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Derrick edited this page Apr 6, 2021 · 1 revision

Build a freestanding libc++ (on Gentoo with LLVM 3.x)

Copied from https://blogs.gentoo.org/gsoc2016-native-clang/2016/05/05/build-a-freestanding-libcxx/ for easier viewing and reference

libc++ is the C++ standard library implemented by LLVM and an essential part of the clang-based toolchain we’re going to build. In this post I’ll demonstrate how to build a freestanding libc++ on Gentoo.

A complete C++ runtime stack consists of three components, from top to bottom:

  • C++ standard library
  • C++ ABI library
  • Stack unwinding library

As stated in my introductory post, in this GSoC project these roles will be taken by libc++, libc++abi and libunwind, respectively. As higher-level libraries depend on lower-level ones, we need to build them from bottom to top, i.e. :

  1. build libunwind
  2. build libc++abi against libunwind
  3. build libc++ against libc++abi

All of the above, of course, should be linked with musl instead of glibc. So first of all, we need a proper toolchain that can link binaries with musl. Luckily, Gentoo’s developers already prepared such a toolchain for us; just type the following commands:

$ emerge layman crossdev && layman -a musl && \
crossdev -t x86_64-pc-linux-musl

Here we use layman to create a layout for musl, and then use crossdev to auto-magically build the toolchain. Check out all needed respositories and we are ready to build the libraries:

$ cd $REPOS
$ svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
$ svn co http://llvm.org/svn/llvm-project/libunwind/trunk libunwind
$ svn co http://llvm.org/svn/llvm-project/libcxxabi/trunk libcxxabi
$ svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx

Build libunwind:

$ cd $REPOS/libunwind && mkdir build && cd build
$ cmake -DCMAKE_C_COMPILER=x86_64-pc-linux-musl-gcc \
-DCMAKE_CXX_COMPILER=x86_64-pc-linux-musl-g++ \
-DLIBUNWIND_ENABLE_SHARED=0 \
-DLLVM_PATH="$REPOS/llvm" ..
$ make

Note: I want to statically link libunwind into libc++abi, so I disable the building of shared library through LIBUNWIND_ENABLE_SHARED. You may safely omit this option if you want a shared version.

Build libc++abi:

$ cd $REPOS/libcxxabi && mkdir build && cd build
$ cmake -DCMAKE_C_COMPILER=x86_64-pc-linux-musl-gcc \
-DCMAKE_CXX_COMPILER=x86_64-pc-linux-musl-g++ \
-DCMAKE_SHARED_LINKER_FLAGS="-L$REPOS/libunwind/build/lib"
-DLIBCXXABI_USE_LLVM_UNWINDER=1 \
-DLIBCXXABI_LIBUNWIND_PATH="$REPOS/libunwind" \
-DLIBCXXABI_LIBCXX_INCLUDES="$REPOS/libcxx/include" \
-DLLVM_PATH="$REPOS/llvm" ..
$ make

Build libc++:

$ cd $REPOS/libcxx && mkdir build && cd build
$ cmake -DCMAKE_C_COMPILER=x86_64-pc-linux-musl-gcc \
-DCMAKE_CXX_COMPILER=x86_64-pc-linux-musl-g++ \
-DLIBCXX_HAS_MUSL_LIBC=1 \
-DLIBCXX_HAS_GCC_S_LIB=0 \
-DLIBCXX_CXX_ABI=libcxxabi \
-DLIBCXX_CXX_ABI_INCLUDE_PATHS="$REPOS/libcxxabi/include" \
-DLIBCXX_CXX_ABI_LIBRARY_PATH="$REPOS/libcxxabi/build/lib" \
-DLLVM_PATH="$REPOS/llvm" \
$ make

Note: libgcc is GCC’s stack unwinding library and should not be used in our C++ runtime stack, so I explicitly disable it through LIBCXX_HAS_GCC_S_LIB ; otherwise it’ll sneak into our library.

Now the C++ runtime stack is complete; it’s time to verify our work:

$ readelf -d $REPOS/libcxx/build/lib/libc++.so.1 | grep NEEDED
0x0000000000000001 (NEEDED) Shared library: [libc++abi.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so]

libunwind is statically linked so is not shown. Dependencies on libc++abi and libc (musl) look correct. So far, so good 🙂

In the following post, I’ll demonstrate how to link an actual C++ program with this freshly built libc++. See you!

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