.. _docs-bazel-integration:

===================================================
Using a Pigweed module in an existing Bazel project
===================================================
This guide explains how to start using a Pigweed module in your existing
Bazel-based C or C++ project. We'll assume you're familiar with the build
system at the level of the `Bazel tutorial <https://bazel.build/start/cpp>`__.

-------------------------------------
Add Pigweed as a WORKSPACE dependency
-------------------------------------
Add Pigweed as a `git_repository
<https://bazel.build/rules/lib/repo/git#git_repository>`__ in your
``WORKSPACE``:

.. code-block:: python

   git_repository(
     name = "pigweed",
     commit = "c00e9e430addee0c8add16c32eb6d8ab94189b9e",
     remote = "https://pigweed.googlesource.com/pigweed/pigweed.git",
   )

(You can find the latest tip-of-tree commit in the History tab in `CodeSearch
<https://cs.opensource.google/pigweed/pigweed>`__.)

If you manage your dependencies as submodules, you can add Pigweed as a
submodule, too, and then add it to the ``WORKSPACE`` as a `local_repository
<https://bazel.build/reference/be/workspace#local_repository>`__:

.. code-block:: python

   local_repository(
     name = "pigweed",
     path = "third_party/pigweed",
   )

We don't yet publish releases that could be pulled in using `http_archive
<https://bazel.build/rules/lib/repo/http#http_archive>`__.

We don't support `bzlmod <https://bazel.build/external/overview#bzlmod>`__ yet.
See http://pwbug.dev/258836641.

If either of these limitations is important to you, please reach out to us on
`chat <https://discord.gg/M9NSeTA>`__.

---------------------------
Use Pigweed in your project
---------------------------
Let's say you want to use ``pw::Vector`` from :ref:`module-pw_containers`, our
embedded-friendly replacement for ``std::vector``.

#. Include the header you want in your code:

   .. code-block:: cpp

      #include "pw_containers/vector.h"

#. Look at the module's `build file
   <https://cs.opensource.google/pigweed/pigweed/+/main:pw_containers/BUILD.bazel>`__
   to figure out which build target you need to provide the header and
   implementation. For ``pw_containers/vector.h``, it's
   ``//pw_containers:vector``.

#. Add this target to the ``deps`` of your
   `cc_library <https://bazel.build/reference/be/c-cpp#cc_library>`__ or
   `cc_binary <https://bazel.build/reference/be/c-cpp#cc_binary>`__:

   .. code-block:: python

      cc_library(
        name = "my_library",
        srcs  = ["my_library.cc"],
        hdrs = ["my_library.h"],
        deps = [
          "@pigweed//pw_containers:vector",  # <-- The new dependency
        ],
      )

#. Add a dependency on ``@pigweed//pw_build:default_link_extra_lib`` to your
   final *binary* target. See :ref:`docs-build_system-bazel_link-extra-lib`
   for a discussion of why this is necessary, and what the alternatives are.

   .. code-block:: python

      cc_binary(
        name = "my_binary",
        srcs  = ["my_binary.cc"],
        deps = [
          ":my_library",
          "@pigweed//pw_build:default_link_extra_lib",  # <-- The new dependency
        ],
      )

--------------------------------------------
Configure backends for facades you depend on
--------------------------------------------
Pigweed makes extensive use of :ref:`docs-facades`, and any module you choose
to use will likely have a transitive dependency on some facade (typically
:ref:`module-pw_assert` or :ref:`module-pw_log`). Continuing with our example,
``pw::Vector`` depends on :ref:`module-pw_assert`.

In Bazel, facades already have a default backend (implementation) that works
for host builds (builds targeting your local development machine). But to build
a binary for your embedded target, you'll need to select a suitable backend
yourself.

Fortunately, the default backend for :ref:`module-pw_assert` is
:ref:`module-pw_assert_basic`, which is a suitable place to start for most
embedded targets, too. But it depends on :ref:`module-pw_sys_io`, another
facade for which you *will* have to choose a backend yourself.

The simplest way to do so is to set the corresponding `label flag
<https://bazel.build/extending/config#label-typed-build-settings>`__ when
invoking Bazel. For example, to use the
:ref:`module-pw_sys_io_baremetal_stm32f429` backend for :ref:`module-pw_sys_io`
provided in upstream Pigweed:

.. code-block:: console

   $ bazel build \
       --@pigweed//targets/pw_sys_io_backend=@pigweed//pw_sys_io_baremetal_stm32f429 \
       //path/to/your:target

You can also define backends within your own project. (If Pigweed doesn't
include a :ref:`module-pw_sys_io` backend suitable for your embedded platform,
that's what you should do now.) See
:ref:`docs-build_system-bazel_configuration` for a tutorial that dives deeper
into facade configuration with Bazel.