Style Guide and Conventions

Tip

Pigweed runs pw format as part of pw presubmit to perform some code formatting checks. To speed up the review process, consider adding pw presubmit as a git push hook using the following command: pw presubmit --install

C++ style

The Pigweed C++ style guide is closely based on Google’s external C++ Style Guide, which is found on the web at https://google.github.io/styleguide/cppguide.html. The Google C++ Style Guide applies to Pigweed except as described in this document.

The Pigweed style guide only applies to Pigweed itself. It does not apply to projects that use Pigweed or to the third-party code included with Pigweed. Non-Pigweed code is free to use features restricted by Pigweed, such as dynamic memory allocation and the entirety of the C++ Standard Library.

Recommendations in the Embedded C++ Guide are considered part of the Pigweed style guide, but are separated out since it covers more general embedded development beyond just C++ style.

Automatic formatting

Pigweed uses clang-format to automatically format Pigweed source code. A .clang-format configuration is provided with the Pigweed repository.

Automatic formatting is essential to facilitate large-scale, automated changes in Pigweed. Therefore, all code in Pigweed is expected to be formatted with clang-format prior to submission. Existing code may be reformatted at any time.

If clang-format formats code in an undesirable or incorrect way, it can be disabled for the affected lines by adding // clang-format off. clang-format must then be re-enabled with a // clang-format on comment.

// clang-format off
constexpr int kMyMatrix[] = {
    100,  23,   0,
      0, 542,  38,
      1,   2, 201,
};
// clang-format on

C Standard Library

In C++ headers, always use the C++ versions of C Standard Library headers (e.g. <cstdlib> instead of <stdlib.h>). If the header is used by both C and C++ code, only the C header should be used.

In C++ code, it is preferred to use C functions from the std namespace. For example, use std::memcpy instead of memcpy. The C++ standard does not require the global namespace versions of the functions to be provided. Using std:: is more consistent with the C++ Standard Library and makes it easier to distinguish Pigweed functions from library functions.

Within core Pigweed, do not use C standard library functions that allocate memory, such as std::malloc. There are exceptions to this for when dynamic allocation is enabled for a system; Pigweed modules are allowed to add extra functionality when a heap is present; but this must be optional.

C++ Standard Library

Much of the C++ Standard Library is not a good fit for embedded software. Many of the classes and functions were not designed with the RAM, flash, and performance constraints of a microcontroller in mind. For example, simply adding the line #include <iostream> can increase the binary size by 150 KB! This is larger than many microcontrollers’ entire internal storage.

However, with appropriate caution, a limited set of standard C++ libraries can be used to great effect. Developers can leverage familiar, well-tested abstractions instead of writing their own. C++ library algorithms and classes can give equivalent or better performance than hand-written C code.

A limited subset of the C++ Standard Library is permitted in Pigweed. To keep Pigweed small, flexible, and portable, functions that allocate dynamic memory must be avoided. Care must be exercised when using multiple instantiations of a template function, which can lead to code bloat.

The following C++ Standard Library headers are always permitted:

  • <array>

  • <complex>

  • <initializer_list>

  • <iterator>

  • <limits>

  • <optional>

  • <random>

  • <ratio>

  • <span>

  • <string_view>

  • <tuple>

  • <type_traits>

  • <utility>

  • <variant>

  • C Standard Library headers (<c*>)

With caution, parts of the following headers can be used:

  • <algorithm> – be wary of potential memory allocation

  • <atomic> – not all MCUs natively support atomic operations

  • <bitset> – conversions to or from strings are disallowed

  • <functional> – do not use std::function

  • <new> – for placement new

  • <numeric> – be wary of code size with multiple template instantiations

Never use any of these headers:

  • Dynamic containers (<list>, <map>, <set>, <vector>, etc.)

  • Streams (<iostream>, <ostream>, <fstream>, etc.)

  • <exception>

  • <future>, <mutex>, <thread>

  • <memory>

  • <regex>

  • <scoped_allocator>

  • <sstream>

  • <stdexcept>

  • <string>

  • <valarray>

Headers not listed here should be carefully evaluated before they are used.

These restrictions do not apply to third party code or to projects that use Pigweed.

Combining C and C++

Prefer to write C++ code over C code, using extern "C" for symbols that must have C linkage. extern "C" functions should be defined within C++ namespaces to simplify referring to other code.

C++ functions with no parameters do not include void in the parameter list. C functions with no parameters must include void.

namespace pw {

bool ThisIsACppFunction() { return true; }

extern "C" int pw_ThisIsACFunction(void) { return -1; }

extern "C" {

int pw_ThisIsAlsoACFunction(void) {
  return ThisIsACppFunction() ? 100 : 0;
}

}  // extern "C"

}  // namespace pw

Comments

Prefer C++-style (//) comments over C-style commments (/* */). C-style comments should only be used for inline comments.

// Use C++-style comments, except where C-style comments are necessary.
// This returns a random number using an algorithm I found on the internet.
#define RANDOM_NUMBER() [] {                \
  return 4;  /* chosen by fair dice roll */ \
}()

Indent code in comments with two additional spaces, making a total of three spaces after the //. All code blocks must begin and end with an empty comment line, even if the blank comment line is the last line in the block.

// Here is an example of code in comments.
//
//   int indentation_spaces = 2;
//   int total_spaces = 3;
//
//   engine_1.thrust = RANDOM_NUMBER() * indentation_spaces + total_spaces;
//
bool SomeFunction();

Control statements

All loops and conditional statements must use braces.

The syntax while (true) is preferred over for (;;) for infinite loops.

Include guards

The first non-comment line of every header file must be #pragma once. Do not use traditional macro include guards. The #pragma once should come directly after the Pigweed copyright block, with no blank line, followed by a blank, like this:

// Copyright 2020 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once

// Header file-level comment goes here...

Memory allocation

Dynamic memory allocation can be problematic. Heap allocations and deallocations occupy valuable CPU cycles. Memory usage becomes nondeterministic, which can result in a system crashing without a clear culprit.

To keep Pigweed portable, core Pigweed code is not permitted to dynamically (heap) allocate memory, such as with malloc or new. All memory should be allocated with automatic (stack) or static (global) storage duration. Pigweed must not use C++ libraries that use dynamic allocation.

Projects that use Pigweed are free to use dynamic allocation, provided they have selected a target that enables the heap.

Naming

Entities shall be named according to the Google style guide, with the following additional requirements.

C++ code
  • All Pigweed C++ code must be in the pw namespace. Namespaces for modules should be nested under pw. For example, pw::string::Format().

  • Whenever possible, private code should be in a source (.cc) file and placed in anonymous namespace nested under pw.

  • If private code must be exposed in a header file, it must be in a namespace nested under pw. The namespace may be named for its subsystem or use a name that designates it as private, such as internal.

  • Template arguments for non-type names (e.g. template <int foo_bar>) should follow the variable naming convention, which means snake case (e.g. foo_bar). This matches the Google C++ style, however the wording in the official style guide isn’t explicit and could be interpreted to use kFooBar style naming. Wide practice establishes that the naming convention is snake_case, and so that is the style we use in Pigweed.

    Note: At time of writing much of Pigweed incorrectly follows the kCamelCase naming for non-type template arguments. This is a bug that will be fixed eventually.

C code In general, C symbols should be prefixed with the module name. If the symbol is not associated with a module, use just pw as the module name. Facade backends may chose to prefix symbols with the facade’s name to help reduce the length of the prefix.

  • Public names used by C code must be prefixed with the module name (e.g. pw_tokenizer_*).

  • If private code must be exposed in a header, private names used by C code must be prefixed with an underscore followed by the module name (e.g. _pw_assert_*).

  • Avoid writing C source (.c) files in Pigweed. Prefer to write C++ code with C linkage using extern "C". Within C source, private C functions and variables must be named with the _pw_my_module_* prefix and should be declared static whenever possible; for example, _pw_my_module_MyPrivateFunction.

  • The C prefix rules apply to

    • C functions (int pw_foo_FunctionName(void);),

    • variables used by C code (int pw_foo_variable_name;),

    • constant variables used by C code (int pw_foo_kConstantName;),

    • structs used by C code (typedef struct {} pw_foo_StructName;), and

    • all of the above for extern "C" names in C++ code.

    The prefix does not apply to struct members, which use normal Google style.

Preprocessor macros
  • Public Pigweed macros must be prefixed with the module name (e.g. PW_MY_MODULE_*).

  • Private Pigweed macros must be prefixed with an underscore followed by the module name (e.g. _PW_MY_MODULE_*).

Example

namespace pw::my_module {
namespace nested_namespace {

// C++ names (types, variables, functions) must be in the pw namespace.
// They are named according to the Google style guide.
constexpr int kGlobalConstant = 123;

// Prefer using functions over extern global variables.
extern int global_variable;

class Class {};

void Function();

extern "C" {

// Public Pigweed code used from C must be prefixed with pw_.
extern const int pw_my_module_kGlobalConstant;

extern int pw_my_module_global_variable;

void pw_my_module_Function(void);

typedef struct {
  int member_variable;
} pw_my_module_Struct;

// Private Pigweed code used from C must be prefixed with _pw_.
extern const int _pw_my_module_kPrivateGlobalConstant;

extern int _pw_my_module_private_global_variable;

void _pw_my_module_PrivateFunction(void);

typedef struct {
  int member_variable;
} _pw_my_module_PrivateStruct;

}  // extern "C"

// Public macros must be prefixed with PW_.
#define PW_MY_MODULE_PUBLIC_MACRO(arg) arg

// Private macros must be prefixed with _PW_.
#define _PW_MY_MODULE_PRIVATE_MACRO(arg) arg

}  // namespace nested_namespace
}  // namespace pw::my_module

Namespace scope formatting

All non-indented blocks (namespaces, extern "C" blocks, and preprocessor conditionals) must have a comment on their closing line with the contents of the starting line.

All nested namespaces should be declared together with no blank lines between them.

#include "some/header.h"

namespace pw::nested {
namespace {

constexpr int kAnonConstantGoesHere = 0;

}  // namespace

namespace other {

const char* SomeClass::yes = "no";

bool ThisIsAFunction() {
#if PW_CONFIG_IS_SET
  return true;
#else
  return false;
#endif  // PW_CONFIG_IS_SET
}

extern "C" {

const int pw_kSomeConstant = 10;
int pw_some_global_variable = 600;

void pw_CFunction() { ... }

}  // extern "C"

}  // namespace
}  // namespace pw::nested

Pointers and references

For pointer and reference types, place the asterisk or ampersand next to the type.

int* const number = &that_thing;
constexpr const char* kString = "theory!"

bool FindTheOneRing(const Region& where_to_look) { ... }

Prefer storing references over storing pointers. Pointers are required when the pointer can change its target or may be nullptr. Otherwise, a reference or const reference should be used. In accordance with the Google C++ style guide, only const references are permitted as function arguments; pointers must be used in place of mutable references when passed as function arguments.

Preprocessor macros

Macros should only be used when they significantly improve upon the C++ code they replace. Macros should make code more readable, robust, and safe, or provide features not possible with standard C++, such as stringification, line number capturing, or conditional compilation. When possible, use C++ constructs like constexpr variables in place of macros. Never use macros as constants, except when a string literal is needed or the value must be used by C code.

When macros are needed, the macros should be accompanied with extensive tests to ensure the macros are hard to use wrong.

Stand-alone statement macros

Macros that are standalone statements must require the caller to terminate the macro invocation with a semicolon. For example, the following does not conform to Pigweed’s macro style:

// BAD! Definition has built-in semicolon.
#define PW_LOG_IF_BAD(mj) \
  CallSomeFunction(mj);

// BAD! Compiles without error; semicolon is missing.
PW_LOG_IF_BAD("foo")

Here’s how to do this instead:

// GOOD; requires semicolon to compile.
#define PW_LOG_IF_BAD(mj) \
  CallSomeFunction(mj)

// GOOD; fails to compile due to lacking semicolon.
PW_LOG_IF_BAD("foo")

For macros in function scope that do not already require a semicolon, the contents can be placed in a do { ... } while (0) loop.

#define PW_LOG_IF_BAD(mj)  \
  do {                     \
    if (mj.Bad()) {        \
      Log(#mj " is bad")   \
    }                      \
  } while (0)

Standalone macros at global scope that do not already require a semicolon can add a static_assert or throwaway struct declaration statement as their last line.

#define PW_NEAT_THING(thing)             \
  bool IsNeat_##thing() { return true; } \
  static_assert(true, "Macros must be terminated with a semicolon")

Private macros in public headers

Private macros in public headers must be prefixed with _PW_, even if they are undefined after use; this prevents collisions with downstream users. For example:

#define _PW_MY_SPECIAL_MACRO(op) ...
...
// Code that uses _PW_MY_SPECIAL_MACRO()
...
#undef _PW_MY_SPECIAL_MACRO

Macros in private implementation files (.cc)

Macros within .cc files that should only used within one file should be undefined after their last use; for example:

#define DEFINE_OPERATOR(op) \
  T operator ## op(T x, T y) { return x op y; } \
  static_assert(true, "Macros must be terminated with a semicolon") \

DEFINE_OPERATOR(+);
DEFINE_OPERATOR(-);
DEFINE_OPERATOR(/);
DEFINE_OPERATOR(*);

#undef DEFINE_OPERATOR

Preprocessor conditional statements

When using macros for conditional compilation, prefer to use #if over #ifdef. This checks the value of the macro rather than whether it exists.

  • #if handles undefined macros equivalently to #ifdef. Undefined macros expand to 0 in preprocessor conditional statements.

  • #if evaluates false for macros defined as 0, while #ifdef evaluates true.

  • Macros defined using compiler flags have a default value of 1 in GCC and Clang, so they work equivalently for #if and #ifdef.

  • Macros defined to an empty statement cause compile-time errors in #if statements, which avoids ambiguity about how the macro should be used.

All #endif statements should be commented with the expression from their corresponding #if. Do not indent within preprocessor conditional statements.

#if USE_64_BIT_WORD
using Word = uint64_t;
#else
using Word = uint32_t;
#endif  // USE_64_BIT_WORD

Unsigned integers

Unsigned integers are permitted in Pigweed. Aim for consistency with existing code and the C++ Standard Library. Be very careful mixing signed and unsigned integers.

Python style

Pigweed uses the standard Python style: PEP8, which is available on the web at https://www.python.org/dev/peps/pep-0008/. All Pigweed Python code should pass yapf when configured for PEP8 style.

Python 3

Pigweed uses Python 3. Some modules may offer limited support for Python 2, but Python 3.6 or newer is required for most Pigweed code.

Build files: GN

Each Pigweed source module will require a build file named BUILD.gn which encapsulates the build targets and specifies their sources and dependencies. The format of this file is simlar in structure to the Bazel/Blaze format (Googlers may also review go/build-style), but with nomenclature specific to Pigweed. For each target specified within the build file there are a list of depdency fields. Those fields, in their expected order, are:

  • <public_config> – external build configuration

  • <public_deps> – necessary public dependencies (ie: Pigweed headers)

  • <public> – exposed package public interface header files

  • <config> – package build configuration

  • <sources> – package source code

  • <deps> – package necessary local dependencies

Assets within each field must be listed in alphabetical order

# Here is a brief example of a GN build file.

import("$dir_pw_unit_test/test.gni")

config("default_config") {
  include_dirs = [ "public" ]
}

source_set("pw_sample_module") {
  public_configs = [ ":default_config" ]
  public_deps = [ dir_pw_status ]
  public = [ "public/pw_sample_module/sample_module.h" ]
  sources = [
    "public/pw_sample_module/internal/sample_module.h",
    "sample_module.cc",
    "used_by_sample_module.cc",
  ]
  deps = [ dir_pw_varint ]
}

pw_test_group("tests") {
  tests = [ ":sample_module_test" ]
}

pw_test("sample_module_test") {
  sources = [ "sample_module_test.cc" ]
  deps = [ ":sample_module" ]
}

pw_doc_group("docs") {
  sources = [ "docs.rst" ]
}