_VIS
# define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)
#endif
#define _LIBCPP_TYPE_VIS _LIBCPP_DLL_VIS
#define _LIBCPP_FUNC_VIS _LIBCPP_DLL_VIS
#define _LIBCPP_EXCEPTION_ABI _LIBCPP_DLL_VIS
#define _LIBCPP_HIDDEN
#define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
#define _LIBCPP_TEMPLATE_VIS
#define _LIBCPP_ENUM_VIS
#endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)
This means when we are building libc++, _LIBCPP_TYPE_VIS becomes __declspec(dllexport). So we can export concrete types to DLL
When we compile code, _LIBCPP_TYPE_VIS becomes __declspec(dllimport). So we can import concrete types from DLL
And experimental/coroutine file includes experimental/__config which includes __config and contains this struct suspend_always definition
struct _LIBCPP_TYPE_VIS suspend_always {
_LIBCPP_INLINE_VISIBILITY
bool await_ready() const _NOEXCEPT { return false; }
_LIBCPP_INLINE_VISIBILITY
void await_suspend(coroutine_handle<>) const _NOEXCEPT {}
_LIBCPP_INLINE_VISIBILITY
void await_resume() const _NOEXCEPT {}
};
So when we build libc++, we have struct __declspec(dllexport) suspend_always. When we compile, we have struct __declspec(dllimport) suspend_always
I even tried hardcoding struct __declspec(dllexport) suspend_always when building libc++, the result is still the same.
Then I came across an idea. I create libcxx-master/src/experimental/coroutine and just make it #include <experimental/coroutine>. Rebuild libc++.dll and it turns out that working fine this time.
My question is whether struct suspend_always (and suspend_never) is really supposed to be exported to c++experimental.lib?
Where can I report this to the implementors?
Seems like a bug. It should be fixed in r358551.