[llvm-dev] Failed to replace stdlibc++ with libc++, linker phase error

[Tom Stellard via llvm-dev]

On 04/12/2019 04:28 PM, AiChi via llvm-dev wrote:
> Hi,
> 
> I'm currently working on one of my team's project to build LLVM full clang toolchain (Clang, libcxx, libcxxabi) on a CentOS machine. 
> 
> Previously we compiled our codebase with llvm-toolset-7/clang++, which by default takes stdlibc++ to compile and link. And now we'd like to switch to use LLVM clang with libc++. I have built libc++ and libc++abi from source (5.0.1 release) and set up related flags to compile our code base, but it turned out having some issue in the "linker" phase:
> 
> Flags set for complier:
> -stdlib=libc++ -std=c++14
> 
> Flags set for linker:
> -stdlib=libc++
> 
> Error:
> /opt/rh/devtoolset-7/root/usr/lib/gcc/x86_64-redhat-linux/7/../../../../bin/ld: product.o: undefined reference to symbol '__cxa_free_exception@@CXXABI_1.3'
> //lib64/libstdc++.so.6: error adding symbols: DSO missing from command line
> clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)
> 
> My question is:
> 1. We plan to replace stdlibc++ with libc++, and we've set up compiler and linker to run with "-stdlib=libc++', why is it complaining about libstdc++?

Can you post the full linker invocation with all the arguments? 

What cmake arguments did you use when building libcxx and libcxxabi?

> 2. How can we resolve this error? (any ideas would be appreciated!)

Not sure exactly, I would try passing -DLIBCXX_CXX_ABI=libcxxabi to cmake
when configuring libcxx if you have not already.

> 3. Is there anything missing if we'd like to replace stdlibc++ with libc++?
> 4. I also noticed when it's in linker phase, these flags were set in the ld command (-lc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc ), which we didn't have them setup anywhere in our project, just out of curiosity, are they added automatically? Why are they needed?

When you are using clang++ or g++ as the linker driver, then it automatically
adds the flags for you.

-Tom

> 
> A little bit more details:
> Environment: CentOS Linux release 7.6.1810 (Core) 
> Clang version: 
> 
>     clang version 5.0.1 (tags/RELEASE_501/final)
>     Target: x86_64-unknown-linux-gnu
>     Thread model: posix
> 
> 
> Thanks for any ideas,
> Aichi
> 
> 
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