Bazel#

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

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.

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",
  ],
)

# 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)"],
)

pw_cc_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. The facade is declared by creating a pw_cc_facade target, which is just a thin wrapper for cc_library. For example,

    pw_cc_facade(
        name = "binary_semaphore_facade",
        # The header that constitues the facade.
        hdrs = [
            "public/pw_sync/binary_semaphore.h",
        ],
        includes = ["public"],
        # Dependencies of this header.
        deps = [
            "//pw_chrono:system_clock",
            "//pw_preprocessor",
        ],
    )
    

    Note

    As pure interfaces, pw_cc_facade targets should not include any source files. Backend-independent source files should be placed in the “library target” instead.

  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). For example,

    cc_library(
        name = "binary_semaphore",
        # A backend-independent source file.
        srcs = [
            "binary_semaphore.cc",
        ],
        # The same header as exposed by the facade.
        hdrs = [
            "public/pw_sync/binary_semaphore.h",
        ],
        deps = [
            # Dependencies of this header
            "//pw_chrono:system_clock",
            "//pw_preprocessor",
            # The backend, hidden behind a label_flag.
            "@pigweed//targets:pw_sync_binary_semaphore_backend",
        ],
    )
    

    Note

    You may be tempted to reduce duplication in the BUILD.bazel files and simply add the facade target to the deps of the library target, instead of re-declaring the facade’s hdrs and deps. Do not do this! It’s a layering check violation: the facade headers provide the module’s interface, and should be directly exposed by the target the users depend on.

  3. The backend label flag. This is a label_flag: a dependency edge in the build graph that can be overridden by downstream projects. For facades defined in upstream Pigweed, the label_flags are collected in //targets/BUILD.bazel.

  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",
        ],
    )
    

    If a project uses only one backend for a given facade, the backend label flag should point at that backend target.

  5. The facade constraint setting and backend constraint values. Every facade has an associated constraint setting (enum used in platform definition), and each backend for this facade has an associated constraint_value (enum value). Example:

    # //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 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:

    1. It allows projects to switch between multiple backends based only on the target platform using a backend multiplexer (see below) instead of setting label flags in their .bazelrc.

    2. It allows tests or libraries that only support a particular backend to express this through the target_compatible_with attribute. Bazel will use this to automatically skip incompatible targets in wildcard builds.

  6. 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 with a select statement mapping constraint values to the appropriate backend targets. For example,

    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 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.

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_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.

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.

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.