Fluoride Style Guide

This document outlines the coding conventions and code style used in Fluoride. Its primary purpose is to provide explicit guidance on style so that developers are consistent with one another and spend less time debating style.

As a rule, we follow the Google C++ Style Guide. Exceptions will be noted below.

Directory structure

Directories at the top-level should consist of major subsystems in Fluoride. Each subsystem's purpose should be documented in the doc/directory_layout.md file, even if it seems obvious from the name.

For a subsystem that contains code, its directory structure should look like:

  Android.mk
  include/
  src/
  test/

Further, the directory structure inside src/ and include/ should be mirrored. In other words, if src/ contains a subdirectory called foo/, include/ must also have a subdirectory named foo/.

Target architecture

Fluoride targets a variety of hardware and cannot make many assumptions about memory layout, sizes, byte order, etc. As a result, some operations are considered unsafe and this section outlines the most important ones to watch out for.

Pointer / integer casts

In general, do not cast pointers to integers or vice versa.

The one exception is if an integer needs to be temporarily converted to a pointer and then back to the original integer. This style of code is typically needed when providing an integral value as the context to a callback, as in the following example.

void my_callback(void *context) {
  uintptr_t arg = context;
}

set_up_callback(my_callback, (uintptr_t)5);

Note, however, that the integral value was written into the pointer and read from the pointer as a uintptr_t to avoid a loss of precision (or to make the loss explicit).

Byte order

It is not safe to assume any particular byte order. When serializing or deserializing data, it is unsafe to memcpy unless both source and destination pointers have the same type.

Language

Fluoride is written in C99 and should take advantage of the features offered by it. However, not all language features lend themselves well to the type of development required by Fluoride. This section provides guidance on some of the features to embrace or avoid.

C Preprocessor

The use of the C preprocessor should be minimized. In particular:

  • use functions or, if absolutely necessary, inline functions instead of macros
  • use static const variables instead of #define
  • use enum for enumerations, not a collection of #defines
  • minimize the use of feature / conditional macros

The last point is perhaps the most contentious. It's well-understood that feature macros are useful in reducing code size but it leads to an exponential explosion in build configurations. Setting up, testing, and verifying each of the 2^n build configurations is untenable for n greater than, say, 4.

C++

Although C++ offers constructs that may make Fluoride development faster, safer, more pleasant, etc. the decision for the time being is to stick with pure C99. The exceptions are when linking against libraries that are written in C++. At the time of writing these libraries are gtest and tinyxml2, where the latter is a dependency that should be eliminated in favor of simpler, non-XML formats.

Variadic functions

Variadic functions are dangerous and should be avoided for most code. The exception is when implementing logging since the benefits of readability outweigh the cost of safety.

Functions with zero arguments

Functions that do not take any arguments (0 arity) should be declared like so:

void function(void);

Note that the function explicitly includes void in its parameter list to indicate to the compiler that it takes no arguments.

Variable declarations

Variables should be declared one per line as close to initialization as possible. In nearly all cases, variables should be declared and initialized on the same line. Variable declarations should not include extra whitespace to line up fields. For example, the following style is preferred:

  int my_long_variable_name = 0;
  int x = 5;

whereas this code is not acceptable:

  int my_long_variable_name = 0;
  int                     x = 5;

As a result of the above rule to declare and initialize variables together, for loops should declare and initialize their iterator variable in the initializer statement:

  for (int i = 0; i < 10; ++i) {
    // use i
  }

Contiguous memory structs

Use C99 flexible arrays as the last member of a struct if the array needs to be allocated in contiguous memory with its containing struct. A flexible array member is writen as array_name[] without a specified size. For example:

typedef struct {
  size_t length;
  uint8_t data[];
} buffer_t;

// Allocate a buffer with 128 bytes available for my_buffer->data.
buffer_t *my_buffer = malloc(sizeof(buffer_t) + 128);
uint8_t *data = my_buffer->data;

Pointer arithmetic

Avoid pointer arithmetic when possible as it results in difficult to read code. Prefer array-indexing syntax over pointer arithmetic.

In particular, do not write code like this:

typedef struct {
  size_t length;
} buffer_t;

buffer_t *my_buffer = malloc(sizeof(buffer_t) + 128);
uint8_t *data = (uint8_t *)(my_buffer + 1);

Instead, use zero-length arrays as described above to avoid pointer arithmetic and array indexing entirely.

Boolean type

Use the C99 bool type with values true and false defined in stdbool.h. Not only is this a standardized type, it is also safer and provides more compile-time checks.

Booleans instead of bitfields

Use booleans to represent boolean state, instead of a set of masks into an integer. It's more transparent and readable, and less error prone.

Function names as strings

C99 defines __func__ as an identifier that represents the function's name in which it is used. The magic identifier __FUNCTION__ should not be used as it is a non-standard language extension and an equivalent standardized mechanism exists. In other words, use __func__ over __FUNCTION__.

Fluoride conventions

This section describes coding conventions that are specific to Fluoride. Whereas the Language section describes the use of language features, this section describes idioms, best practices, and conventions that are independent of language features.

Memory management

Use osi_malloc or osi_calloc to allocate bytes instead of plain malloc. Likewise, use osi_free over free. These wrapped functions provide additional lightweight memory bounds checks that can help track down memory errors.

By convention, functions that contain *_new in their name are allocation routines and objects returned from those functions must be freed with the corresponding *_free function. For example, list objects returned from list_new should be freed with list_free and no other freeing routine.

Asserts

Use CHECK liberally throughout the code to enforce invariants. Assertions should not have any side-effects and should be used to detect programming logic errors. Please do not use assert.

At minimum, every function should assert expectations on its arguments. The following example demonstrates the kinds of assertions one should make on function arguments.

  size_t open_and_read_file(const char *filename, void *target_buffer, size_t max_bytes) {
    CHECK(filename != NULL);
    CHECK(filename[0] != '\0');
    CHECK(target_buffer != NULL);
    CHECK(max_bytes > 0);

    // function implementation begins here
  }

Header files

In general, every source file (.c or .cpp) in a src/ directory should have a corresponding header (.h) in the include/ directory.

Template header file

[copyright header]

#pragma once

#include <system/a.h>
#include <system/b.h>

#include "subsystem/include/a.h"
#include "subsystem/include/b.h"

typedef struct alarm_t alarm_t;
typedef struct list_t list_t;

// This comment describes the following function. It is not a structured
// comment, it's English prose. Function arguments can be referred to as
// |param|. This function returns true if a new object was created, false
// otherwise.
bool template_new(const list_t *param);

// Each public function must have a comment describing its semantics. In
// particular, edge cases, and whether a pointer argument may or may not be
// NULL.
void template_use_alarm(alarm_t *alarm);

License header

Each header file must begin with the following Apache 2.0 License with <year> and <owner> replaced with the year in which the file was authored and the owner of the copyright, respectively.

/******************************************************************************
 *
 *  Copyright <year> <owner>
 *
 *  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:
 *
 *  http://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.
 *
 ******************************************************************************/

Include guard

After the license header, each header file must contain the include guard:

#pragma once

This form is used over traditional #define-based include guards as it is less error-prone, doesn't pollute the global namespace, is more compact, and can result in faster compilation.

Formatting

Code formatting is done automatically using clang-format.

The style file is located at the root of the source tree in .clang-format. The -style=file option instructs clang-format to look for this file. You may find clang-format --help useful for more advanced usage. The Online clang-format Documentation can also be helpful.

clang-format -style=file -i path_to_files/filename_or_*

My Patch Doesn't Apply Anymore!

Choose one of the options below. The fewer patches that have been applied to the tree since the formatting change was applied, the better. In this short guide, commands you type will be marked as code, with output in italics.

Option 1: The Best Case

Use this option if your patch touches only a few files with few intermediate patches.

Find the formatting patch

git log --oneline path_to_files/filename_or_* | grep clang-format | head -n 5

15ce1bd subtree: Apply clang-format for the first time*

Revert the formatting patch

git revert HASH -n

(Replace HASH with 15ce1bd in this example.)

Check for conflicts with your patch

git status | grep both.modified

If this list contains files modified by your patch, you should give up

git revert --abort

and try a different method.

If this list contains files not modified by your patch, you should unstage them

git reset HEAD both_modified_file

and remove their changes

git checkout both_modified_file

Apply your patch

git cherry-pick your_patch_that_used_to_apply_cleanly

Reformat the code

clang-format -style=file -i path_to_files/filename_or_*

Commit the code that your patch touched

git add path_to_files/filename_or_*

git commit --amend

Clean up any other files

git checkout .

Review your new patch

git diff

Option 2: Reformat your patch

Start with a tree with your patch applied to the tip of tree

git log --oneline | head -n 1

dc5f0e2 Unformatted but vital patch*

(Remember the HASH from this step)

Create a new branch starting from the first unrelated patch

git checkout HEAD^ -b reformat_my_patch_branch

git log --oneline | head -n 1

15ce1bd First Unrelated patch*

Reformat the code

clang-format -style=file -i path_to_files/filename_or_*

Commit your temporary formatting patch

git add path_to_files/filename_or_*

git commit -m tmp_format_patch

Revert your temporary formatting patch (Bear with me!)

git revert HEAD --no-edit

Apply your patch

git cherry-pick HASH

(The HASH of your patch, dc5f0e2 in this case)

Reformat the code

clang-format -style=file -i path_to_files/filename_or_*

Commit your second temporary formatting patch

git add path_to_files/filename_or_*

git commit -m tmp_format_patch_2

Check to see that everything looks right

git log --oneline | head -n 5

04c97cf tmp_format_patch_2

cf8560c Unformatted but vital patch

04c97cf Revert "tmp_format_patch"

d66bb6f tmp_format_patch

15ce1bd First Unrelated patch

Squash the last three patches with git rebase

git rebase -i HEAD^^^

pick 04c97cf tmp_format_patch_2

squash cf8560c Unformatted but vital patch

squash 04c97cf Revert "tmp_format_patch"

Remember to edit the commit message!

clang-format -style=file -i path_to_files/filename_or_*

Check to see that everything looks right

git log --oneline | head -n 2

523078f Reformatted vital patch

d66bb6f tmp_format_patch

Review your new patch

git show

Checkout the current tree and cherry pick your reformatted patch!

git checkout aosp/master

git cherry-pick HASH

(HASH is 523078f in this example)






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