libc/docs/full_host_build.rst

.. _full_host_build:

===============
Full Host Build
===============

.. contents:: Table of Contents
   :depth: 1
   :local:

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 <https://clang.llvm.org/>`_ compiler, the
`lld <https://lld.llvm.org/>`_ linker and the
`compiler-rt <https://compiler-rt.llvm.org/>`_ 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++ <https://libcxx.llvm.org/>`_ in the sysroot.

.. note:: When the libc is complete enough, we should be able to include
   `libc++ <https://libcxx.llvm.org/>`_, 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.

.. code-block:: sh

   $> cd llvm-project  # The llvm-project checkout
   $> mkdir build
   $> cd build
   $> cmake ../llvm  \
      -G Ninja  \ # Generator
      -DLLVM_ENABLE_PROJECTS="clang;libc;lld;compiler-rt"   \ # Enabled projects
      -DCMAKE_BUILD_TYPE=<Debug|Release>  \ # Select build type
      -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
      -DLLVM_LIBC_FULL_BUILD=ON      \  # We want the full libc
      -DLLVM_LIBC_INCLUDE_SCUDO=ON   \  # Include Scudo in the libc
      -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
      -DCOMPILER_RT_BUILD_GWP_ASAN=OFF                       \
      -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=OFF        \
      -DCMAKE_INSTALL_PREFIX=<SYSROOT>  # Specify a sysroot directory

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 <https://llvm.org/docs/ScudoHardenedAllocator.html>`_ 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.
* **The 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:

.. code-block:: sh
   
   $> 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. It is left to the reader to figure out the best
way to install Linux headers on the system they want to use the full libc on.