Bazel

pw_build: Integrations for Bazel, GN, and CMake

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

Wrapper rules

The built-in Bazel rules cc_binary, cc_test, py_binary, and py_test are wrapped with pw_cc_binary, pw_cc_test, pw_py_binary, and pw_py_test, respectively. To access a wrapper, load its individual .bzl file. For example, to access pw_cc_binary:

load("@pigweed//pw_build:pw_cc_binary.bzl", "pw_cc_binary")

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.

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 $(execpath :some_linker_script)"],
)

pw_facade

In Bazel, a facade module has a few components:

1. 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.

2. 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:

   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.

3. The backend label flag. This is a label_flag: a dependency edge in the build graph that can be overridden by downstream projects.

4. 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,

   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",
       ],
   )
   

The backend label flag should point at the backend target. Typically, the backend you want to use depends on the platform you are building for. See the Facades and backends tutorial for advice on how to set this up.

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 GN 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

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

#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

#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

pw_cc_binary

pw_cc_binary is a wrapper of cc_binary. It’s implemented at //pw_build/pw_cc_binary.bzl. Usage of this wrapper is optional; downstream Pigweed projects can instead use cc_binary if preferred.

Basic usage:

load("@pigweed//pw_build:pw_cc_binary.bzl", "pw_cc_binary")

pw_cc_binary(
    name = "…",
    srcs = ["…"],
    deps = [
        "…",
    ],
)

Pros of using pw_cc_binary:

• It simplifies link-time dependency. Projects using cc_binary must set up (and document) link-time dependency themselves.

Cons of using pw_cc_binary:

• It makes the configuration of pw_log and pw_assert a bit more magical.

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.

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.

pw_elf_to_bin

The pw_elf_to_bin rule takes in a binary executable target and produces a file using the -Obinary option to objcopy. This is only suitable for use with binaries where all the segments are non-overlapping. A common use case for this type of file is booting directly on hardware with no bootloader.

load("@pigweed//pw_build:binary_tools.bzl", "pw_elf_to_bin")

pw_elf_to_bin(
  name = "bin",
  elf_input = ":main",
  bin_out = "main.bin",
)

pw_elf_to_dump

The pw_elf_to_dump rule takes in a binary executable target and produces a text file containing the output of the toolchain’s objdump -xd command. This contains the full binary layout, symbol table and disassembly which is often useful when debugging embedded firmware.

load("@pigweed//pw_build:binary_tools.bzl", "pw_elf_to_dump")

pw_elf_to_dump(
  name = "dump",
  elf_input = ":main",
  dump_out = "main.dump",
)

pw_copy_and_patch_file

Provides the ability to patch a file as part of the build.

The source file will not be patched in place, but instead copied before patching. The output of this target will be the patched file.

Arguments

• name: The name of the target.

• source: The source file to be patched.

• out: The output file containing the patched contents.

• patch_file: The patch file.

Example

To apply the patch changes.patch to the file data/file.txt which is located in the bazel dependency @external-sdk//.

pw_copy_and_patch_file(
    name = "apply_patch",
    src = "@external-sdk//data/file.txt",
    out = "data/patched_file.txt",
    patch_file = "changes.patch",
)

Platform compatibility rules

Macros and rules related to platform compatibility are provided in //pw_build:compatibility.bzl.

boolean_constraint_value

This macro is syntactic sugar for declaring a constraint setting with just two possible constraint values. The only exposed target is the constraint_value corresponding to True; the default value of the setting is False.

This macro is meant to simplify declaring Module-specific constraints.

host_backend_alias

An alias that resolves to the backend for host platforms. This is useful when declaring a facade that provides a default backend for host platform use.

Flag merging rules

Macros that help with using platform-based flags are in //pw_build:merge_flags.bzl. These are useful, for example, when you wish to Provide default backend collections as dicts.

Miscellaneous utilities

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 Libraries required at linktime 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.

glob_dirs

A starlark helper for performing glob() operations strictly on directories.

glob_dirs(include: List[str], exclude: List[str] = [], allow_empty: bool = True) → List[str]

Matches the provided glob pattern to identify a list of directories.

This helper follows the same semantics as Bazel’s native glob() function, but only matches directories.

Args:

include: A list of wildcard patterns to match against. exclude: A list of wildcard patterns to exclude. allow_empty: Whether or not to permit an empty list of matches.

Returns:

List of directory paths that match the specified constraints.

match_dir

A starlark helper for using a wildcard pattern to match a single directory

match_dir(include: List[str], exclude: List[str] = [], allow_empty: bool = True) → str | None

Identifies a single directory using a wildcard pattern.

This helper follows the same semantics as Bazel’s native glob() function, but only matches a single directory. If more than one match is found, this will fail.

Args:

include: A list of wildcard patterns to match against. exclude: A list of wildcard patterns to exclude. allow_empty: Whether or not to permit returning None.

Returns:

Path to a single directory that matches the specified constraints, or None if no match is found and allow_empty is True.

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.