Full Host Build

In this document, we will present a recipe to build the full libc for the host. When we say build the libc for the host, the goal is to build the libc for the same system on which the libc is being built. Also, we will take this opportunity to demonstrate how one can set up a sysroot (see the documentation of the --sysroot option here: https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html) which includes not only the components of LLVM’s libc, but also a full LLVM only toolchain consisting of the clang compiler, the lld linker and the compiler-rt runtime libraries. LLVM’s libc is not yet complete enough to allow using and linking a C++ application against a C++ standard library (like libc++). Hence, we do not include libc++ in the sysroot.


When the libc is complete enough, we should be able to include libc++, libcxx-abi and libunwind in the LLVM only toolchain and use them to build and link C++ applications.

Configure the full libc build

Below is the list of commands for a simple recipe to build and install the libc components along with other components of an LLVM only toolchain. In this we’ve set the Ninja generator, enabled a full compiler suite, set the build type to “Debug”, and enabled the Scudo allocator. The build also tells clang to use the freshly built lld and compiler-rt.

$> cd llvm-project  # The llvm-project checkout
$> mkdir build
$> cd build
$> SYSROOT=/path/to/sysroot # Remember to set this!
$> cmake ../llvm  \
   -G Ninja  \
   -DLLVM_ENABLE_PROJECTS="clang;libc;lld;compiler-rt"   \
   -DCOMPILER_RT_BUILD_GWP_ASAN=OFF                       \
   -DCLANG_DEFAULT_RTLIB=compiler-rt \

We will go over some of the special options passed to the cmake command above.

  • Enabled Projects - Since we want to build and install clang, lld and compiler-rt along with the libc, we specify clang;libc;lld;compiler-rt as the list of enabled projects.

  • The full build option - Since we want to do build the full libc, we pass -DLLVM_LIBC_FULL_BUILD=ON.

  • Scudo related options - LLVM’s libc uses Scudo as its allocator. So, when building the full libc, we should specify that we want to include Scudo in the libc. Since the libc currently only supports static linking, we also specify that we do not want to build the Scudo shared library.

  • Default sysroot and install prefix - This is the path to the tool chain install directory. This is the directory where you intend to set up the sysroot.

Build and install

After configuring the build with the above cmake command, one can build and install the libc, clang (and its support libraries and builtins), lld and compiler-rt, with the following command:

$> ninja install-clang install-builtins install-compiler-rt  \
   install-core-resource-headers install-libc install-lld

Once the above command completes successfully, the $SYSROOT directory you have specified with the CMake configure step above will contain a full LLVM-only toolchain with which you can build practical/real-world C applications. See https://github.com/llvm/llvm-project/tree/main/libc/examples for examples of how to start using this new toolchain.

Linux Headers

If you are using the full libc on Linux, then you will also need to install Linux headers in your sysroot. The way to do this varies per system.

These instructions should work on a Debian-based x86_64 system:

$> apt download linux-libc-dev
$> dpkg -x linux-libc-dev*deb .
$> mv usr/include/* /path/to/sysroot/include
$> rm -rf usr linux-libc-dev*deb
$> ln -s x86_64-linux-gnu/asm ~/Programming/sysroot/include/asm

Using your newly built libc

You can now use your newly built libc nearly like you would use any compiler invocation:

$> /path/to/sysroot/bin/clang -static main.c


Because the libc does not yet support dynamic linking, the -static parameter must be added to all clang invocations.