Coding Conventions

General

Code shall be C99 compliant.
Avoid dynamic memory allocation (malloc/free, new, etc.)! It will break real-time guarantees, increase code complexity, and make it more likely to use more memory than available.
Avoid the use of floating point arithmetic. Not every MCU has a FPU and software floating point libraries cause unnecessary overhead. Instead use fixed-point integers and transform equations so that they stay within the range of integer math. An easy way to ensure this is by multiplying by a constant factor, ideally a power of two - this is a simple shift operation. Take care that intermediate values do not exceed the range of the data type you are using. When writing drivers, do not convert the measurement data into float, but instead choose an appropriate integer format / SI prefix.
Please obey the Linux coding style as described in https://www.kernel.org/doc/Documentation/process/coding-style.rst with the following modifications and additions:
- Line length: aim for no more than 80 characters per line, the absolute maximum should be 100 characters per line.
- All line endings shall be set to LF (\n). (How to handle line endings in Git: https://help.github.com/articles/dealing-with-line-endings)
- There must be no trailing whitespace in any line. The script /dist/tools/whitespacecheck/check.sh master || exit can be used to detect whitespaces at the end of line(s) that would lead to Murdock build error(s).
- Use empty braces for empty while loops waiting for a hardware register instead of semicolon. while (HW_STATUS != STATUS_OK) {} is correct, while (HW_STATUS != STATUS_OK); is wrong.
- Our policy regarding typedefs is completely different (see below) (BTW: Do we have any reason to do so?)
- Comments should be C-style comments (see below)
In order to follow Linux's recommendation on conditional compilation make use of IS_ACTIVE and IS_USED macros from kernel_defines.h with C conditionals. If a symbol is not going to be defined under a certain condition, the usage of preprocessor #if defined() is fine.
You can use uncrustify with the provided option files: https://github.com/RIOT-OS/RIOT/blob/master/uncrustify-riot.cfg

Types

Be careful with platform dependent type sizes like int or long. Use data types that include the bit length in the name like uint16_t when you need to make sure that a certain variable is exactly of this length.
The use of typedefs for structs and pointers is allowed.
Type definitions (using typedef) always end on "_t".
If a typedef is used for a struct, it has to be specified at the struct definition (i.e., not as a separate line). E.g.:

    typedef struct {
        uint8_t a;
        uint8_t b;
    } foobar_t;

Use of a separate line typedef for structs is allowed for forward declarations, e.g.,

    typedef struct mystruct mystruct_t;
    [...]
    struct mystruct {
         [...]
    };

Guidelines for pointer types (as long as it is reasonable):
- use char * for strings and only for strings
- use uint8_t[] as type for arbitrary byte buffers, but use void * to pass them around. uint8_t[] because we're dealing with bytes and not characters, void * to avoid unnecessary casting shall the need arise to have their content to have a certain type
- use uint8_t * to pass "typed" byte buffers, e.g., link-layer addresses, where it avoids unnecessary temporary variable
- use void * for generic typing

Variables

Do NOT use global variables unless it is unavoidable.
If you declare a variable within a header file, you MUST use the keyword extern.

Functions

Every function needs a prototype in addition to its definition. If a prototype is specified within a .c file it has to be declared BEFORE any function definitions.
If the scope of a function is limited to one file, it MUST be declared static.
Functions without parameters must be specified with (void).
Keep functions short! As a rule of thumb, the function's body should not exceed one screen.
Do NOT use global macros defining more than one line of code. Use inline functions instead.

Return values

Any function must return one of the following values:
- logical value (zero or not zero)
- an error code (given as a negative number or zero) or a positive status value
- the count of read or written bytes/values for I/O functions
- the position or address (for search functions)
- a pointer
NULL indicates an error case, too.
Do NOT return structs or other larger types! These would get copied to the stack, resulting in expensive operations. Moreover, some compilers have trouble with larger return types. Use pointers to structs instead and take care of the structs lifetime.
If possible, prefer signed types over unsigned ones in order to be able to add error codes later on.

Naming

Names of all public functions and variables must start with the name of the corresponding library, e.g.:

    thread_getpid(void);
    hwtimer_init_comp(uint32_t fcpu);
    int transceiver_pid;

Private functions and variables do NOT have this library prefix.
Do NOT use CamelCase. Function, variable and file names as well as enums, structs or typedefs are written in lowercase with underscores.

    /* instead of: */
    void CamelCaseNamedFunction(int camelCaseNamedVar);

    /* write: */
    void camel_case_named_function(int camel_case_named_var);

When implementing constants or variables that are defined in third party documents such as RFCs, add a prefix to those names based on the RIOT coding conventions. If you use a name in the RIOT code that is different from the one in the third party document, you must add a reference to the original name of the constant or variable in the Doxygen documentation.

Indentation and braces

Indentations are four spaces (i.e., NO tab characters).
As an exception to the Linux coding style, the closing brace is empty on a line of its own when followed by an else, too. When followed by a while in a do-statement, it goes into the same line.
Use curly braces even for one-line blocks. This improves debugging and later additions.

    /* instead of: */
    if (debug) println("DEBUG");
    else println("DEBUG ELSE");

    /* write: */
    if (debug) {
        println("DEBUG");
    }
    else {
        println("DEBUG ELSE");
    }

Commas are always followed by a space.
For complex statements it is always good to use more parentheses - or split up the statement and simplify it.

Includes

The include of system headers (in <>-brackets) always precedes RIOT specific includes (in quotes).
Optional headers must only be included if their corresponding module is selected/being build. In other words: always put an #ifdef MODULE_... statement around includes of optional headers:

#ifdef MODULE_ABC
#include "abc.h"
#endif

Header Guards

All files are required to have header guards of the form

#ifndef PATH_TO_FILE_FILENAME_H
#define PATH_TO_FILE_FILENAME_H

...
#endif /* PATH_TO_FILE_FILENAME_H */

Rules for generating the guard name:

take the file name
if there's include/ in the file's pathname, include the path from there on.
replace "/" and "." with "_"
convert to uppercase letters
if the produced guard starts with "_", prefix "PRIV"

Examples:

"core/include/msg.h" -> "MSG_H"
"sys/include/net/gnrc/pkt.h" -> NET_GNRC_PKT_H
"drivers/abcd0815/abcd0815_params.h" -> ABCD0815_PARAMS_H
"sys/module/_internal.h" -> PRIV_INTERNAL_H

Note: these rules will be enforced by the CI.

C++ compatibility

C Header files should be always wrapped for C++ compatibility to prevent issues with name mangling, i.e. mark all the containing functions and definitions as extern "C"

#ifdef __cplusplus
extern "C" {
#endif

... all your function declarations, global variables and defines belong here

#ifdef __cplusplus
}
#endif

use __restrict instead of restrict in headers (compare RIOT-OS#2042)

Absolute values

Absolute values must be specified as macros or enums, not as literals, i.e. instead of

int timeout = 7 * 1000000;

write

int timeout = TIMEOUT_INTERVAL * USEC_PER_SEC;

Comments

All comments should be written as C-style comments.

E.g:

/* This is a C-style comment */

Wrong:

// C++ comment here

Documentation

All documentation must be in English.
All files contain the copyright note and the author.
Doxygen documentation is mandatory for all header files.
Every header file includes a general description about the provided functionality.
Every function must be documented - including parameters and return value.

An examplary doxygen documentation in a header file can look like this.

/*
 * Copyright (C) 2014 Peter Schmerzl <peter@schmerzl-os.org>
 *
 * This file is subject to the terms and conditions of the GNU Lesser General
 * Public License v2.1. See the file LICENSE in the top level directory for more
 * details.
 */

/**
 * @ingroup     foobar
 * @{
 *
 * @file
 * @brief       Definitions for foo and bar functions.
 *
 * More detailed information about the file and the functionality implemented.
 *
 * @author      Peter Schmerzl <peter@schmerzl-os.org>
 *
 */

/**
 * @brief   Set the state of foobar.
 *
 * @param[in]  state      The new state of foobar.
 * @param[out] old_state  The old state of foobar is written to this variable.
 *
 * @return 1 if setting the state was successful, 0 otherwise.
 */
 int set_foobar(int state, int *old_state);

Common compilation warnings

Some solutions to correctly handle compilation warnings.

-Wformat

Solution for string formatting errors:

When printing a size_t
- use %u and cast the variable to (unsigned) because newlib-nano does not support %zu example
When printing an unsigned char/uint8_t
- Use %u because newlib-nano does not support %hu/PRIu8 example
When printing an uint32_t
- Use PRIu32 print format example
When printing 64bit variables
- It is not correctly supported by newlib-nano as said here. It is recommended to use fmt module for these. Example

-Wformat-nonliteral

For a printf style function with the following error: error: format string is not a string literal.

Function using a variable number of arguments:
- Use __attribute__((__format__ (__printf__, 3, 4))), where here 3 is the number of the argument with the format and 4 the format arguments, starting from 1. See example
Function using va_list:
- Use __attribute__((__format__ (__printf__, 1, 0))), where here 1 is the number of the argument with the format and 0 as there is no variable numbers of arguments. See example

Git

Make one commit per change.
The first line of the commit message describes the main feature of the commit.

Continuous Integration

If the CI tests fail due to errors these errors need to be addressed.
If the CI tests fail due to warnings/errors emitted by cppcheck you should try to fix the error. If the error is definitely a false positive there is the possibility to suppress this warning/error. You MUST do so by adding a comment, including a rationale why it is a false positive and why the code can't be fixed otherwise, in the following format:

    /* cppcheck-suppress <category of error/warning>
     * (reason: cppcheck is being really silly. this is certainly not a
     * null-pointer dereference */

Python coding convention

Code shall be compliant with Python 3.5 minimum because this is the default Python 3 version in Ubuntu 16.04 (used as the reference system for CI)
Code shall report no error when running the Flake8 tool, e.g:
- for style checks described in PEP8,
- for lint checks provided by Pyflakes,
- for complexity checks provided by the McCabe project
A line length of maximum of 120 is allowed instead of the pep8 79: this increases tests readability as they can expects long line of output.
Only runnable scripts shall start with #!/usr/bin/env python3
Runnable scripts shall use the following scheme:

#!/usr/bin/env python3

# Copyright (C) <your copyright>
#
# This file is subject to the terms and conditions of the GNU Lesser
# General Public License v2.1. See the file LICENSE in the top level
# directory for more details.

# put the module imports first
# see https://www.python.org/dev/peps/pep-0008/#imports
# for more details
import module1
import module2

# Optional global variables
GLOBAL_VARIABLE = "I'm global"


# local functions, if required
def local_func():
    # Put your local function code here


# The main function
def main_func():
    # Put your main code here


if __name__ == "__main__":
    # Call the main function from here:
    main_func()

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CODING_CONVENTIONS.md

CODING_CONVENTIONS.md

Coding Conventions

General

Types

Variables

Functions

Return values

Naming

Indentation and braces

Includes

Header Guards

C++ compatibility

Absolute values

Comments

Documentation

Common compilation warnings

-Wformat

-Wformat-nonliteral

Git

Continuous Integration

Python coding convention

Files

CODING_CONVENTIONS.md

Latest commit

History

CODING_CONVENTIONS.md

File metadata and controls

Coding Conventions

General

Types

Variables

Functions

Return values

Naming

Indentation and braces

Includes

Header Guards

C++ compatibility

Absolute values

Comments

Documentation

Common compilation warnings

-Wformat

-Wformat-nonliteral

Git

Continuous Integration

Python coding convention