Clang link libc++ statically - c++

Question

On Ubuntu 20.04 I need to use libc++ since the default libstdc++ is lacking required support. In order to not have to ship libc++ I need to statically link against libc++.
I have a static version of libc++ available which has been created locally using the following series of commands (as per https://libcxx.llvm.org/BuildingLibcxx.html):
git clone --single-branch --branch release/15.x --depth 1 https://github.com/llvm/llvm-project.git
mkdir /llvm-project/build
cmake -G "Ninja" -S /llvm-project/runtimes -B /llvm-project/build \
-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
-DLIBCXX_INCLUDE_BENCHMARKS=OFF \
-DLIBCXX_INCLUDE_TESTS=OFF \
-DLIBCXX_ENABLE_SHARED=ON \
-DLIBCXX_ENABLE_STATIC=ON \
-DLIBCXXABI_ENABLE_SHARED=ON \
-DLIBCXXABI_ENABLE_STATIC=ON \
-DLIBCXX_CXX_ABI=libcxxabi \
-DCMAKE_INSTALL_PREFIX=/usr \
-DCMAKE_BUILD_TYPE=Release
ninja -C /llvm-project/build cxx cxx-abi unwind
ninja -C /llvm-project/build install-cxx install-cxxabi install-unwind
I fail to make it achieve the desired results of not of not having any runtime dependency on libc++.
Trying to compile the following simple example:
#include <iostream>
int main(int, const char**) {
std::cout << "Clang " << __clang_version__ << '\n';
#ifdef _LIBCPP_VERSION
std::cout << " Using libc++ " << _LIBCPP_VERSION << '\n';
#endif
return 0;
}
Using clang++ test.cpp -std=c++20 -static-libstdc++ -static-libgcc will result in no undesired runtime dependencies (only linux-vdso, libm, libc, ld-linux), however it is using stdlibc++ which will not work for the actual code.
Using clang++ test.cpp -stdlib=libc++ -std=c++20 -static-libstdc++ -static-libgcc results in lots of linker errors about missing standard stuff.
Using clang++ test.cpp -stdlib=libc++ -std=c++20 -static-libstdc++ -static-libgcc -lc++abi followed by an ldd a.out will show it having a dependency on libc++abi.so and libgcc_s.so.1 which are both undesired, the former more so than the latter.
(Why do I have to explicitly link c++abi anyway, should it not just do it by itself like it does with c++?)
In an attempt to not have the pesky runtime dependency doing clang++ test.cpp -stdlib=libc++ -std=c++20 -static-libstdc++ -static-libgcc -lc++abi -static will result in
ld: error: duplicate symbol: __lll_lock_wait_private
>>> defined at libc-lowlevellock.o:(__lll_lock_wait_private) in archive /lib/x86_64-linux-gnu/libc.a
>>> defined at lowlevellock.c:27
>>> lowlevellock.o:(.text+0x0) in archive /lib/x86_64-linux-gnu/libpthread.a
No idea why it suddenly decides to have pthread in there, as I never specified that and it would not seem to be required given the example code but whatever.
How can one convince clang to actually statically link libc++ (and libc++abi), without errors, so that it will not have non-standard runtime dependencies?

Answer 1

The entire C++ standard library (libc++/libstdc++ etc) all rely on the standard C library and pthreads among others.
C++ standard libs on Linux are just a tiny shell around the infrastructure that the C language provides, the real workhorse.
std::thread is basically just syntax sugar on top of pthreads. This is a hard reality most C++ developers find very hard to stomach.
See examples:
std::threadlibstdc++ libc++
std::chrono libstdc++ libc++
std::filesystem libstdc++ libc++