Bluedroid Style Guide

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

Directory structure

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

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

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

Bluedroid 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.


Bluedroid 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 Bluedroid. 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.


Although C++ offers constructs that may make Bluedroid 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__.

Bluedroid conventions

This section describes coding conventions that are specific to Bluedroid. 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.


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

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) {
    assert(filename != NULL);
    assert(filename[0] != '\0');
    assert(target_buffer != NULL);
    assert(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 (C) <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:
 *  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.


Code formatting is pretty arbitrary, but the codebase is easier to follow if everyone uses the same style. Individuals may not agree with every aspect of the formatting rules, and some of the rules may take some getting used to, but it is important that all engineers follow the formatting rules so we can all understand and read the code easily.

White space

  • use only spaces, indent 2 spaces at a time
  • no trailing whitespaces at the end of a line
  • no tab characters
  • use one blank line to separate logical code blocks, function definitions, and sections of a file
// Space after keyword in conditionals and loops.
// No space immeidately before or after parentheses.
if (foo)

// Space surrounding binary operators.
if (foo < 5)

// Space after comma.
for (int x = 0, y = 0; x; ++y)

// No space between unary operators and their argument.
z = -y;

// No space between function name and open parenthesis.
call_my_fn(arg1, arg2);

// Space before * in variable declaration.
int *x = NULL;

// Space after // beginning a comment.
// Notice the space between "//" and "N".

Use only spaces, and indent 2 spaces at a time. Do not use tab characters in the codebase.

Use a single blank line to separate logical code blocks, function definitions, and sections of a file.

Brace style

// Open curly braces are never on a line by themselves.
void my_function(void) {
  // Conditional statements with only one child statement should elide braces.
  // The child statement must be on a new line, indented by 2 spaces.
  if (foo)

  // Conditionals with a branch containing more than one child statement forces
  // braces on both branches.
  if (foo) {
  } else {