pw_build/docs.rst

.. _module-pw_build:

--------
pw_build
--------
Pigweed's modules aim to be easily integratable into both new and existing
embedded projects. To that goal, the ``pw_build`` module provides support for
multiple build systems. Our personal favorite is `GN`_/`Ninja`_, which is used
by upstream developers for its speed and flexibility. `CMake`_ and `Bazel`_
build files are also provided by all modules, allowing Pigweed to be added to a
project with minimal effort.

.. _GN: https://gn.googlesource.com/gn/
.. _Ninja: https://ninja-build.org/
.. _CMake: https://cmake.org/
.. _Bazel: https://bazel.build/

Beyond just compiling code, Pigweed’s GN build system can also:

* Generate HTML documentation, via our Sphinx integration (with ``pw_docgen``)
* Display memory usage report cards (with ``pw_bloat``)
* Incrementally run unit tests after code changes (with ``pw_target_runner``)
* And more!

These are only supported in the GN build, so we recommend using it if possible.

GN / Ninja
==========
The GN / Ninja build system is the primary build system used for upstream
Pigweed development, and is the most tested and feature-rich build system
Pigweed offers.

This module's ``build.gn`` file contains a number of C/C++ ``config``
declarations that are used by upstream Pigweed to set some architecture-agnostic
compiler defaults. (See Pigweed's ``//BUILDCONFIG.gn``)

``pw_build`` also provides several useful GN templates that are used throughout
Pigweed.

Build system philosophies
-------------------------
While Pigweed's GN build is not hermetic, it strives to adhere to principles of
`hermeticity <https://bazel.build/concepts/hermeticity>`_. Some guidelines to
move towards the ideal of hermeticity include:

* Only rely on pre-compiled tools provided by CIPD (or some other versioned,
  pre-compiled binary distribution mechanism). This eliminates build artifact
  differences caused by different tool versions or variations (e.g. same tool
  version built with slightly different compilation flags).
* Do not use absolute paths in Ninja commands. Typically, these appear when
  using ``rebase_path("//path/to/my_script.py")``. Most of the time, Ninja
  steps should be passed paths rebased relative to the build directory (i.e.
  ``rebase_path("//path/to/my_script.py", root_build_dir)``). This ensures build
  commands are the same across different machines.
* Prevent produced artifacts from relying on or referencing system state. This
  includes time stamps, writing absolute paths to generated artifacts, or
  producing artifacts that reference system state in a way that prevents them
  from working the same way on a different machine.
* Isolate build actions to the build directory. In general, the build system
  should not add or modify files outside of the build directory. This can cause
  confusion to users, and makes the concept of a clean build more ambiguous.

Target types
------------
.. code-block::

  import("$dir_pw_build/target_types.gni")

  pw_source_set("my_library") {
    sources = [ "lib.cc" ]
  }

Pigweed defines wrappers around the four basic GN binary types ``source_set``,
``executable``, ``static_library``, and ``shared_library``. These wrappers apply
default arguments to each target, as defined in ``pw_build/default.gni``.
Arguments may be added or removed globally using the ``default_configs``,
``default_public_deps``, and ``remove_default_configs`` build args.
Additionally, arguments may be removed on a per-target basis with the
``remove_configs`` and ``remove_public_deps`` variables. These target types may
also be set to have restricted visibility by default via
``pw_build_DEFAULT_VISIBILITY`` for when projects want to selectively control
which Pigweed libraries are used and where.

The ``pw_executable`` template provides additional functionality around building
complete binaries. As Pigweed is a collection of libraries, it does not know how
its final targets are built. ``pw_executable`` solves this by letting each user
of Pigweed specify a global executable template for their target, and have
Pigweed build against it. This is controlled by the build variable
``pw_executable_config.target_type``, specifying the name of the executable
template for a project.

In some uncommon cases, a project's ``pw_executable`` template definition may
need to stamp out some ``pw_source_set``s. Since a pw_executable template can't
import ``$dir_pw_build/target_types.gni`` due to circular imports, it should
import ``$dir_pw_build/cc_library.gni`` instead.

Additionally ``pw_executable``, ``pw_source_set``, ``pw_static_library``, and
``pw_shared_library`` track source files via the ``pw_source_files`` field the
target's
`GN metadata <https://gn.googlesource.com/gn/+/main/docs/reference.md#metadata_collection>`_.
This list can be writen to a file at build time using ``generated_file``.  The
primary use case for this is to generate a token database containing all the
source files.  This allows PW_ASSERT to emit filename tokens even though it
can't add them to the elf file because of the resons described at
:ref:`module-pw_assert-assert-api`.

.. note::
  ``pw_source_files``, if not rebased will default to outputing module relative
  paths from a ``generated_file`` target.  This is likely not useful.  Adding
  a ``rebase`` argument to ``generated_file`` such as
  ``rebase = root_build_dir`` will result in usable paths.  For an example,
  see `//pw_tokenizer/database.gni`'s `pw_tokenizer_filename_database` template.

.. tip::

  Prefer to use ``pw_executable`` over plain ``executable`` targets to allow
  cleanly building the same code for multiple target configs.

**Arguments**

All of the ``pw_*`` target type overrides accept any arguments, as they simply
forward them through to the underlying target.

.. _module-pw_build-link-deps:

Link-only deps
--------------
It may be necessary to specify additional link-time dependencies that may not be
explicitly depended on elsewhere in the build. One example of this is a
``pw_assert`` backend, which may need to leave out dependencies to avoid
circular dependencies. Its dependencies need to be linked for executables and
libraries, even if they aren't pulled in elsewhere.

The ``pw_build_LINK_DEPS`` build arg is a list of dependencies to add to all
``pw_executable``, ``pw_static_library``, and ``pw_shared_library`` targets.
This should only be used as a last resort when dependencies cannot be properly
expressed in the build.

Python packages
---------------
GN templates for :ref:`Python build automation <docs-python-build>` are
described in :ref:`module-pw_build-python`.

.. toctree::
  :hidden:

  python


.. _module-pw_build-cc_blob_library:

pw_cc_blob_library
------------------
The ``pw_cc_blob_library`` template is useful for embedding binary data into a
program. The template takes in a mapping of symbol names to file paths, and
generates a set of C++ source and header files that embed the contents of the
passed-in files as arrays of ``std::byte``.

The blob byte arrays are constant initialized and are safe to access at any
time, including before ``main()``.

``pw_cc_blob_library`` is also available in the CMake build. It is provided by
``pw_build/cc_blob_library.cmake``.

**Arguments**

* ``blobs``: A list of GN scopes, where each scope corresponds to a binary blob
  to be transformed from file to byte array. This is a required field. Blob
  fields include:

  * ``symbol_name``: The C++ symbol for the byte array.
  * ``file_path``: The file path for the binary blob.
  * ``linker_section``: If present, places the byte array in the specified
    linker section.
  * ``alignas``: If present, uses the specified string or integer verbatim in
    the ``alignas()`` specifier for the byte array.

* ``out_header``: The header file to generate. Users will include this file
  exactly as it is written here to reference the byte arrays.
* ``namespace``: An optional (but highly recommended!) C++ namespace to place
  the generated blobs within.

Example
^^^^^^^

**BUILD.gn**

.. code-block::

  pw_cc_blob_library("foo_bar_blobs") {
    blobs: [
      {
        symbol_name: "kFooBlob"
        file_path: "${target_out_dir}/stuff/bin/foo.bin"
      },
      {
        symbol_name: "kBarBlob"
        file_path: "//stuff/bin/bar.bin"
        linker_section: ".bar_section"
      },
    ]
    out_header: "my/stuff/foo_bar_blobs.h"
    namespace: "my::stuff"
    deps = [ ":generate_foo_bin" ]
  }

.. note:: If the binary blobs are generated as part of the build, be sure to
          list them as deps to the pw_cc_blob_library target.

**Generated Header**

.. code-block::

  #pragma once

  #include <array>
  #include <cstddef>

  namespace my::stuff {

  extern const std::array<std::byte, 100> kFooBlob;

  extern const std::array<std::byte, 50> kBarBlob;

  }  // namespace my::stuff

**Generated Source**

.. code-block::

  #include "my/stuff/foo_bar_blobs.h"

  #include <array>
  #include <cstddef>

  #include "pw_preprocessor/compiler.h"

  namespace my::stuff {

  const std::array<std::byte, 100> kFooBlob = { ... };

  PW_PLACE_IN_SECTION(".bar_section")
  const std::array<std::byte, 50> kBarBlob = { ... };

  }  // namespace my::stuff

.. _module-pw_build-facade:

pw_facade
---------
In their simplest form, a :ref:`facade<docs-module-structure-facades>` is a GN
build arg used to change a dependency at compile time. Pigweed targets configure
these facades as needed.

The ``pw_facade`` template bundles a ``pw_source_set`` with a facade build arg.
This allows the facade to provide header files, compilation options or anything
else a GN ``source_set`` provides.

The ``pw_facade`` template declares two targets:

* ``$target_name``: the public-facing ``pw_source_set``, with a ``public_dep``
  on the backend
* ``$target_name.facade``: target used by the backend to avoid circular
  dependencies

.. code-block::

  # Declares ":foo" and ":foo.facade" GN targets
  pw_facade("foo") {
    backend = pw_log_BACKEND
    public_configs = [ ":public_include_path" ]
    public = [ "public/pw_foo/foo.h" ]
  }

Low-level facades like ``pw_assert`` cannot express all of their dependencies
due to the potential for dependency cycles. Facades with this issue may require
backends to place their implementations in a separate build target to be listed
in ``pw_build_LINK_DEPS`` (see :ref:`module-pw_build-link-deps`). The
``require_link_deps`` variable in ``pw_facade`` asserts that all specified build
targets are present in ``pw_build_LINK_DEPS`` if the facade's backend variable
is set.

.. _module-pw_build-python-action:

pw_python_action
----------------
The ``pw_python_action`` template is a convenience wrapper around GN's `action
function <https://gn.googlesource.com/gn/+/main/docs/reference.md#func_action>`_
for running Python scripts. The main benefit it provides is resolution of GN
target labels to compiled binary files. This allows Python scripts to be written
independently of GN, taking only filesystem paths as arguments.

Another convenience provided by the template is to allow running scripts without
any outputs. Sometimes scripts run in a build do not directly produce output
files, but GN requires that all actions have an output. ``pw_python_action``
solves this by accepting a boolean ``stamp`` argument which tells it to create a
placeholder output file for the action.

**Arguments**

``pw_python_action`` accepts all of the arguments of a regular ``action``
target. Additionally, it has some of its own arguments:

* ``module``: Run the specified Python module instead of a script. Either
  ``script`` or ``module`` must be specified, but not both.
* ``capture_output``: Optional boolean. If true, script output is hidden unless
  the script fails with an error. Defaults to true.
* ``stamp``: Optional variable indicating whether to automatically create a
  placeholder output file for the script. This allows running scripts without
  specifying ``outputs``. If ``stamp`` is true, a generic output file is
  used. If ``stamp`` is a file path, that file is used as a stamp file. Like any
  output file, ``stamp`` must be in the build directory. Defaults to false.
* ``environment``: Optional list of strings. Environment variables to set,
  passed as NAME=VALUE strings.
* ``working_directory``: Optional file path. When provided the current working
  directory will be set to this location before the Python module or script is
  run.
* ``venv``: Optional gn target of the pw_python_venv that should be used to run
  this action.

.. _module-pw_build-python-action-expressions:

Expressions
^^^^^^^^^^^

``pw_python_action`` evaluates expressions in ``args``, the arguments passed to
the script. These expressions function similarly to generator expressions in
CMake. Expressions may be passed as a standalone argument or as part of another
argument. A single argument may contain multiple expressions.

Generally, these expressions are used within templates rather than directly in
BUILD.gn files. This allows build code to use GN labels without having to worry
about converting them to files.

.. note::

  We intend to replace these expressions with native GN features when possible.
  See `b/234886742 <http://issuetracker.google.com/234886742>`_.

The following expressions are supported:

.. describe:: <TARGET_FILE(gn_target)>

  Evaluates to the output file of the provided GN target. For example, the
  expression

  .. code-block::

    "<TARGET_FILE(//foo/bar:static_lib)>"

  might expand to

  .. code-block::

    "/home/User/project_root/out/obj/foo/bar/static_lib.a"

  ``TARGET_FILE`` parses the ``.ninja`` file for the GN target, so it should
  always find the correct output file, regardless of the toolchain's or target's
  configuration. Some targets, such as ``source_set`` and ``group`` targets, do
  not have an output file, and attempting to use ``TARGET_FILE`` with them
  results in an error.

  ``TARGET_FILE`` only resolves GN target labels to their outputs. To resolve
  paths generally, use the standard GN approach of applying the
  ``rebase_path(path, root_build_dir)`` function. This function
  converts the provided GN path or list of paths to be relative to the build
  directory, from which all build commands and scripts are executed.

.. describe:: <TARGET_FILE_IF_EXISTS(gn_target)>

  ``TARGET_FILE_IF_EXISTS`` evaluates to the output file of the provided GN
  target, if the output file exists. If the output file does not exist, the
  entire argument that includes this expression is omitted, even if there is
  other text or another expression.

  For example, consider this expression:

  .. code-block::

    "--database=<TARGET_FILE_IF_EXISTS(//alpha/bravo)>"

  If the ``//alpha/bravo`` target file exists, this might expand to the
  following:

  .. code-block::

    "--database=/home/User/project/out/obj/alpha/bravo/bravo.elf"

  If the ``//alpha/bravo`` target file does not exist, the entire
  ``--database=`` argument is omitted from the script arguments.

.. describe:: <TARGET_OBJECTS(gn_target)>

  Evaluates to the object files of the provided GN target. Expands to a separate
  argument for each object file. If the target has no object files, the argument
  is omitted entirely. Because it does not expand to a single expression, the
  ``<TARGET_OBJECTS(...)>`` expression may not have leading or trailing text.

  For example, the expression

  .. code-block::

    "<TARGET_OBJECTS(//foo/bar:a_source_set)>"

  might expand to multiple separate arguments:

  .. code-block::

    "/home/User/project_root/out/obj/foo/bar/a_source_set.file_a.cc.o"
    "/home/User/project_root/out/obj/foo/bar/a_source_set.file_b.cc.o"
    "/home/User/project_root/out/obj/foo/bar/a_source_set.file_c.cc.o"

**Example**

.. code-block::

  import("$dir_pw_build/python_action.gni")

  pw_python_action("postprocess_main_image") {
    script = "py/postprocess_binary.py"
    args = [
      "--database",
      rebase_path("my/database.csv", root_build_dir),
      "--binary=<TARGET_FILE(//firmware/images:main)>",
    ]
    stamp = true
  }

.. _module-pw_build-evaluate-path-expressions:

pw_evaluate_path_expressions
----------------------------
It is not always feasible to pass information to a script through command line
arguments. If a script requires a large amount of input data, writing to a file
is often more convenient. However, doing so bypasses ``pw_python_action``'s GN
target label resolution, preventing such scripts from working with build
artifacts in a build system-agnostic manner.

``pw_evaluate_path_expressions`` is designed to address this use case. It takes
a list of input files and resolves target expressions within them, modifying the
files in-place.

Refer to ``pw_python_action``'s :ref:`module-pw_build-python-action-expressions`
section for the list of supported expressions.

.. note::

  ``pw_evaluate_path_expressions`` is typically used as an intermediate
  sub-target of a larger template, rather than a standalone build target.

**Arguments**

* ``files``: A list of file paths to process.

**Example**

The following template defines an executable target which additionally outputs
the list of object files from which it was compiled, making use of
``pw_evaluate_path_expressions`` to resolve their paths.

.. code-block::

  import("$dir_pw_build/evaluate_path_expressions.gni")

  template("executable_with_artifacts") {
    executable("${target_name}.exe") {
      sources = invoker.sources
      if defined(invoker.deps) {
        deps = invoker.deps
      }
    }

    _artifacts_file = "$target_gen_dir/${target_name}_artifacts.json"
    _artifacts = {
      binary = "<TARGET_FILE(:${target_name}.exe)>"
      objects = "<TARGET_OBJECTS(:${target_name}.exe)>"
    }
    write_file(_artifacts_file, _artifacts, "json")

    pw_evaluate_path_expressions("${target_name}.evaluate") {
      files = [ _artifacts_file ]
    }

    group(target_name) {
      deps = [
        ":${target_name}.exe",
        ":${target_name}.evaluate",
      ]
    }
  }

.. _module-pw_build-pw_exec:

pw_exec
-------
``pw_exec`` allows for execution of arbitrary programs. It is a wrapper around
``pw_python_action`` but allows for specifying the program to execute.

.. note:: Prefer to use ``pw_python_action`` instead of calling out to shell
  scripts, as the python will be more portable. ``pw_exec`` should generally
  only be used for interacting with legacy/existing scripts.

**Arguments**

* ``program``: The program to run. Can be a full path or just a name (in which
  case $PATH is searched).
* ``args``: Optional list of arguments to the program.
* ``deps``: Dependencies for this target.
* ``public_deps``: Public dependencies for this target. In addition to outputs
  from this target, outputs generated by public dependencies can be used as
  inputs from targets that depend on this one. This is not the case for private
  deps.
* ``inputs``: Optional list of build inputs to the program.
* ``outputs``: Optional list of artifacts produced by the program's execution.
* ``env``: Optional list of key-value pairs defining environment variables for
  the program.
* ``env_file``: Optional path to a file containing a list of newline-separated
  key-value pairs defining environment variables for the program.
* ``args_file``: Optional path to a file containing additional positional
  arguments to the program. Each line of the file is appended to the
  invocation. Useful for specifying arguments from GN metadata.
* ``skip_empty_args``: If args_file is provided, boolean indicating whether to
  skip running the program if the file is empty. Used to avoid running
  commands which error when called without arguments.
* ``capture_output``: If true, output from the program is hidden unless the
  program exits with an error. Defaults to true.
* ``working_directory``: The working directory to execute the subprocess with.
  If not specified it will not be set and the subprocess will have whatever
  the parent current working directory is.
* ``visibility``: GN visibility to apply to the underlying target.

**Example**

.. code-block::

  import("$dir_pw_build/exec.gni")

  pw_exec("hello_world") {
    program = "/bin/sh"
    args = [
      "-c",
      "echo hello \$WORLD",
    ]
    env = [
      "WORLD=world",
    ]
  }

pw_input_group
--------------
``pw_input_group`` defines a group of input files which are not directly
processed by the build but are still important dependencies of later build
steps. This is commonly used alongside metadata to propagate file dependencies
through the build graph and force rebuilds on file modifications.

For example ``pw_docgen`` defines a ``pw_doc_group`` template which outputs
metadata from a list of input files. The metadata file is not actually part of
the build, and so changes to any of the input files do not trigger a rebuild.
This is problematic, as targets that depend on the metadata should rebuild when
the inputs are modified but GN cannot express this dependency.

``pw_input_group`` solves this problem by allowing a list of files to be listed
in a target that does not output any build artifacts, causing all dependent
targets to correctly rebuild.

**Arguments**

``pw_input_group`` accepts all arguments that can be passed to a ``group``
target, as well as requiring one extra:

* ``inputs``: List of input files.

**Example**

.. code-block::

  import("$dir_pw_build/input_group.gni")

  pw_input_group("foo_metadata") {
    metadata = {
      files = [
        "x.foo",
        "y.foo",
        "z.foo",
      ]
    }
    inputs = metadata.files
  }

Targets depending on ``foo_metadata`` will rebuild when any of the ``.foo``
files are modified.

pw_zip
------
``pw_zip`` is a target that allows users to zip up a set of input files and
directories into a single output ``.zip`` file—a simple automation of a
potentially repetitive task.

**Arguments**

* ``inputs``: List of source files as well as the desired relative zip
  destination. See below for the input syntax.
* ``dirs``: List of entire directories to be zipped as well as the desired
  relative zip destination. See below for the input syntax.
* ``output``: Filename of output ``.zip`` file.
* ``deps``: List of dependencies for the target.

**Input Syntax**

Inputs all need to follow the correct syntax:

#. Path to source file or directory. Directories must end with a ``/``.
#. The delimiter (defaults to ``>``).
#. The desired destination of the contents within the ``.zip``. Must start
   with ``/`` to indicate the zip root. Any number of subdirectories are
   allowed. If the source is a file it can be put into any subdirectory of the
   root. If the source is a file, the zip copy can also be renamed by ending
   the zip destination with a filename (no trailing ``/``).

Thus, it should look like the following: ``"[source file or dir] > /"``.

**Example**

Let's say we have the following structure for a ``//source/`` directory:

.. code-block::

  source/
  ├── file1.txt
  ├── file2.txt
  ├── file3.txt
  └── some_dir/
      ├── file4.txt
      └── some_other_dir/
          └── file5.txt

And we create the following build target:

.. code-block::

  import("$dir_pw_build/zip.gni")

  pw_zip("target_name") {
    inputs = [
      "//source/file1.txt > /",             # Copied to the zip root dir.
      "//source/file2.txt > /renamed.txt",  # File renamed.
      "//source/file3.txt > /bar/",         # File moved to the /bar/ dir.
    ]

    dirs = [
      "//source/some_dir/ > /bar/some_dir/",  # All /some_dir/ contents copied
                                              # as /bar/some_dir/.
    ]

    # Note on output: if the specific output directory isn't defined
    # (such as output = "zoo.zip") then the .zip will output to the
    # same directory as the BUILD.gn file that called the target.
    output = "//$target_out_dir/foo.zip"  # Where the foo.zip will end up
  }

This will result in a ``.zip`` file called ``foo.zip`` stored in
``//$target_out_dir`` with the following structure:

.. code-block::

  foo.zip
  ├── bar/
  │   ├── file3.txt
  │   └── some_dir/
  │       ├── file4.txt
  │       └── some_other_dir/
  │           └── file5.txt
  ├── file1.txt
  └── renamed.txt

.. _module-pw_build-relative-source-file-names:

pw_relative_source_file_names
-----------------------------
This template recursively walks the listed dependencies and collects the names
of all the headers and source files required by the targets, and then transforms
them such that they reflect the ``__FILE__`` when pw_build's ``relative_paths``
config is applied. This is primarily intended for side-band generation of
pw_tokenizer tokens so file name tokens can be utilized in places where
pw_tokenizer is unable to embed token information as part of C/C++ compilation.

This template produces a JSON file containing an array of strings (file paths
with ``-ffile-prefix-map``-like transformations applied) that can be used to
:ref:`generate a token database <module-pw_tokenizer-database-creation>`.

**Arguments**

* ``deps``: A required list of targets to recursively extract file names from.
* ``outputs``: A required array with a single element: the path to write the
  final JSON file to.

**Example**

Let's say we have the following project structure:

.. code-block::

  project root
  ├── foo/
  │   ├── foo.h
  │   └── foo.cc
  ├── bar/
  │   ├── bar.h
  │   └── bar.cc
  ├── unused/
  │   ├── unused.h
  │   └── unused.cc
  └── main.cc

And a BUILD.gn at the root:

.. code-block::

  pw_source_set("bar") {
    public_configs = [ ":bar_headers" ]
    public = [ "bar/bar.h" ]
    sources = [ "bar/bar.cc" ]
  }

  pw_source_set("foo") {
    public_configs = [ ":foo_headers" ]
    public = [ "foo/foo.h" ]
    sources = [ "foo/foo.cc" ]
    deps = [ ":bar" ]
  }


  pw_source_set("unused") {
    public_configs = [ ":unused_headers" ]
    public = [ "unused/unused.h" ]
    sources = [ "unused/unused.cc" ]
    deps = [ ":bar" ]
  }

  pw_executable("main") {
    sources = [ "main.cc" ]
    deps = [ ":foo" ]
  }

  pw_relative_source_file_names("main_source_files") {
    deps = [ ":main" ]
    outputs = [ "$target_gen_dir/main_source_files.json" ]
  }

The json file written to `out/gen/main_source_files.json` will contain:

.. code-block::

  [
    "bar/bar.cc",
    "bar/bar.h",
    "foo/foo.cc",
    "foo/foo.h",
    "main.cc"
  ]

Since ``unused`` isn't a transitive dependency of ``main``, its source files
are not included. Similarly, even though ``bar`` is not a direct dependency of
``main``, its source files *are* included because ``foo`` brings in ``bar`` as
a transitive dependency.

Note how the file paths in this example are relative to the project root rather
than being absolute paths (e.g. ``/home/user/ralph/coding/my_proj/main.cc``).
This is a result of transformations applied to strip absolute pathing prefixes,
matching the behavior of pw_build's ``$dir_pw_build:relative_paths`` config.

Build time errors: pw_error and pw_build_assert
-----------------------------------------------
In Pigweed's complex, multi-toolchain GN build it is not possible to build every
target in every configuration. GN's ``assert`` statement is not ideal for
enforcing the correct configuration because it may prevent the GN build files or
targets from being referred to at all, even if they aren't used.

The ``pw_error`` GN template results in an error if it is executed during the
build. These error targets can exist in the build graph, but cannot be depended
on without an error.

``pw_build_assert`` evaluates to a ``pw_error`` if a condition fails or nothing
(an empty group) if the condition passes. Targets can add a dependency on a
``pw_build_assert`` to enforce a condition at build time.

The templates for build time errors are defined in ``pw_build/error.gni``.

CMake
=====
Pigweed's `CMake`_ support is provided primarily for projects that have an
existing CMake build and wish to integrate Pigweed without switching to a new
build system.

The following command generates Ninja build files for a host build in the
``out/cmake_host`` directory:

.. code-block:: sh

  cmake -B out/cmake_host -S "$PW_ROOT" -G Ninja -DCMAKE_TOOLCHAIN_FILE=$PW_ROOT/pw_toolchain/host_clang/toolchain.cmake

The ``PW_ROOT`` environment variable must point to the root of the Pigweed
directory. This variable is set by Pigweed's environment setup.

Tests can be executed with the ``pw_run_tests.GROUP`` targets. To run Pigweed
module tests, execute ``pw_run_tests.modules``:

.. code-block:: sh

  ninja -C out/cmake_host pw_run_tests.modules

:ref:`module-pw_watch` supports CMake, so you can also run

.. code-block:: sh

  pw watch -C out/cmake_host pw_run_tests.modules

CMake functions
---------------
CMake convenience functions are defined in ``pw_build/pigweed.cmake``.

* ``pw_add_library`` -- The base helper used to instantiate CMake libraries.
  This is meant for use in downstream projects as upstream Pigweed modules are
  expected to use ``pw_add_module_library`` and ``pw_add_facade``.
* ``pw_add_module_library`` -- Add an upstream Pigweed library.
* ``pw_add_facade`` -- Declare a module facade.
* ``pw_set_backend`` -- Set the backend library to use for a facade.
* ``pw_auto_add_simple_module`` -- For modules with only one library,
  automatically declare the library and its tests. This has been deprecated,
  please use ``pw_add_module_library`` instead.
* ``pw_add_test`` -- Declare a test target.
* ``pw_target_link_targets`` -- Helper wrapper around ``target_link_libraries``
  which only supports CMake targets and detects when the target does not exist.
  Note that generator expressions are not supported.
* ``pw_add_global_compile_options`` -- Applies compilation options to all
  targets in the build. This should only be used to add essential compilation
  options, such as those that affect the ABI. Use ``pw_add_library`` or
  ``target_compile_options`` to apply other compile options.

See ``pw_build/pigweed.cmake`` for the complete documentation of these
functions.

Special libraries that do not fit well with these functions are created with the
standard CMake functions, such as ``add_library`` and ``target_link_libraries``.

Facades and backends
--------------------
The CMake build uses CMake cache variables for configuring
:ref:`facades<docs-module-structure-facades>` and backends. Cache variables are
similar to GN's build args set with ``gn args``. Unlike GN, CMake does not
support multi-toolchain builds, so these variables have a single global value
per build directory.

The ``pw_add_facade`` function declares a cache variable named
``<module_name>_BACKEND`` for each facade. Cache variables can be awkward to
work with, since their values only change when they're assigned, but then
persist accross CMake invocations. These variables should be set in one of the
following ways:

* Call ``pw_set_backend`` to set backends appropriate for the target in the
  target's toolchain file. The toolchain file is provided to ``cmake`` with
  ``-DCMAKE_TOOLCHAIN_FILE=<toolchain file>``.
* Call ``pw_set_backend`` in the top-level ``CMakeLists.txt`` before other
  CMake code executes.
* Set the backend variable at the command line with the ``-D`` option.

  .. code-block:: sh

    cmake -B out/cmake_host -S "$PW_ROOT" -G Ninja \
        -DCMAKE_TOOLCHAIN_FILE=$PW_ROOT/pw_toolchain/host_clang/toolchain.cmake \
        -Dpw_log_BACKEND=pw_log_basic

* Temporarily override a backend by setting it interactively with ``ccmake`` or
  ``cmake-gui``.

If the backend is set to a build target that does not exist, there will be an
error message like the following:

.. code-block::

  CMake Error at pw_build/pigweed.cmake:244 (add_custom_target):
  Error evaluating generator expression:

    $<TARGET_PROPERTY:my_backend_that_does_not_exist,TYPE>

  Target "my_backend_that_does_not_exist" not found.

Toolchain setup
---------------
In CMake, the toolchain is configured by setting CMake variables, as described
in the `CMake documentation <https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html>`_.
These variables are typically set in a toolchain CMake file passed to ``cmake``
with the ``-D`` option (``-DCMAKE_TOOLCHAIN_FILE=path/to/file.cmake``).
For Pigweed embedded builds, set ``CMAKE_SYSTEM_NAME`` to the empty string
(``""``).

Toolchains may set the ``pw_build_WARNINGS`` variable to a list of ``INTERFACE``
libraries with compilation options for Pigweed's upstream libraries. This
defaults to a strict set of warnings. Projects may need to use less strict
compilation warnings to compile backends exposed to Pigweed code (such as
``pw_log``) that cannot compile with Pigweed's flags. If desired, Projects can
access these warnings by depending on ``pw_build.warnings``.

Third party libraries
---------------------
The CMake build includes third-party libraries similarly to the GN build. A
``dir_pw_third_party_<library>`` cache variable is defined for each third-party
dependency. The variable must be set to the absolute path of the library in
order to use it. If the variable is empty
(``if("${dir_pw_third_party_<library>}" STREQUAL "")``), the dependency is not
available.

Third-party dependencies are not automatically added to the build. They can be
manually added with ``add_subdirectory`` or by setting the
``pw_third_party_<library>_ADD_SUBDIRECTORY`` option to ``ON``.

Third party variables are set like any other cache global variable in CMake. It
is recommended to set these in one of the following ways:

* Set with the CMake ``set`` function in the toolchain file or a
  ``CMakeLists.txt`` before other CMake code executes.

  .. code-block:: cmake

    set(dir_pw_third_party_nanopb ${CMAKE_CURRENT_SOURCE_DIR}/external/nanopb CACHE PATH "" FORCE)

* Set the variable at the command line with the ``-D`` option.

  .. code-block:: sh

    cmake -B out/cmake_host -S "$PW_ROOT" -G Ninja \
        -DCMAKE_TOOLCHAIN_FILE=$PW_ROOT/pw_toolchain/host_clang/toolchain.cmake \
        -Ddir_pw_third_party_nanopb=/path/to/nanopb

* Set the variable interactively with ``ccmake`` or ``cmake-gui``.

Use Pigweed from an existing CMake project
------------------------------------------
To use Pigweed libraries form a CMake-based project, simply include the Pigweed
repository from a ``CMakeLists.txt``.

.. code-block:: cmake

  add_subdirectory(path/to/pigweed pigweed)

All module libraries will be available as ``module_name`` or
``module_name.sublibrary``.

If desired, modules can be included individually.

.. code-block:: cmake

  add_subdirectory(path/to/pigweed/pw_some_module pw_some_module)
  add_subdirectory(path/to/pigweed/pw_another_module pw_another_module)

Bazel
=====
Bazel is currently very experimental, and only builds for host and ARM Cortex-M
microcontrollers.

The common configuration for Bazel for all modules is in the ``pigweed.bzl``
file. The built-in Bazel rules ``cc_binary``, ``cc_library``, and ``cc_test``
are wrapped with ``pw_cc_binary``, ``pw_cc_library``, and ``pw_cc_test``.
These wrappers add parameters to calls to the compiler and linker.

In addition to wrapping the built-in rules, Pigweed also provides a custom
rule for handling linker scripts with Bazel. e.g.

.. code-block:: python

  pw_linker_script(
    name = "some_linker_script",
    linker_script = ":some_configurable_linker_script.ld",
    defines = [
        "PW_BOOT_FLASH_BEGIN=0x08000200",
        "PW_BOOT_FLASH_SIZE=1024K",
        "PW_BOOT_HEAP_SIZE=112K",
        "PW_BOOT_MIN_STACK_SIZE=1K",
        "PW_BOOT_RAM_BEGIN=0x20000000",
        "PW_BOOT_RAM_SIZE=192K",
        "PW_BOOT_VECTOR_TABLE_BEGIN=0x08000000",
        "PW_BOOT_VECTOR_TABLE_SIZE=512",
    ],
  )

  pw_cc_binary(
    name = "some_binary",
    srcs = ["some_source.c"],
    additional_linker_inputs = [":some_linker_script"],
    linkopts = ["-T $(location :some_linker_script)"],
  )

Currently Pigweed is making use of a set of
`open source <https://github.com/silvergasp/bazel-embedded>`_ toolchains. The
host builds are only supported on Linux/Mac based systems. Additionally the
host builds are not entirely hermetic, and will make use of system
libraries and headers. This is close to the default configuration for Bazel,
though slightly more hermetic. The host toolchain is based around clang-11 which
has a system dependency on 'libtinfo.so.5' which is often included as part of
the libncurses packages. On Debian based systems this can be installed using the
command below:

.. code-block:: sh

  sudo apt install libncurses5

The host toolchain does not currently support native Windows, though using WSL
is a viable alternative.

The ARM Cortex-M Bazel toolchains are based around gcc-arm-non-eabi and are
entirely hermetic. You can target Cortex-M, by using the platforms command line
option. This set of toolchains is supported from hosts; Windows, Mac and Linux.
The platforms that are currently supported are listed below:

.. code-block:: sh

  bazel build //:your_target --platforms=@pigweed//pw_build/platforms:cortex_m0
  bazel build //:your_target --platforms=@pigweed//pw_build/platforms:cortex_m1
  bazel build //:your_target --platforms=@pigweed//pw_build/platforms:cortex_m3
  bazel build //:your_target --platforms=@pigweed//pw_build/platforms:cortex_m4
  bazel build //:your_target --platforms=@pigweed//pw_build/platforms:cortex_m7
  bazel build //:your_target \
    --platforms=@pigweed//pw_build/platforms:cortex_m4_fpu
  bazel build //:your_target \
    --platforms=@pigweed//pw_build/platforms:cortex_m7_fpu


The above examples are cpu/fpu oriented platforms and can be used where
applicable for your application. There some more specific platforms for the
types of boards that are included as examples in Pigweed. It is strongly
encouraged that you create your own set of platforms specific for your project,
that implement the constraint_settings in this repository. e.g.

New board constraint_value:

.. code-block:: python

  #your_repo/build_settings/constraints/board/BUILD
  constraint_value(
    name = "nucleo_l432kc",
    constraint_setting = "@pigweed//pw_build/constraints/board",
  )

New chipset constraint_value:

.. code-block:: python

  # your_repo/build_settings/constraints/chipset/BUILD
  constraint_value(
    name = "stm32l432kc",
    constraint_setting = "@pigweed//pw_build/constraints/chipset",
  )

New platforms for chipset and board:

.. code-block:: python

  #your_repo/build_settings/platforms/BUILD
  # Works with all stm32l432kc
  platforms(
    name = "stm32l432kc",
    parents = ["@pigweed//pw_build/platforms:cortex_m4"],
    constraint_values =
      ["@your_repo//build_settings/constraints/chipset:stm32l432kc"],
  )

  # Works with only the nucleo_l432kc
  platforms(
    name = "nucleo_l432kc",
    parents = [":stm32l432kc"],
    constraint_values =
      ["@your_repo//build_settings/constraints/board:nucleo_l432kc"],
  )

In the above example you can build your code with the command line:

.. code-block:: python

  bazel build //:your_target_for_nucleo_l432kc \
    --platforms=@your_repo//build_settings:nucleo_l432kc


You can also specify that a specific target is only compatible with one
platform:

.. code-block:: python

  cc_library(
    name = "compatible_with_all_stm32l432kc",
    srcs = ["tomato_src.c"],
    target_compatible_with =
      ["@your_repo//build_settings/constraints/chipset:stm32l432kc"],
  )

  cc_library(
    name = "compatible_with_only_nucleo_l432kc",
    srcs = ["bbq_src.c"],
    target_compatible_with =
      ["@your_repo//build_settings/constraints/board:nucleo_l432kc"],
  )