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.

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. First, we will explain how to build for developing LLVM-libc, then we will explain how to build LLVM-libc as part of a complete toolchain.

Configure the build for development

Below is the list of commands for a simple recipe to build LLVM-libc for development. In this we’ve set the Ninja generator, set the build type to “Debug”, and enabled the Scudo allocator. This build also enables generating the documentation and verbose cmake logging, which are useful development features.

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)

$> cd llvm-project  # The llvm-project checkout
$> mkdir build
$> cd build
$> cmake ../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

Build and test

After configuring the build with the above cmake command, one can build test libc with the following command:

$> ninja libc libm check-libc

To build the docs run this command:

$> ninja docs-libc-html

To run a specific test, use the following:

$> ninja libc.test.src.<HEADER>.<FUNCTION>_test.__unit__
$> ninja libc.test.src.ctype.isalpha_test.__unit__ # EXAMPLE

Configure the complete toolchain build

For a complete toolchain we recommend creating 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-libc is not quite 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.

Note

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.

Below is the cmake command for a bootstrapping build of LLVM. This will build clang and lld with the current system’s toolchain, then build compiler-rt and LLVM-libc with that freshly built clang. This ensures that LLVM-libc can take advantage of the latest clang features and optimizations.

This build also uses Ninja as cmake’s generator, and sets lld and compiler-rt as the default for the fresh clang. Those settings are recommended, but the build should still work without them. The compiler-rt options are required for building Scudo as the allocator for LLVM-libc.

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/<TARGET TRIPLE>/asm. Installing the gcc-multilib package creates this symlink, or you can do it manually with this command: sudo ln -s /usr/include/<TARGET TRIPLE>/asm /usr/include/asm

$> cd llvm-project  # The llvm-project checkout
$> mkdir build
$> cd build
$> SYSROOT=/path/to/sysroot # Remember to set this!
$> cmake ../llvm  \
   -G Ninja  \
   -DLLVM_ENABLE_PROJECTS="clang;lld"   \
   -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
   -DCMAKE_BUILD_TYPE=Release  \
   -DCMAKE_C_COMPILER=clang \
   -DCMAKE_CXX_COMPILER=clang++ \
   -DLLVM_LIBC_FULL_BUILD=ON \
   -DLLVM_LIBC_INCLUDE_SCUDO=ON \
   -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
   -DCOMPILER_RT_BUILD_GWP_ASAN=OFF                       \
   -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=OFF        \
   -DCLANG_DEFAULT_LINKER=lld \
   -DCLANG_DEFAULT_RTLIB=compiler-rt \
   -DCMAKE_INSTALL_PREFIX=$SYSROOT

Build and install

Warning

Running these install commands without setting a $SYSROOT will install them into your system include path, which may break your system. If you’re just trying to develop libc, then just run ninja check-libc to build the libc and run the tests. If you’ve already accidentally installed the headers, you may need to delete them from /usr/local/include.

After configuring the build with the above cmake command, one can build and install the toolchain with

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

or

$> ninja install

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 .
$> cp -r usr/* /path/to/sysroot/
$> rm -r usr linux-libc-dev*deb
$> ln -s /path/to/sysroot/include/x86_64-linux-gnu/asm /path/to/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

Warning

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

You can make sure you’re using the newly built toolchain by trying out features that aren’t yet supported by the system toolchain, such as fixed point. The following is an example program that demonstrates the difference:

// $ $SYSROOT/bin/clang example.c -static -ffixed-point --sysroot=$SYSROOT

#include <stdio.h>
int main() {
   printf("Hello, World!\n%.9f\n%.9lK\n",
      4294967295.000000001,
      4294967295.000000001ulK);
   return 0;
}