Coding Style for Apache Mynewt Core

This document is meant to define the coding style for Apache Mynewt, and all subprojects of Apache Mynewt. This covers C and Assembly coding conventions, only. Other languages (such as Go), have their own coding conventions.


  • All files that are newly written, should have the Apache License clause at the top of them.

  • For files that are copied from another source, but contain an Apache compatible license, the original license header shall be maintained.

  • For more information on applying the Apache license, the definitive source is here:

  • The Apache License clause for the top of files is as follows:

 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.

Whitespace and Braces

  • Code must be indented to 4 spaces, tabs should not be used.

  • Do not add whitespace at the end of a line.

  • Put space after keywords (for, if, return, switch, while).

  • for, else, if, while statements must have braces around their code blocks, i.e., do:

    if (x) {
    } else {


    if (x) 
  • Braces for statements must be on the same line as the statement. Good:
    for (i = 0; i < 10; i++) {
        if (i == 5) {
        } else {


    for (i = 0; i < 10; i++) 
    { <-- brace must be on same line as for
        if (i == 5) {
        } <-- no new line between else
        else {
  • After a function declaration, the braces should be on a newline, i.e. do:
    static void *
    function(int var1, int var2)


    static void *
    function(int var1, int var2) {

Line Length and Wrap

  • Line length should never exceed 79 columns.

  • When you have to wrap a long statement, put the operator at the end of the line. i.e.:

    if (x &&
        y == 10 &&


    if (x
        && y == 10
        && b)


  • No C++ style comments allowed.

  • When using a single line comment, put it above the line of code that you intend to comment, i.e., do:

    /* check variable */
    if (a) {


    if (a) { /* check variable */
  • All public APIs should be commented with Doxygen style comments describing purpose, parameters and return values. Private APIs need not be documented.

Header files

  • Header files must contain the following structure:

    • Apache License (see above)
    • #ifdef aliasing, to prevent multiple includes
    • #include directives for other required header files
    • #ifdef __cplusplus wrappers to maintain C++ friendly APIs
    • Contents of the header file
  • #ifdef aliasing, shall be in the following format, where the package name is “os” and the file name is “callout.h”:

#ifndef _OS_CALLOUT_H
#define _OS_CALLOUT_H
  • #include directives must happen prior to the cplusplus wrapper.

  • The cplusplus wrapper must have the following format, and precedes any contents of the header file:

#ifdef __cplusplus
#extern "C" {


  • Names of functions, structures and variables must be in all lowercase.

  • Names should be as short as possible, but no shorter.

  • Globally visible names must be prefixed with the name of the module, followed by the ‘_’ character, i.e.:





  • No spaces after function names when calling a function, i.e, do:
    rc = function(a)


    rc = function (a)
  • Arguments to function calls should have spaces between the comma, i.e. do:
    rc = function(a, b)


    rc = function(a,b)
  • The function type must be on a line by itself preceding the function, i.e. do:
    static void *
    function(int var1, int var2)


    static void *function(int var1, int var2)
  • In general, for functions that return values that denote success or error, 0 shall be success, and non-zero shall be the failure code.

Variables and Macros

  • Do not use typedefs for structures. This makes it impossible for applications to use pointers to those structures opaquely.

  • typedef may be used for non-structure types, where it is beneficial to hide or alias the underlying type used (e.g. os_time_t.) Indicate typedefs by applying the _t marker to them.

  • Place all function-local variable definitions at the top of the function body, before any statements.

Compiler Directives

  • Code must compile cleanly with -Wall enabled.