bt/doc/style_guide.md

# 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](https://google.github.io/styleguide/cppguide.html).
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 `#define`s
* 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](http://clang.llvm.org/docs/ClangFormat.html) 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)