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.. _module-pw_build-bazel:
Bazel
=====
Bazel is currently very experimental, and only builds for host and ARM Cortex-M
microcontrollers.
.. _module-pw_build-bazel-wrapper-rules:
Wrapper rules
-------------
The common configuration for Bazel for all modules is in the ``pigweed.bzl``
file. The built-in Bazel rules ``cc_binary``, ``cc_test`` are wrapped with
``pw_cc_binary`` and ``pw_cc_test``.
.. _module-pw_build-bazel-pw_linker_script:
pw_linker_script
----------------
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",
],
deps = [":some_header_library"],
)
# You can include headers provided by targets specified in deps.
cc_library(
name = "some_header_library",
hdrs = ["test_header.h"],
includes = ["."],
)
# You can include the linker script in the deps.
cc_binary(
name = "some_binary",
srcs = ["some_source.cc"],
deps = [":some_linker_script"],
)
# Alternatively, you can use additional_linker_inputs and linkopts. This
# allows you to explicitly specify the command line order of linker scripts,
# and may be useful if your project defines more than one.
cc_binary(
name = "some_binary",
srcs = ["some_source.cc"],
additional_linker_inputs = [":some_linker_script"],
linkopts = ["-T $(location :some_linker_script)"],
)
.. _module-pw_build-bazel-pw_facade:
pw_facade
---------
In Bazel, a :ref:`facade <docs-module-structure-facades>` module has a few
components:
#. The **facade target**, i.e. the interface to the module. This is what
*backend implementations* depend on to know what interface they're supposed
to implement.
#. The **library target**, i.e. both the facade (interface) and backend
(implementation). This is what *users of the module* depend on. It's a
regular ``cc_library`` that exposes the same headers as the facade, but
has a dependency on the "backend label flag" (discussed next). It may also
include some source files (if these are backend-independent).
Both the facade and library targets are created using the
``pw_facade`` macro. For example, consider the following
macro invocation:
.. code-block:: python
pw_facade(
name = "binary_semaphore",
# A backend-independent source file.
srcs = [
"binary_semaphore.cc",
],
# The facade header.
hdrs = [
"public/pw_sync/binary_semaphore.h",
],
# Dependencies of this header.
deps = [
"//pw_chrono:system_clock",
"//pw_preprocessor",
],
# The backend, hidden behind a label_flag; see below.
backend = [
":binary_semaphore_backend",
],
)
This macro expands to both the library target, named ``binary_semaphore``,
and the facade target, named ``binary_semaphore.facade``.
#. The **backend label flag**. This is a `label_flag
<https://bazel.build/extending/config#label-typed-build-settings>`_: a
dependency edge in the build graph that can be overridden by downstream projects.
#. The **backend target** implements a particular backend for a facade. It's
just a plain ``cc_library``, with a dependency on the facade target. For example,
.. code-block:: python
cc_library(
name = "binary_semaphore",
srcs = [
"binary_semaphore.cc",
],
hdrs = [
"public/pw_sync_stl/binary_semaphore_inline.h",
"public/pw_sync_stl/binary_semaphore_native.h",
"public_overrides/pw_sync_backend/binary_semaphore_inline.h",
"public_overrides/pw_sync_backend/binary_semaphore_native.h",
],
includes = [
"public",
"public_overrides",
],
deps = [
# Dependencies of the backend's headers and sources.
"//pw_assert",
"//pw_chrono:system_clock",
# A dependency on the facade target, which defines the interface
# this backend target implements.
"//pw_sync:binary_semaphore.facade",
],
)
If a project uses only one backend for a given facade, the backend label
flag should point at that backend target.
#. The **facade constraint setting** and **backend constraint values**. Every
facade has an associated `constraint setting
<https://bazel.build/concepts/platforms#api-review>`_ (enum used in platform
definition), and each backend for this facade has an associated
``constraint_value`` (enum value). Example:
.. code-block:: python
# //pw_sync/BUILD.bazel
constraint_setting(
name = "binary_semaphore_backend_constraint_setting",
)
# //pw_sync_stl/BUILD.bazel
constraint_value(
name = "binary_semaphore_backend",
constraint_setting = "//pw_sync:binary_semaphore_backend_constraint_setting",
)
# //pw_sync_freertos/BUILD.bazel
constraint_value(
name = "binary_semaphore_backend",
constraint_setting = "//pw_sync:binary_semaphore_backend_constraint_setting",
)
`Target platforms <https://bazel.build/extending/platforms>`_ for Pigweed
projects should indicate which backend they select for each facade by
listing the corresponding ``constraint_value`` in their definition. This can
be used in a couple of ways:
#. It allows projects to switch between multiple backends based only on the
`target platform <https://bazel.build/extending/platforms>`_ using a
*backend multiplexer* (see below) instead of setting label flags in
their ``.bazelrc``.
#. It allows tests or libraries that only support a particular backend to
express this through the `target_compatible_with
<https://bazel.build/reference/be/common-definitions#common.target_compatible_with>`_
attribute. Bazel will use this to `automatically skip incompatible
targets in wildcard builds
<https://bazel.build/extending/platforms#skipping-incompatible-targets>`_.
#. The **backend multiplexer**. If a project uses more than one backend for a
given facade (e.g., it uses different backends for host and embedded target
builds), the backend label flag will point to a target that resolves to the
correct backend based on the target platform. This will typically be an
`alias <https://bazel.build/reference/be/general#alias>`_ with a ``select``
statement mapping constraint values to the appropriate backend targets. For
example,
.. code-block:: python
alias(
name = "pw_sync_binary_semaphore_backend_multiplexer",
actual = select({
"//pw_sync_stl:binary_semaphore_backend": "@pigweed//pw_sync_stl:binary_semaphore",
"//pw_sync_freertos:binary_semaphore_backend": "@pigweed//pw_sync_freertos:binary_semaphore_backend",
# If we're building for a host OS, use the STL backend.
"@platforms//os:macos": "@pigweed//pw_sync_stl:binary_semaphore",
"@platforms//os:linux": "@pigweed//pw_sync_stl:binary_semaphore",
"@platforms//os:windows": "@pigweed//pw_sync_stl:binary_semaphore",
# Unless the target platform is the host platform, it must
# explicitly specify which backend to use. The unspecified_backend
# is not compatible with any platform; taking this branch will produce
# an informative error.
"//conditions:default": "@pigweed//pw_build:unspecified_backend",
}),
)
pw_cc_blob_library
------------------
The ``pw_cc_blob_library`` rule is useful for embedding binary data into a
program. The rule 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 :ref:`GN <module-pw_build-cc_blob_library>`
and CMake builds.
Arguments
^^^^^^^^^
* ``blobs``: A list of ``pw_cc_blob_info`` targets, where each target
corresponds to a binary blob to be transformed from file to byte array. This
is a required field. ``pw_cc_blob_info`` attributes 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 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``: C++ namespace to place the generated blobs within.
* ``alwayslink``: Whether this library should always be linked. Defaults to false.
Example
^^^^^^^
**BUILD.bazel**
.. code-block::
pw_cc_blob_info(
name = "foo_blob",
file_path = "foo.bin",
symbol_name = "kFooBlob",
)
pw_cc_blob_info(
name = "bar_blob",
file_path = "bar.bin",
symbol_name = "kBarBlob",
linker_section = ".bar_section",
)
pw_cc_blob_library(
name = "foo_bar_blobs",
blobs = [
":foo_blob",
":bar_blob",
],
out_header = "my/stuff/foo_bar_blobs.h",
namespace = "my::stuff",
)
.. 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-bazel-pw_cc_binary_with_map:
pw_cc_binary_with_map
---------------------
The ``pw_cc_binary_with_map`` rule can be used to build a binary like
``cc_binary`` does but also generate a .map file from the linking step.
.. code-block::
pw_cc_binary_with_map(
name = "test",
srcs = ["empty_main.cc"],
)
This should result in a ``test.map`` file generated next to the ``test`` binary.
Note that it's only partially compatible with the ``cc_binary`` interface and
certain things are not implemented like make variable substitution.
Miscellaneous utilities
-----------------------
.. _module-pw_build-bazel-empty_cc_library:
empty_cc_library
^^^^^^^^^^^^^^^^
This empty library is used as a placeholder for label flags that need to point
to a library of some kind, but don't actually need the dependency to amount to
anything.
default_link_extra_lib
^^^^^^^^^^^^^^^^^^^^^^
This library groups together all libraries commonly required at link time by
Pigweed modules. See :ref:`docs-build_system-bazel_link-extra-lib` for more
details.
unspecified_backend
^^^^^^^^^^^^^^^^^^^
A special target used instead of a cc_library as the default condition in
backend multiplexer select statements to signal that a facade is in an
unconfigured state. This produces better error messages than e.g. using an
invalid label.
Toolchains and platforms
------------------------
Pigweed provides clang-based host toolchains for Linux and Mac Arm gcc
toolchain. The clang-based Linux and Arm gcc toolchains are entirely hermetic.
We don't currently provide a host toolchain for Windows.