1.. _docs-module-structure: 2 3---------------- 4Module Structure 5---------------- 6The Pigweed module structure is designed to keep as much code as possible for a 7particular slice of functionality in one place. That means including the code 8from multiple languages, as well as all the related documentation and tests. 9 10Additionally, the structure is designed to limit the number of places a file 11could go, so that when reading callsites it is obvious where a header is from. 12That is where the duplicated ``<module>`` occurrences in file paths comes from. 13 14Example module structure 15------------------------ 16.. code-block:: python 17 18 pw_foo/... 19 20 docs.rst # If there is just 1 docs file, call it docs.rst 21 README.md # All modules must have a short README for gittiles 22 23 BUILD.gn # GN build required 24 BUILD # Bazel build required 25 26 # C++ public headers; the repeated module name is required 27 public/pw_foo/foo.h 28 public/pw_foo/baz.h 29 30 # Exposed private headers go under internal/ 31 public/pw_foo/internal/bar.h 32 public/pw_foo/internal/qux.h 33 34 # Public override headers must go in 'public_overrides' 35 public_overrides/gtest/gtest.h 36 public_overrides/string.h 37 38 # Private headers go into <module>_*/... 39 pw_foo_internal/zap.h 40 pw_foo_private/zip.h 41 pw_foo_secret/alxx.h 42 43 # C++ implementations go in the root 44 foo_impl.cc 45 foo.cc 46 baz.cc 47 bar.cc 48 zap.cc 49 zip.cc 50 alxx.cc 51 52 # C++ tests also go in the root 53 foo_test.cc 54 bar_test.cc 55 zip_test.cc 56 57 # Python files go into 'py/<module>/...' 58 py/BUILD.gn # Python packages are declared in GN using pw_python_package 59 py/setup.py # Python files are structured as standard Python packages 60 py/foo_test.py # Tests go in py/ but outside of the Python package 61 py/bar_test.py 62 py/pw_foo/__init__.py 63 py/pw_foo/__main__.py 64 py/pw_foo/bar.py 65 py/pw_foo/py.typed # Indicates that this package has type annotations 66 67 # Go files go into 'go/...' 68 go/... 69 70 # Examples go in examples/, mixing different languages 71 examples/demo.py 72 examples/demo.cc 73 examples/demo.go 74 examples/BUILD.gn 75 examples/BUILD 76 77 # Size reports go under size_report/ 78 size_report/BUILD.gn 79 size_report/base.cc 80 size_report/use_case_a.cc 81 size_report/use_case_b.cc 82 83 # Protobuf definition files go into <module>_protos/... 84 pw_foo_protos/foo.proto 85 pw_foo_protos/internal/zap.proto 86 87 # Other directories are fine, but should be private. 88 data/... 89 graphics/... 90 collection_of_tests/... 91 code_relating_to_subfeature/... 92 93Module name 94----------- 95Pigweed upstream modules are always named with a prefix ``pw_`` to enforce 96namespacing. Projects using Pigweed that wish to make their own modules can use 97whatever name they like, but we suggest picking a short prefix to namespace 98your product (e.g. for an Internet of Toast project, perhaps the prefix could 99be ``it_``). 100 101C++ module structure 102-------------------- 103 104C++ public headers 105~~~~~~~~~~~~~~~~~~ 106Located ``{pw_module_dir}/public/<module>``. These are headers that must be 107exposed due to C++ limitations (i.e. are included from the public interface, 108but are not intended for public use). 109 110**Public headers** should take the form: 111 112``{pw_module_dir}/public/<module>/*.h`` 113 114**Exposed private headers** should take the form: 115 116``{pw_module_dir}/public/<module>/internal/*.h`` 117 118Examples: 119 120.. code-block:: 121 122 pw_foo/... 123 public/pw_foo/foo.h 124 public/pw_foo/a_header.h 125 public/pw_foo/baz.h 126 127For headers that must be exposed due to C++ limitations (i.e. are included from 128the public interface, but are not intended for use), place the headers in a 129``internal`` subfolder under the public headers directory; as 130``{pw_module_dir}/public/<module>/internal/*.h``. For example: 131 132.. code-block:: 133 134 pw_foo/... 135 public/pw_foo/internal/secret.h 136 public/pw_foo/internal/business.h 137 138.. note:: 139 140 These headers must not override headers from other modules. For 141 that, there is the ``public_overrides/`` directory. 142 143C++ public override headers 144~~~~~~~~~~~~~~~~~~~~~~~~~~~ 145Located ``{pw_module_dir}/public_overrides/<module>``. In general, the Pigweed 146philosophy is to avoid having "things hiding under rocks", and having header 147files with the same name that can override each other is considered a rock 148where surprising things can hide. Additionally, a design goal of the Pigweed 149module structure is to make it so there is ideally exactly one obvious place 150to find a header based on an ``#include``. 151 152However, in some cases header overrides are necessary to enable flexibly 153combining modules. To make this as explicit as possible, headers which override 154other headers must go in 155 156``{pw_module_dir}/public_overrides/...``` 157 158For example, the ``pw_unit_test`` module provides a header override for 159``gtest/gtest.h``. The structure of the module is (omitting some files): 160 161.. code-block:: 162 163 pw_unit_test/... 164 165 public_overrides/gtest 166 public_overrides/gtest/gtest.h 167 168 public/pw_unit_test 169 public/pw_unit_test/framework.h 170 public/pw_unit_test/simple_printing_event_handler.h 171 public/pw_unit_test/event_handler.h 172 173Note that the overrides are in a separate directory ``public_overrides``. 174 175C++ implementation files 176~~~~~~~~~~~~~~~~~~~~~~~~ 177Located ``{pw_module_dir}/``. C++ implementation files go at the top level of 178the module. Implementation files must always use "" style includes. 179 180Example: 181 182.. code-block:: 183 184 pw_unit_test/... 185 main.cc 186 framework.cc 187 test.gni 188 BUILD.gn 189 README.md 190 191Compile-time configuration 192~~~~~~~~~~~~~~~~~~~~~~~~~~ 193Pigweed modules are intended to be used in a wide variety of environments. 194In support of this, some modules expose compile-time configuration options. 195Pigweed has an established pattern for declaring and overriding module 196configuration. 197 198.. tip:: 199 200 Compile-time configuration provides flexibility, but also imposes 201 restrictions. A module can only have one configuration in a given build. 202 This makes testing modules with compile-time configuration more difficult. 203 Where appropriate, consider alternatives such as C++ templates or runtime 204 configuration. 205 206Declaring configuration 207^^^^^^^^^^^^^^^^^^^^^^^ 208Configuration options are declared in a header file as macros. If the macro is 209not already defined, a default definition is provided. Otherwise, nothing is 210done. Configuration headers may include ``static_assert`` statements to validate 211configuration values. 212 213.. code-block:: c++ 214 215 // Example configuration header 216 217 #ifndef PW_FOO_INPUT_BUFFER_SIZE_BYTES 218 #define PW_FOO_INPUT_BUFFER_SIZE_BYTES 128 219 #endif // PW_FOO_INPUT_BUFFER_SIZE_BYTES 220 221 static_assert(PW_FOO_INPUT_BUFFER_SIZE_BYTES >= 64); 222 223The configuration header may go in one of three places in the module, depending 224on whether the header should be exposed by the module or not. 225 226.. code-block:: 227 228 pw_foo/... 229 230 # Publicly accessible configuration header 231 public/pw_foo/config.h 232 233 # Internal configuration header that is included by other module headers 234 public/pw_foo/internal/config.h 235 236 # Internal configuration header 237 pw_foo_private/config.h 238 239The configuration header is provided by a build system library. This library 240acts as a :ref:`facade<docs-module-structure-facades>`. The facade uses a 241variable such as ``pw_foo_CONFIG``. In upstream Pigweed, all config facades 242default to the ``pw_build_DEFAULT_MODULE_CONFIG`` backend. In the GN build 243system, the config facade is declared as follows: 244 245.. code-block:: 246 247 declare_args() { 248 # The build target that overrides the default configuration options for this 249 # module. This should point to a source set that provides defines through a 250 # public config (which may -include a file or add defines directly). 251 pw_foo_CONFIG = pw_build_DEFAULT_MODULE_CONFIG 252 } 253 254 # An example source set for each potential config header location follows. 255 256 # Publicly accessible configuration header (most common) 257 pw_source_set("config") { 258 public = [ "public/pw_foo/config.h" ] 259 public_configs = [ ":public_include_path" ] 260 public_deps = [ pw_foo_CONFIG ] 261 } 262 263 # Internal configuration header that is included by other module headers 264 pw_source_set("config") { 265 sources = [ "public/pw_foo/internal/config.h" ] 266 public_configs = [ ":public_include_path" ] 267 public_deps = [ pw_foo_CONFIG ] 268 visibility = [":*"] # Only allow this module to depend on ":config" 269 friend = [":*"] # Allow this module to access the config.h header. 270 } 271 272 # Internal configuration header 273 pw_source_set("config") { 274 public = [ "pw_foo_private/config.h" ] 275 public_deps = [ pw_foo_CONFIG ] 276 visibility = [":*"] # Only allow this module to depend on ":config" 277 } 278 279Overriding configuration 280^^^^^^^^^^^^^^^^^^^^^^^^ 281As noted above, all module configuration facades default to the same backend 282(``pw_build_DEFAULT_MODULE_CONFIG``). This allows projects to override 283configuration values for multiple modules from a single configuration backend, 284if desired. The configuration values may also be overridden individually by 285setting backends for the individual module configurations (e.g. in GN, 286``pw_foo_CONFIG = "//configuration:my_foo_config"``). 287 288Configurations options are overridden by setting macros in the config backend. 289These macro definitions can be provided through compilation options, such as 290``-DPW_FOO_INPUT_BUFFER_SIZE_BYTES=256``. Configuration macro definitions may 291also be set in a header file. The header file is included using the ``-include`` 292compilation option. 293 294This example shows two ways to configure a module in the GN build system. 295 296.. code-block:: 297 298 # In the toolchain, set either pw_build_DEFAULT_MODULE_CONFIG or pw_foo_CONFIG 299 pw_build_DEFAULT_MODULE_CONFIG = get_path_info(":define_overrides", "abspath") 300 301 # This configuration sets PW_FOO_INPUT_BUFFER_SIZE_BYTES using the -D flag. 302 pw_source_set("define_overrides") { 303 public_configs = [ ":define_options" ] 304 } 305 306 config("define_options") { 307 defines = [ "PW_FOO_INPUT_BUFFER_SIZE_BYTES=256" ] 308 } 309 310 # This configuration sets PW_FOO_INPUT_BUFFER_SIZE_BYTES in a header file. 311 pw_source_set("include_overrides") { 312 public_configs = [ ":set_options_in_header_file" ] 313 314 # Header file with #define PW_FOO_INPUT_BUFFER_SIZE_BYTES 256 315 sources = [ "my_config_overrides.h" ] 316 } 317 318 config("set_options_in_header_file") { 319 cflags = [ 320 "-include", 321 rebase_path("my_config_overrides.h"), 322 ] 323 } 324 325.. admonition:: Why this config pattern is preferred 326 327 Alternate patterns for configuring a module include overriding the module's 328 config header or having that header optionally include a header at a known 329 path (e.g. ``pw_foo/config_overrides.h``). There are a few downsides to these 330 approaches: 331 332 * The module needs its own config header that defines, provides defaults for, 333 and validates the configuration options. Replacing this header with a 334 user-defined header would require defining all options in the user's header, 335 which is cumbersome and brittle, and would bypass validation in the module's 336 header. 337 * Including a config override header at a particular path would prevent 338 multiple modules from sharing the same configuration file. Multiple headers 339 could redirect to the same configuration file, but this would still require 340 creating a separate header and setting the config backend variable for each 341 module. 342 * Optionally including a config override header requires boilerplate code that 343 would have to be duplicated in every configurable module. 344 * An optional config override header file would silently be excluded if the 345 file path were accidentally misspelled. 346 347Python module structure 348----------------------- 349Python code is structured as described in the :ref:`docs-python-build-structure` 350section of :ref:`docs-python-build`. 351 352.. _docs-module-structure-facades: 353 354Facades 355------- 356In Pigweed, facades represent a dependency that can be swapped at compile time. 357Facades are similar in concept to a virtual interface, but the implementation is 358set by the build system. Runtime polymorphism with facades is not 359possible, and each facade may only have one implementation (backend) per 360toolchain compilation. 361 362In the simplest sense, a facade is just a dependency represented by a variable. 363For example, the ``pw_log`` facade is represented by the ``pw_log_BACKEND`` 364build variable. Facades typically are bundled with a build system library that 365depends on the backend. 366 367Facades are essential in some circumstances: 368 369* Low-level, platform-specific features (:ref:`module-pw_cpu_exception`). 370* Features that require a macro or non-virtual function interface 371 (:ref:`module-pw_log`, :ref:`module-pw_assert`). 372* Highly leveraged code where a virtual interface or callback is too costly or 373 cumbersome (:ref:`module-pw_tokenizer`). 374 375.. caution:: 376 377 Modules should only use facades when necessary. Facades are permanently locked 378 to a particular implementation at compile time. Multiple backends cannot be 379 used in one build, and runtime dependency injection is not possible, which 380 makes testing difficult. Where appropriate, modules should use other 381 mechanisms, such as virtual interfaces, callbacks, or templates, in place of 382 facades. 383 384The GN build system provides the 385:ref:`pw_facade template<module-pw_build-facade>` as a convenient way to declare 386facades. 387 388Documentation 389------------- 390Documentation should go in the root module folder, typically in the 391``docs.rst`` file. There must be a docgen entry for the documentation in the 392``BUILD.gn`` file with the target name ``docs``; so the full target for the 393docs would be ``<module>:docs``. 394 395.. code-block:: 396 397 pw_example_module/... 398 399 docs.rst 400 401For modules with more involved documentation, create a separate directory 402called ``docs/`` under the module root, and put the ``.rst`` files and other 403related files (like images and diagrams) there. 404 405.. code-block:: 406 407 pw_example_module/... 408 409 docs/docs.rst 410 docs/bar.rst 411 docs/foo.rst 412 docs/image/screenshot.png 413 docs/image/diagram.svg 414 415Creating a new Pigweed module 416----------------------------- 417To create a new Pigweed module, follow the below steps. 418 419.. tip:: 420 421 Connect with the Pigweed community (by `mailing the Pigweed list 422 <https://groups.google.com/forum/#!forum/pigweed>`_ or `raising your idea 423 in the Pigweed chat <https://discord.gg/M9NSeTA>`_) to discuss your module 424 idea before getting too far into the implementation. This can prevent 425 accidentally duplicating work, or avoiding writing code that won't get 426 accepted. 427 4281. Create module folder following `Module name`_ guidelines 4292. Add `C++ public headers`_ files in 430 ``{pw_module_dir}/public/{pw_module_name}/`` 4313. Add `C++ implementation files`_ files in ``{pw_module_dir}/`` 4324. Add module documentation 433 434 - Add ``{pw_module_dir}/README.md`` that has a module summary 435 - Add ``{pw_module_dir}/docs.rst`` that contains the main module 436 documentation 437 4385. Add build support inside of new module 439 440 - Add GN with ``{pw_module_dir}/BUILD.gn`` 441 - Add Bazel with ``{pw_module_dir}/BUILD`` 442 - Add CMake with ``{pw_module_dir}/CMakeLists.txt`` 443 4446. Add folder alias for new module variable in ``/modules.gni`` 445 446 - ``dir_pw_new = get_path_info("pw_new", "abspath")`` 447 4487. Add new module to main GN build 449 450 - in ``/BUILD.gn`` to ``group("pw_modules")`` using folder alias variable 451 4528. Add test target for new module in ``/BUILD.gn`` to 453 ``pw_test_group("pw_module_tests")`` 4549. Add new module to CMake build 455 456 - In ``/CMakeLists.txt`` add ``add_subdirectory(pw_new)`` 457 45810. Add the new module to docs module 459 460 - Add in ``docs/BUILD.gn`` to ``group("module_docs")`` 461 46211. Run :ref:`module-pw_module-module-check` 463 464 - ``$ pw module-check {pw_module_dir}`` 465 46612. Contribute your module to upstream Pigweed (optional but encouraged!) 467
