Coding Style Guidelines — libcamera v0.2.0 documentation

# Coding Style Guidelines

These coding guidelines are meant to ensure code quality. As a contributor you are expected to follow them in all code submitted to the project. While strict compliance is desired, exceptions are tolerated when justified with good reasons. Please read the whole coding guidelines and use common sense to decide when departing from them is appropriate.

libcamera is written in C++, a language that has seen many revisions and offers an extensive set of features that are easy to abuse. These coding guidelines establish the subset of C++ used by the project.

## Coding Style

Even if the programming language in use is different, the project embraces the Linux Kernel Coding Style with a few exception and some C++ specificities.

In particular, from the kernel style document, the following section are adopted:

- 1 “Indentation”

- 2 “Breaking Long Lines” striving to fit code within 80 columns and accepting up to 120 columns when necessary

- 3 “Placing Braces and Spaces”

- 3.1 “Spaces”

- 8 “Commenting” with the exception that in-function comments are not always un-welcome.

While libcamera uses the kernel coding style for all typographic matters, the project is a user space library, developed in a different programming language, and the kernel guidelines fall short for this use case.

For this reason, rules and guidelines from the Google C++ Style Guide have been adopted as well as most coding principles specified therein, with a few exceptions and relaxed limitations on some subjects.

The following exceptions apply to the naming conventions specified in the document:

- File names: libcamera uses the .cpp extensions for C++ source files and the .h extension for header files

- Variables, function parameters, function names and class members use camel case style, with the first letter in lower-case (as in ‘camelCase’ and not ‘CamelCase’)

- Types (classes, structs, type aliases, and type template parameters) use camel case, with the first letter in capital case (as in ‘CamelCase’ and not ‘camelCase’)

- Enum members use ‘CamelCase’, while macros are in capital case with underscores in between

- All formatting rules specified in the selected sections of the Linux kernel Code Style for indentation, braces, spacing, etc

- Header guards are formatted as ‘__LIBCAMERA_FILE_NAME_H__’

### Order of Includes

Headers shall be included at the beginning of .c, .cpp and .h files, right after the file description comment block and, for .h files, the header guard macro. For .cpp files, if the file implements an API declared in a header file, that header file shall be included first in order to ensure it is self-contained.

While the following list is extensive, it documents the expected behaviour defined by the clang-format configuration and tooling should assist with ordering.

The headers shall be grouped and ordered as follows:

1. The header declaring the API being implemented (if any)

2. The C and C++ system and standard library headers

3. Linux kernel headers

4. The libcamera base private header if required

5. The libcamera base library headers

6. The libcamera public API headers

7. The libcamera IPA interfaces

8. The internal libcamera headers

9. Other libraries’ headers, with one group per library

10. Local headers grouped by subdirectory

11. Any local headers

Groups of headers shall be separated by a single blank line. Headers within each group shall be sorted alphabetically.

System and library headers shall be included with angle brackets. Project headers shall be included with angle brackets for the libcamera public API headers, and with double quotes for internal libcamera headers.

## C++ Specific Rules

The code shall be implemented in C++17, with the following caveats:

- Type inference (auto and decltype) shall be used with caution, to avoid drifting towards an untyped language.

- The explicit, override and final specifiers are to be used where applicable.

- Smart pointers, as well as shared pointers and weak pointers, shall not be overused.

- Classes are encouraged to define move constructors and assignment operators where applicable, and generally make use of the features offered by rvalue references.

### Object Ownership

libcamera creates and destroys many objects at runtime, for both objects internal to the library and objects exposed to the user. To guarantee proper operation without use after free, double free or memory leaks, knowing who owns each object at any time is crucial. The project has enacted a set of rules to make object ownership tracking as explicit and fool-proof as possible.

In the context of this section, the terms object and instance are used interchangeably and both refer to an instance of a class. The term reference refers to both C++ references and C++ pointers in their capacity to refer to an object. Passing a reference means offering a way to a callee to obtain a reference to an object that the caller has a valid reference to. Borrowing a reference means using a reference passed by a caller without ownership transfer based on the assumption that the caller guarantees the validity of the reference for the duration of the operation that borrows it.

1. Single Owner Objects

   - By default an object has a single owner at any time.

   - Storage of single owner objects varies depending on how the object ownership will evolve through the lifetime of the object.

     - Objects whose ownership needs to be transferred shall be stored as std::unique_ptr<> as much as possible to emphasize the single ownership.

     - Objects whose owner doesn’t change may be embedded in other objects, or stored as pointer or references. They may be stored as std::unique_ptr<> for automatic deletion if desired.

   - Ownership is transferred by passing the reference as a std::unique_ptr<> and using std::move(). After ownership transfer the former owner has no valid reference to the object anymore and shall not access it without first obtaining a valid reference.

   - Objects may be borrowed by passing an object reference from the owner to the borrower, providing that

     - the owner guarantees the validity of the reference for the whole duration of the borrowing, and

     - the borrower doesn’t access the reference after the end of the borrowing.

     When borrowing from caller to callee for the duration of a function call, this implies that the callee shall not keep any stored reference after it returns. These rules apply to the callee and all the functions it calls, directly or indirectly.

     When the object is stored in a std::unique_ptr<>, borrowing passes a reference to the object, not to the std::unique_ptr<>, as

     - a ‘const &’ when the object doesn’t need to be modified and may not be null.

     - a pointer when the object may be modified or may be null. Unless otherwise specified, pointers passed to functions are considered as borrowed references valid for the duration of the function only.

2. Shared Objects

   - Objects that may have multiple owners at a given time are called shared objects. They are reference-counted and live as long as any references to the object exist.

   - Shared objects are created with std::make_shared<> or std::allocate_shared<> and stored in an std::shared_ptr<>.

   - Ownership is shared by creating and passing copies of any valid std::shared_ptr<>. Ownership is released by destroying the corresponding std::shared_ptr<>.

   - When passed to a function, std::shared_ptr<> are always passed by value, never by reference. The caller can decide whether to transfer its ownership of the std::shared_ptr<> with std::move() or retain it. The callee shall use std::move() if it needs to store the shared pointer.

   - Do not over-use std::move(), as it may prevent copy-elision. In particular a function returning a std::shared_ptr<> value shall not use std::move() in its return statements, and its callers shall not wrap the function call with std::move().

   - Borrowed references to shared objects are passed as references to the objects themselves, not to the std::shared_ptr<>, with the same rules as for single owner objects.

These rules match the object ownership rules from the Chromium C++ Style Guide.

Attention

Long term borrowing of single owner objects is allowed. Example use cases are implementation of the singleton pattern (where the singleton guarantees the validity of the reference forever), or returning references to global objects whose lifetime matches the lifetime of the application. As long term borrowing isn’t marked through language constructs, it shall be documented explicitly in details in the API.

### Global Variables

The order of initializations and destructions of global variables cannot be reasonably controlled. This can cause problems (including segfaults) when global variables depend on each other, directly or indirectly. For example, if the declaration of a global variable calls a constructor which uses another global variable that hasn’t been initialized yet, incorrect behavior is likely. Similar issues may occur when the library is unloaded and global variables are destroyed.

Global variables that are statically initialized and have trivial destructors (such as an integer constant) do not cause any issue. Other global variables shall be avoided when possible, but are allowed when required (for instance to implement factories with auto-registration). They shall not depend on any other global variable, should run a minimal amount of code in the constructor and destructor, and code that contains dependencies should be moved to a later point in time.

### Error Handling

Proper error handling is crucial to the stability of libcamera. The project follows a set of high-level rules:

- Make errors impossible through API design. The best way to handle errors is to prevent them from happening in the first place. The preferred option is thus to prevent error conditions at the API design stage when possible.

- Detect errors at compile time. Compile-test checking of errors not only reduces the runtime complexity, but also ensures that errors are caught early on during development instead of during testing or, worse, in production. The static_assert() declaration should be used where possible for this purpose.

- Validate all external API contracts. Explicit pre-condition checks shall be used to validate API contracts. Whenever possible, appropriate errors should be returned directly. As libcamera doesn’t use exceptions, errors detected in constructors shall result in the constructed object being marked as invalid, with a public member function available to check validity. The checks should be thorough for the public API, and may be lighter for internal APIs when pre-conditions can reasonably be considered to be met through other means.

- Use assertions for fatal issues only. The ASSERT() macro causes a program abort when compiled in debug mode, and is a no-op otherwise. It is useful to abort execution synchronously with the error check instead of letting the error cause problems (such as segmentation faults) later, and to provide a detailed backtrace. Assertions shall only be used to catch conditions that are never supposed to happen without a serious bug in libcamera that would prevent safe recovery. They shall never be used to validate API contracts. The assertion conditions shall not cause any side effect as they are compiled out in non-debug mode.

### C Compatibility Headers

The C++ standard defines a set of C++ standard library headers, and for some of them, defines C compatibility headers. The former have a name of the form <cxxx> while the later are named <xxx.h>. The C++ headers declare names in the std namespace, and may declare the same names in the global namespace. The C compatibility headers declare names in the global namespace, and may declare the same names in the std namespace. Code shall not rely on the optional declaration of names in the global or std namespace.

Usage of the C compatibility headers is preferred, except for the math.h header. Where math.h defines separate functions for different argument types (e.g. abs(int), labs(long int), fabs(double) and fabsf(float)) and requires the developer to pick the right function, cmath defines overloaded functions (std::abs(int), std::abs(long int), std::abs(double) and std::abs(float) to let the compiler select the right function. This avoids potential errors such as calling abs(int) with a float argument, performing an unwanted implicit integer conversion. For this reason, cmath is preferred over math.h.

## Documentation

All public and protected classes, structures, enumerations, macros, functions and variables shall be documented with a Doxygen comment block, using the Javadoc style with C-style comments. When documenting private member functions and variables the same Doxygen style shall be used as for public and protected members.

Documentation relates to header files, but shall be stored in the .cpp source files in order to group the implementation and documentation. Every documented header file shall have a file documentation block in the .cpp source file.

The following comment block shows an example of correct documentation for a member function of the PipelineHandler class.

    /**
     * \fn PipelineHandler::start()
     * \brief Start capturing from a group of streams
     * \param[in] camera The camera to start
     *
     * Start the group of streams that have been configured for capture by
     * \a configureStreams(). The intended caller of this function is the Camera
     * class which will in turn be called from the application to indicate that
     * it has configured the streams and is ready to capture.
     *
     * \return 0 on success or a negative error code otherwise
     */

The comment block shall be placed right before the function it documents. If the function is defined inline in the class definition in the header file, the comment block shall be placed alone in the .cpp source file in the same order as the function definitions in the header file and shall start with an fn line. Otherwise no fn line shall be present.

The brief directive shall be present. If the function takes parameters, param directives shall be present, with the appropriate [in], [out] or [inout] specifiers. Only when the direction of the parameters isn’t known (for instance when defining a template function with variadic arguments) the direction specifier shall be omitted. The return directive shall be present when the function returns a value, and shall be omitted otherwise.

The long description is optional. When present it shall be surrounded by empty lines and may span multiple paragraphs. No blank lines shall otherwise be added between the fn, brief, param and return directives.

## Tools

The ‘clang-format’ code formatting tool can be used to reformat source files with the libcamera coding style, defined in the .clang-format file at the root of the source tree.

As clang-format is a code formatter, it operates on full files and outputs reformatted source code. While it can be used to reformat code before sending patches, it may generate unrelated changes. To avoid this, libcamera provides a ‘checkstyle.py’ script wrapping the formatting tools to only retain related changes. This should be used to validate modifications before submitting them for review.

The script operates on one or multiple git commits specified on the command line. It does not modify the git tree, the index or the working directory and is thus safe to run at any point.

Commits are specified using the same revision range syntax as ‘git log’. The most usual use cases are to specify a single commit by sha1, branch name or tag name, or a commit range with the <from>..<to> syntax. When no arguments are given, the topmost commit of the current branch is selected.

    $ ./utils/checkstyle.py cc7d204b2c51
    ----------------------------------------------------------------------------------
    cc7d204b2c51853f7d963d144f5944e209e7ea29 libcamera: Use the logger instead of cout
    ----------------------------------------------------------------------------------
    No style issue detected

When operating on a range of commits, style checks are performed on each commit from oldest to newest.

    $ ../utils/checkstyle.py 3b56ddaa96fb~3..3b56ddaa96fb
    ----------------------------------------------------------------------------------
    b4351e1a6b83a9cfbfc331af3753602a02dbe062 libcamera: log: Fix Doxygen documentation
    ----------------------------------------------------------------------------------
    No style issue detected

    --------------------------------------------------------------------------------------
    6ab3ff4501fcfa24db40fcccbce35bdded7cd4bc libcamera: log: Document the LogMessage class
    --------------------------------------------------------------------------------------
    No style issue detected

    ---------------------------------------------------------------------------------
    3b56ddaa96fbccf4eada05d378ddaa1cb6209b57 build: Add 'std=c++11' cpp compiler flag
    ---------------------------------------------------------------------------------
    Commit doesn't touch source files, skipping

Commits that do not touch any .c, .cpp or .h files are skipped.

    $ ./utils/checkstyle.py edbd2059d8a4
    ----------------------------------------------------------------------
    edbd2059d8a4bd759302ada4368fa4055638fd7f libcamera: Add initial logger
    ----------------------------------------------------------------------
    --- src/libcamera/include/log.h
    +++ src/libcamera/include/log.h
    @@ -21,11 +21,14 @@
     {
     public:
            LogMessage(const char *fileName, unsigned int line,
    -                 LogSeverity severity);
    -       LogMessage(const LogMessage&) = delete;
    +                  LogSeverity severity);
    +       LogMessage(const LogMessage &) = delete;
            ~LogMessage();

    -       std::ostream& stream() { return msgStream; }
    +       std::ostream &stream()
    +       {
    +               return msgStream;
    +       }

     private:
            std::ostringstream msgStream;

    --- src/libcamera/log.cpp
    +++ src/libcamera/log.cpp
    @@ -42,7 +42,7 @@

     static const char *log_severity_name(LogSeverity severity)
     {
    -       static const char * const names[] = {
    +       static const char *const names[] = {
                    "INFO",
                    "WARN",
                    " ERR",

    ---
    2 potential style issues detected, please review

When potential style issues are detected, they are displayed in the form of a diff that fixes the issues, on top of the corresponding commit. As the script is in early development false positive are expected. The flagged issues should be reviewed, but the diff doesn’t need to be applied blindly.

Execution of checkstyle.py can be automated through git commit hooks. Example of pre-commit and post-commit hooks are available in utils/hooks/pre-commit and utils/hooks/post-commit. You can install either hook by copying it to .git/hooks/. The post-commit hook is easier to start with as it will only flag potential issues after committing, while the pre-commit hook will abort the commit if issues are detected and requires usage of git commit –no-verify to ignore false positives.

Happy hacking, libcamera awaits your patches!