rtems-docs/eng/coding-formatting.rst

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 2018.
.. COMMENT: RTEMS Foundation, The RTEMS Documentation Project

Formatting
**********

Rules
-----

* Adhere to the :ref:`EightyCharacterLineLimit`.

* Use spaces instead of tabs.

* Use two spaces for one indentation level.

* Put function return types and names on one line if they fit.

* Put function calls on one line if they fit.

* No space between a function name or function-like macro and
  the opening parenthesis.

* Put braces on the same line as and one space after the
  conditional expression ends.

* Put the opening brace of a function definition one line after the closing
  parenthesis of its prototype.

* Put a single space inside and outside of each parenthesis of a conditional
  expression.  Exception: never put a space before a closing semi-colon.

* Put a single space before and after ternary operators.

* Put a single space before and after binary operators.

* Put no space between unary operators (e.g. ``*``, ``&``, ``!``, ``~``, ``++``, ``--``)
  and their operands.

* No spaces around dereferencing operators (``->`` and ``.``).

* Do not use more than one blank line in a row.

* Do not use trailing white space at the end of a line.

.. _EightyCharacterLineLimit:

Eighty Character Line Limit
---------------------------

Code should look good for everyone under some standard width assumptions.
Where a line wraps should be the same for anyone reading the code.  For
historical reasons, RTEMS uses 80 characters as the maximum width for each line
of code.  The newline (``\n``) character terminating the line does not count
for the 80 character limit.

If you find yourself with code longer than 80 characters, first ask yourself
whether the nesting level is too deep, names too long, compound expressions too
complicated, or if some other guideline for improving readability can help to
shrink the line length.  Refactoring nested blocks into functions can help to
alleviate code width problems while improving code readability.  Making names
descriptive yet terse can also improve readability.  If absolutely necessary to
have a long line, follow the rules on this page to break the line up to adhere
to the 80 characters per line rule.

Breaking Long Lines
-------------------

The ``if``, ``while``, and ``for`` control statements have their condition
expressions aligned and broken on separate lines.  When the conditions have to
be broken, none go on the first line with the ``if``, ``while``, or ``for``
statement, and none go on the last line with the closing parenthesis and the
curly brace.  Long statements are broken up and indented at operators, with an
operator always being the last token on a line.  No blank spaces should be left
at the end of any line.  The continuation of a broken line is indented by one
level.  Here is an example with a ``for`` loop.

.. code-block:: c

  for ( initialization = statement; a + really + longish + statement + that + evaluates + to < a + boolean; another + statement ) {
    some_variable = a + really + longish + statement + that + needs + two + lines + gets + indented + four + more + spaces + on + the + second + and + subsequent + lines + and + broken + up + at + operators;
  }

Should be replaced with

.. code-block:: c

  for (
    initialization = statement;
    a + really + longish + statement + that + evaluates + to <
      a + boolean;
    another + statement
  ) {
    some_variable = a + really + longish + statement + that + needs +
      two + lines + gets + indented + four + more +
      spaces + on + the + second + and + subsequent +
      lines + and + broken + up + at + operators;
  }

Similarly,

.. code-block:: c

  if ( this + that < those && this + these < that && this + those < these && this < those && those < that ) {

should be broken up like

.. code-block:: c

  if (
    this + that < those &&
    this + these < that &&
    this + those < these &&
    this < those &&
    those < that
  ) {

Note that each expression that resolves to a boolean goes on its own line.
Where you place the boolean operator is a matter of choice.

When a line is long because of a comment at the end, move the comment to
just before the line, for example

.. code-block:: c

  #define A_LONG_MACRO_NAME (AND + EXPANSION) /* Plus + a + really + long + comment */

can be replaced with

.. code-block:: c

  /* Plus + a + really + long + comment */
  #define A_LONG_MACRO_NAME (AND + EXPANSION)

C Preprocessor macros need to be broken up with some care, because the
preprocessor does not understand that it should eat newline characters. So

.. code-block:: c

  #define A_LONG_MACRO_NAME (AND + EXCESSIVELY + LONG + EXPANSION + WITH + LOTS + OF + EXTRA + STUFF + DEFINED)

would become

.. code-block:: c

  #define A_LONG_MACRO_NAME ( \
    AND + EXCESSIVELY + LONG + EXPANSION + WITH + LOTS + OF + EXTRA + STUFF + \
    DEFINED \
  )

Notice that each line is terminated by a backslash.
The backslash tells the preprocessor to eat the newline. Of course, if you have
such a long macro, you should consider not using a macro.

Function declarations can be broken up at each argument, for example

.. code-block:: c

  int this_is_a_function( int arg1, int arg2, int arg3, int arg4, int arg5, int arg6, int arg7, int arg8, int arg9 );

would be broken up as

.. code-block:: c

  int this_is_a_function(
    int arg1,
    int arg2,
    int arg3,
    int arg4,
    int arg5,
    int arg6,
    int arg7,
    int arg8,
    int arg9
  );

Excessively long comments should be broken up at a word boundary or somewhere
that makes sense, for example

.. code-block:: c

  /* Excessively long comments should be broken up at a word boundary or somewhere that makes sense, for example */

would be

.. code-block:: c

  /*
   * Excessively long comments should be broken up at a word boundary or
   * somewhere that makes sense, for example.
   */

Note that multiline comments have a single asterisk aligned with the asterisk
in the opening ``/*``.  The closing ``*/`` should appear on a line by itself at
the end.