clang - Install libc++ on ubuntu

Question

I am wondering what is the right/easy way to install a binary libc++ on Ubuntu, in my case Trusty aka 14.04?

On the LLVM web site there are apt packages http://apt.llvm.org/ and I have used these to install 3.9. However these packages don't seem to include libc++. I install the libc++-dev package but that seems to be a really old version. There are also binaries that can be downloaded http://llvm.org/releases/download.html#3.9.0. These do seem to contain libc++ but I'm not sure if I can just copy bits of this into places like /usr/include/c++/v1, in fact I'm not really sure what bits I would need to copy. I am aware I can use libc++ from an alternate location as documented here http://libcxx.llvm.org/docs/UsingLibcxx.html which I have tried. However I can't modify the build system of the large code base I work on to do this.

So is three any reason the apt packages don't include libc++ and any pointers to installing a binary would be gratefully recieved.

Answer 1

How to build libc++ on Ubuntu 16.04

I had a similar issue as you do. While testing clang with libstdc++ worked fine with C++11 and C++14 there still might be licensing issues with libstdc++. So I ended up installing Clang toolchain from their repos and compiling libc++ on Ubuntu 16.04.

Disclaimer: This post is summary of long search on how to build the libc++ on Ubuntu Linux. Many of the posts I found in 2017 were either outdated or described a partial solution on other systems e.g. CentOS. Links to these posts are:

• Hacking with Clang llvm abi and llvm libc
• Building Clang and libc++ on Ubuntu Linux
• How to Build libcxx and libcxxabi by clang on CentOS 7

Here are the steps to build LLVM + Clang + libc++ from the 4.0 release branch:

1. Install the key of LLVM Repositories

   # apt-get update && apt-get dist-upgrade -y && apt-get install -y vim curl && curl -q https://apt.llvm.org/llvm-snapshot.gpg.key |apt-key add -

2. Create a new new APT Repository File (you can also exclude 2 lines referring to v3.9 repos)

   # cat > /etc/apt/sources.list.d/llvm-repos.list <<EOF deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial main deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.9 main deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.9 main deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-4.0 main deb-src http://apt.llvm.org/xenial/ llvm-toolchain-xenial-4.0 main EOF

3. Install Clang and all Packages needed to build libc++ from LLVM repos

   # apt-get update && apt-get install -y clang-4.0 clang-4.0-doc libclang-common-4.0-dev libclang-4.0-dev libclang1-4.0 libclang1-4.0-dbg libllvm4.0 libllvm4.0-dbg lldb-4.0 llvm-4.0 llvm-4.0-dev llvm-4.0-runtime clang-format-4.0 python-clang-4.0 liblldb-4.0-dev lld-4.0 libfuzzer-4.0-dev subversion cmake

4. Create an alternative for C++ compiler and linker. This is not a must, but lets you switch compilers or linkers if needed. Also some build files needed cc or c++ or clang++ as far as I remember. Keep in mind, that we switch to LLD linker as default:

   update-alternatives --install /usr/bin/cc cc /usr/bin/clang-4.0 100 && update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-4.0 100 && update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-4.0 100 && update-alternatives --install /usr/bin/clang clang /usr/bin/clang-4.0 100 && update-alternatives --install /usr/bin/ld ld /usr/bin/ld.lld-4.0 10 && update-alternatives --install /usr/bin/ld ld /usr/bin/ld.gold 20 && update-alternatives --install /usr/bin/ld ld /usr/bin/ld.bfd 30 && ld --version && echo 3 | update-alternatives --config ld && ld --version

5. Checkout sources of libc++ and libc++abi:

   $ cd /tmp $ svn co http://llvm.org/svn/llvm-project/libcxx/branches/release_40/ libcxx $ svn co http://llvm.org/svn/llvm-project/libcxxabi/branches/release_40/ libcxxabi $ mkdir -p libcxx/build libcxxabi/build

6. To run libc++ on Linux one needs ABI compatibility to the standard library, e.g. libstdc++. This is where libc++abi comes into game. The only problem is that it needs libc++ to be on the system for which it is build. Thus libc++ is built in 2 steps. First: without any ABI compatibility. But it will be used for bootstrapping of ABI lib and than the second step is to recompile libc++ with the proper ABI present on system:

   Bootstraping => build libc++ without proper ABI:

   cd /tmp/libcxx/build cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_CONFIG_PATH=/usr/bin/llvm-config-4.0 -DCMAKE_INSTALL_PREFIX=/usr .. && make install

   Building libc++abi with libstdc++ compatible ABI:

   cd /tmp/libcxxabi/build CPP_INCLUDE_PATHS=echo | c++ -Wp,-v -x c++ - -fsyntax-only 2>&1 |grep ' /usr'|tr ' ' ' '|tr -s ' ' |tr ' ' ';' CPP_INCLUDE_PATHS="/usr/include/c++/v1/;$CPP_INCLUDE_PATHS" cmake -G "Unix Makefiles" -DLIBCXX_CXX_ABI=libstdc++ -DLIBCXX_LIBSUPCXX_INCLUDE_PATHS="$CPP_INCLUDE_PATHS" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DLLVM_CONFIG_PATH=/usr/bin/llvm-config-4.0 -DLIBCXXABI_LIBCXX_INCLUDES=../../libcxx/include .. make install

   Rebuild libc++ with proper ABI lib deployed on system:

   cd /tmp/libcxx/build cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DLIBCXX_CXX_ABI=libcxxabi -DLLVM_CONFIG_PATH=/usr/bin/llvm-config-4.0 -DLIBCXX_CXX_ABI_INCLUDE_PATHS=../../libcxxabi/include .. && make install

7. Create a test file to check whether everything works fine. IMO you should also test cerr stream, as previously it was not supported with the libc++abi and there were some segfaults. Please refer to this question.

   create a test.cpp file:

   #include <iostream> int main() { using namespace std; cout << "[OK] Hello world to cout!" << endl; cerr << "[OK] Hello world to cerr!" << endl; clog << "[OK] Hello world to clog!" << endl; return 0; }

   And compile it and run it using this command line:

   clang++ -std=c++11 -stdlib=libc++ -lc++abi test.cpp && ./a.out

Reason there is no package

I found libc++ packages for Ubuntu but they are a bit behind recent version: https://packages.ubuntu.com/xenial/libc++-dev

Why they are not current, I can't answer, but my guess is that LLVM+Clang can work with mostly any Standard Library, whereas libc++ as you see must be linked to particular runtime ABI and might heavily depend on available C runtime library. I agree there should be a package which covers 90% of the cases. May be this is just the lack of resources. Searching the mailing archive did not bring up anything special.