Full Host Build

Note

Fullbuild requires running headergen, which is a python program that depends on pyyaml. The minimum versions are listed on the Generating Public and Internal headers page, as well as additional information.

Standard Building and Testing

Note

If your build fails with an error saying the compiler can’t find <asm/unistd.h> or similar then you’re probably missing the symlink from /usr/include/asm to /usr/include/<HOST TRIPLE>/asm. Installing the gcc-multilib package creates this symlink, or you can do it manually with this command: sudo ln -s /usr/include/<HOST TRIPLE>/asm /usr/include/asm (your host triple will probably be similar to x86_64-linux-gnu)

For basic development, such as adding new functions or fixing bugs, you can build and test the libc directly without setting up a full sysroot. This approach is faster and sufficient for most contributors.

To configure the build, create a build directory and run cmake:

cmake \
   -B build \
   -S runtimes \
   -G Ninja \
   -DCMAKE_C_COMPILER=clang \
   -DCMAKE_CXX_COMPILER=clang++ \
   -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DCMAKE_BUILD_TYPE=Debug \
   -DLLVM_LIBC_INCLUDE_SCUDO=ON \
   -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
   -DCOMPILER_RT_BUILD_GWP_ASAN=OFF                       \
   -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=OFF        \
   -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
   -DLLVM_ENABLE_SPHINX=ON -DLIBC_INCLUDE_DOCS=ON \
   -DLIBC_CMAKE_VERBOSE_LOGGING=ON

After configuring the build, you can build the libc, math library, and run the tests with the following command:

ninja -C build libc libm check-libc

To run a specific unit test for a function, you can target it directly using its full name:

ninja -C build libc.test.src.<HEADER>.<FUNCTION>_test.__unit__

For example, to run the test for isalpha in ctype.h:

ninja -C build libc.test.src.ctype.isalpha_test.__unit__

Building Documentation

If you have Sphinx installed, you can build the libc documentation locally. The build configuration above already includes the necessary flags (-DLLVM_ENABLE_SPHINX=ON -DLIBC_INCLUDE_DOCS=ON).

To generate the HTML documentation:

ninja -C build docs-libc-html

The generated documentation will be available in the docs/libc/html directory within your build folder.

Building a Simple Sysroot

Warning

The LLVM libc is missing many critical functions needed to build non-trivial applications. If you are not currently working on porting the libc, we recommend sticking with your system libc. However, ignoring warnings like this are how most of us got into this business. So: Speak friend and enter.

This document describes how to set up a simple sysroot and a compiler that uses it from scratch. These are not full cross-compilation instructions. We make a few assumptions:

  • The host and target are the same architecture and OS. For example, building a Linux x86-64 libc on a Linux x86-64 host.

  • The host has a working and recent Clang toolchain. Clang 21 has been tested.

  • Your container is using Debian Testing or a derived distribution. Other distributions likely work but the package names and paths may differ.

  • You have root access to your machine to set up the compiler wrapper.

For more comprehensive instructions on setting up a sysroot, see the official LLVM guide.

Step 1: Preparation

First, set up the environment variables for your sysroot path and the major version of your host Clang.

SYSROOT=$(readlink -f ~/sysroot)

Step 2: Linux Headers

Next, install the Linux kernel headers into your sysroot. For this guide, we’ll copy the headers from the host system’s /usr/include directory. This includes linux, asm-generic, and the architecture-specific asm headers.

# Create the include directory
mkdir -p $SYSROOT/usr/include

# Copy the header directories
cp -R /usr/include/linux $SYSROOT/usr/include/
cp -R /usr/include/asm-generic $SYSROOT/usr/include/
# Use -L to dereference the asm symlink and copy the actual files
cp -R -L /usr/include/asm $SYSROOT/usr/include/

Note

For a more production-ready sysroot, you would typically download a specific kernel version and install the headers using make headers_install configured for the target architecture and installation path.

Step 3: Build and Install Runtimes

Now, configure the build for LLVM libc and compiler-rt. We’re building with llvm instead of runtimes because we need to install the clang-resource-headers that provide stdarg.h, stddef.h and others.

cmake \
   -S llvm \
   -B build-runtimes \
   -G Ninja \
   -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF \
   -DCMAKE_INSTALL_PREFIX=$SYSROOT/usr \
   -DLLVM_ENABLE_PROJECTS="clang"   \
   -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
   -DCMAKE_BUILD_TYPE=Release \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLIBC_INCLUDE_DOCS=OFF \
   -DLLVM_LIBC_INCLUDE_SCUDO=ON \
   -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
   -DCOMPILER_RT_BUILD_GWP_ASAN=OFF                       \
   -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=OFF        \
   -DCOMPILER_RT_BUILD_BUILTINS:BOOL=TRUE \
   -DCOMPILER_RT_BUILD_CRT:BOOL=TRUE \
   -DCOMPILER_RT_BUILD_GWP_ASAN:BOOL=FALSE

After configuring, build and install the necessary components:

ninja -C build-runtimes install-clang-resource-headers install-libc install-compiler-rt install-builtins

Step 4: Configure the Compiler Wrapper

To make using the new toolchain easier, you can create a Clang configuration file. This allows you to avoid passing long command line arguments every time you compile a program.

  1. Identify the directory where your Clang binary is located:

CLANG_DIR=$(dirname $(readlink -f /usr/bin/clang))

  1. Create a symlink to clang named llvm-libc-clang in that directory:

sudo ln -sf $CLANG_DIR/clang /usr/bin/llvm-libc-clang

  1. Create the configuration file in the same directory. Clang automatically looks for a file named <executable-name>.cfg in the same directory as the executable. Use the following command to generate it with your environment variables:

CLANG_VERSION=$(build-runtimes/bin/clang -dumpversion | cut -d. -f1)

cat <<EOF | sudo tee $CLANG_DIR/llvm-libc-clang.cfg
--target=x86_64-unknown-linux-llvm
--sysroot=$SYSROOT
-resource-dir=$SYSROOT/usr/lib/clang/$CLANG_VERSION
--rtlib=compiler-rt
--unwindlib=none
-static
EOF

Step 5: Verification

You can now use your newly built toolchain by running your wrapper.

// hello.c
#include <stdio.h>
int main() {
   printf("Hello, World!\n");
   return 0;
}

Compile and run the example:

llvm-libc-clang hello.c
./a.out