2016-09-19 19:07:56 +08:00
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.. _codingstyle:
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2016-09-19 19:07:44 +08:00
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Linux kernel coding style
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=========================
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2005-04-17 06:20:36 +08:00
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This is a short document describing the preferred coding style for the
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2016-09-19 19:07:46 +08:00
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linux kernel. Coding style is very personal, and I won't **force** my
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2005-04-17 06:20:36 +08:00
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views on anybody, but this is what goes for anything that I have to be
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able to maintain, and I'd prefer it for most other things too. Please
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at least consider the points made here.
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First off, I'd suggest printing out a copy of the GNU coding standards,
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and NOT read it. Burn them, it's a great symbolic gesture.
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Anyway, here goes:
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2016-09-19 19:07:44 +08:00
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1) Indentation
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--------------
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2005-04-17 06:20:36 +08:00
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Tabs are 8 characters, and thus indentations are also 8 characters.
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There are heretic movements that try to make indentations 4 (or even 2!)
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characters deep, and that is akin to trying to define the value of PI to
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be 3.
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Rationale: The whole idea behind indentation is to clearly define where
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a block of control starts and ends. Especially when you've been looking
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at your screen for 20 straight hours, you'll find it a lot easier to see
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how the indentation works if you have large indentations.
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Now, some people will claim that having 8-character indentations makes
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the code move too far to the right, and makes it hard to read on a
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80-character terminal screen. The answer to that is that if you need
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more than 3 levels of indentation, you're screwed anyway, and should fix
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your program.
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In short, 8-char indents make things easier to read, and have the added
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benefit of warning you when you're nesting your functions too deep.
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Heed that warning.
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2006-12-10 18:18:56 +08:00
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The preferred way to ease multiple indentation levels in a switch statement is
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2016-09-19 19:07:45 +08:00
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to align the ``switch`` and its subordinate ``case`` labels in the same column
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instead of ``double-indenting`` the ``case`` labels. E.g.:
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2006-12-10 18:18:56 +08:00
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2006-12-10 18:18:56 +08:00
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switch (suffix) {
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case 'G':
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case 'g':
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mem <<= 30;
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break;
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case 'M':
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case 'm':
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mem <<= 20;
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break;
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case 'K':
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case 'k':
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mem <<= 10;
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2019-10-06 00:46:43 +08:00
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fallthrough;
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2006-12-10 18:18:56 +08:00
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default:
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break;
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}
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2005-04-17 06:20:36 +08:00
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Don't put multiple statements on a single line unless you have
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something to hide:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2005-04-17 06:20:36 +08:00
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if (condition) do_this;
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do_something_everytime;
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2020-08-25 12:55:58 +08:00
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Don't use commas to avoid using braces:
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.. code-block:: c
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if (condition)
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do_this(), do_that();
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Always uses braces for multiple statements:
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.. code-block:: c
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if (condition) {
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do_this();
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do_that();
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}
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2006-12-10 18:18:56 +08:00
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Don't put multiple assignments on a single line either. Kernel coding style
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is super simple. Avoid tricky expressions.
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2020-08-25 12:55:58 +08:00
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2005-04-17 06:20:36 +08:00
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Outside of comments, documentation and except in Kconfig, spaces are never
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used for indentation, and the above example is deliberately broken.
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Get a decent editor and don't leave whitespace at the end of lines.
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2016-09-19 19:07:44 +08:00
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2) Breaking long lines and strings
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----------------------------------
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2005-04-17 06:20:36 +08:00
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Coding style is all about readability and maintainability using commonly
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available tools.
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2020-05-30 07:12:21 +08:00
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The preferred limit on the length of a single line is 80 columns.
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Statements longer than 80 columns should be broken into sensible chunks,
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unless exceeding 80 columns significantly increases readability and does
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not hide information.
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2020-06-01 18:00:49 +08:00
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Descendants are always substantially shorter than the parent and
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2020-05-30 07:12:21 +08:00
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are placed substantially to the right. A very commonly used style
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is to align descendants to a function open parenthesis.
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These same rules are applied to function headers with a long argument list.
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However, never break user-visible strings such as printk messages because
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that breaks the ability to grep for them.
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2011-08-04 03:19:07 +08:00
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2005-04-17 06:20:36 +08:00
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2016-09-19 19:07:44 +08:00
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3) Placing Braces and Spaces
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----------------------------
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2005-04-17 06:20:36 +08:00
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The other issue that always comes up in C styling is the placement of
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braces. Unlike the indent size, there are few technical reasons to
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choose one placement strategy over the other, but the preferred way, as
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shown to us by the prophets Kernighan and Ritchie, is to put the opening
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brace last on the line, and put the closing brace first, thusly:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2005-04-17 06:20:36 +08:00
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if (x is true) {
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we do y
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}
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2006-12-10 18:18:56 +08:00
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This applies to all non-function statement blocks (if, switch, for,
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while, do). E.g.:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2006-12-10 18:18:56 +08:00
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switch (action) {
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case KOBJ_ADD:
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return "add";
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case KOBJ_REMOVE:
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return "remove";
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case KOBJ_CHANGE:
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return "change";
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default:
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return NULL;
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}
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2005-04-17 06:20:36 +08:00
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However, there is one special case, namely functions: they have the
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opening brace at the beginning of the next line, thus:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2005-04-17 06:20:36 +08:00
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int function(int x)
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{
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body of function
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}
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Heretic people all over the world have claimed that this inconsistency
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is ... well ... inconsistent, but all right-thinking people know that
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2016-09-19 19:07:46 +08:00
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(a) K&R are **right** and (b) K&R are right. Besides, functions are
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2005-04-17 06:20:36 +08:00
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special anyway (you can't nest them in C).
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2016-09-19 19:07:46 +08:00
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Note that the closing brace is empty on a line of its own, **except** in
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2005-04-17 06:20:36 +08:00
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the cases where it is followed by a continuation of the same statement,
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2016-09-19 19:07:45 +08:00
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ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
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2005-04-17 06:20:36 +08:00
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this:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2005-04-17 06:20:36 +08:00
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do {
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body of do-loop
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} while (condition);
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and
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2005-04-17 06:20:36 +08:00
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if (x == y) {
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..
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} else if (x > y) {
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...
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} else {
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....
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}
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Rationale: K&R.
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Also, note that this brace-placement also minimizes the number of empty
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(or almost empty) lines, without any loss of readability. Thus, as the
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supply of new-lines on your screen is not a renewable resource (think
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25-line terminal screens here), you have more empty lines to put
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comments on.
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2007-05-08 15:30:34 +08:00
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Do not unnecessarily use braces where a single statement will do.
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2015-02-17 01:26:18 +08:00
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if (condition)
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action();
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2007-05-08 15:30:34 +08:00
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2011-03-23 07:35:01 +08:00
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and
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2016-09-19 19:07:44 +08:00
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.. code-block:: none
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2015-02-17 01:26:18 +08:00
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if (condition)
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do_this();
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else
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do_that();
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2011-03-23 07:35:01 +08:00
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2011-11-05 02:22:19 +08:00
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This does not apply if only one branch of a conditional statement is a single
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statement; in the latter case use braces in both branches:
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2007-05-08 15:30:34 +08:00
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2015-02-17 01:26:18 +08:00
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if (condition) {
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do_this();
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do_that();
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} else {
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otherwise();
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}
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2007-05-08 15:30:34 +08:00
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2018-03-15 06:21:38 +08:00
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Also, use braces when a loop contains more than a single simple statement:
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.. code-block:: c
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while (condition) {
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if (test)
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do_something();
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}
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2016-09-19 19:07:44 +08:00
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3.1) Spaces
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***********
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2006-12-10 18:18:56 +08:00
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Linux kernel style for use of spaces depends (mostly) on
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function-versus-keyword usage. Use a space after (most) keywords. The
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notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
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somewhat like functions (and are usually used with parentheses in Linux,
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2016-09-19 19:07:44 +08:00
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although they are not required in the language, as in: ``sizeof info`` after
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``struct fileinfo info;`` is declared).
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2006-12-10 18:18:56 +08:00
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2016-09-19 19:07:44 +08:00
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So use a space after these keywords::
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2015-02-17 01:26:18 +08:00
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2006-12-10 18:18:56 +08:00
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if, switch, case, for, do, while
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2015-02-17 01:26:18 +08:00
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2006-12-10 18:18:56 +08:00
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but not with sizeof, typeof, alignof, or __attribute__. E.g.,
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2015-02-17 01:26:18 +08:00
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2006-12-10 18:18:56 +08:00
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s = sizeof(struct file);
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Do not add spaces around (inside) parenthesized expressions. This example is
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2016-09-19 19:07:44 +08:00
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**bad**:
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.. code-block:: c
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2006-12-10 18:18:56 +08:00
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s = sizeof( struct file );
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When declaring pointer data or a function that returns a pointer type, the
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2016-09-19 19:07:45 +08:00
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preferred use of ``*`` is adjacent to the data name or function name and not
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2006-12-10 18:18:56 +08:00
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adjacent to the type name. Examples:
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2016-09-19 19:07:44 +08:00
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.. code-block:: c
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2006-12-10 18:18:56 +08:00
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char *linux_banner;
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unsigned long long memparse(char *ptr, char **retptr);
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char *match_strdup(substring_t *s);
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Use one space around (on each side of) most binary and ternary operators,
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2016-09-19 19:07:44 +08:00
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such as any of these::
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2006-12-10 18:18:56 +08:00
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= + - < > * / % | & ^ <= >= == != ? :
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2016-09-19 19:07:44 +08:00
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but no space after unary operators::
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2015-02-17 01:26:18 +08:00
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2006-12-10 18:18:56 +08:00
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& * + - ~ ! sizeof typeof alignof __attribute__ defined
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2016-09-19 19:07:44 +08:00
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no space before the postfix increment & decrement unary operators::
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2015-02-17 01:26:18 +08:00
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2006-12-10 18:18:56 +08:00
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++ --
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2016-09-19 19:07:44 +08:00
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no space after the prefix increment & decrement unary operators::
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2015-02-17 01:26:18 +08:00
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2006-12-10 18:18:56 +08:00
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++ --
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2016-09-19 19:07:45 +08:00
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and no space around the ``.`` and ``->`` structure member operators.
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2006-12-10 18:18:56 +08:00
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2007-07-16 14:41:37 +08:00
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Do not leave trailing whitespace at the ends of lines. Some editors with
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2016-09-19 19:07:45 +08:00
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``smart`` indentation will insert whitespace at the beginning of new lines as
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2007-07-16 14:41:37 +08:00
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appropriate, so you can start typing the next line of code right away.
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However, some such editors do not remove the whitespace if you end up not
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putting a line of code there, such as if you leave a blank line. As a result,
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you end up with lines containing trailing whitespace.
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Git will warn you about patches that introduce trailing whitespace, and can
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optionally strip the trailing whitespace for you; however, if applying a series
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of patches, this may make later patches in the series fail by changing their
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context lines.
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2005-04-17 06:20:36 +08:00
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2016-09-19 19:07:44 +08:00
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4) Naming
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---------
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2005-04-17 06:20:36 +08:00
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2020-02-17 08:08:26 +08:00
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C is a Spartan language, and your naming conventions should follow suit.
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Unlike Modula-2 and Pascal programmers, C programmers do not use cute
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names like ThisVariableIsATemporaryCounter. A C programmer would call that
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2016-09-19 19:07:45 +08:00
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variable ``tmp``, which is much easier to write, and not the least more
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2005-04-17 06:20:36 +08:00
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difficult to understand.
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HOWEVER, while mixed-case names are frowned upon, descriptive names for
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2016-09-19 19:07:45 +08:00
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global variables are a must. To call a global function ``foo`` is a
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2005-04-17 06:20:36 +08:00
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shooting offense.
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2016-09-19 19:07:46 +08:00
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GLOBAL variables (to be used only if you **really** need them) need to
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2005-04-17 06:20:36 +08:00
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have descriptive names, as do global functions. If you have a function
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that counts the number of active users, you should call that
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2016-09-19 19:07:46 +08:00
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``count_active_users()`` or similar, you should **not** call it ``cntusr()``.
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2005-04-17 06:20:36 +08:00
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Encoding the type of a function into the name (so-called Hungarian
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2020-02-17 08:08:26 +08:00
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notation) is asinine - the compiler knows the types anyway and can check
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2021-02-08 23:04:48 +08:00
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those, and it only confuses the programmer.
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2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
LOCAL variable names should be short, and to the point. If you have
|
2016-09-19 19:07:45 +08:00
|
|
|
some random integer loop counter, it should probably be called ``i``.
|
|
|
|
Calling it ``loop_counter`` is non-productive, if there is no chance of it
|
|
|
|
being mis-understood. Similarly, ``tmp`` can be just about any type of
|
2005-04-17 06:20:36 +08:00
|
|
|
variable that is used to hold a temporary value.
|
|
|
|
|
|
|
|
If you are afraid to mix up your local variable names, you have another
|
|
|
|
problem, which is called the function-growth-hormone-imbalance syndrome.
|
2006-12-10 18:18:56 +08:00
|
|
|
See chapter 6 (Functions).
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2020-07-04 14:54:35 +08:00
|
|
|
For symbol names and documentation, avoid introducing new usage of
|
|
|
|
'master / slave' (or 'slave' independent of 'master') and 'blacklist /
|
|
|
|
whitelist'.
|
|
|
|
|
|
|
|
Recommended replacements for 'master / slave' are:
|
|
|
|
'{primary,main} / {secondary,replica,subordinate}'
|
|
|
|
'{initiator,requester} / {target,responder}'
|
|
|
|
'{controller,host} / {device,worker,proxy}'
|
|
|
|
'leader / follower'
|
|
|
|
'director / performer'
|
|
|
|
|
|
|
|
Recommended replacements for 'blacklist/whitelist' are:
|
|
|
|
'denylist / allowlist'
|
|
|
|
'blocklist / passlist'
|
|
|
|
|
|
|
|
Exceptions for introducing new usage is to maintain a userspace ABI/API,
|
|
|
|
or when updating code for an existing (as of 2020) hardware or protocol
|
|
|
|
specification that mandates those terms. For new specifications
|
|
|
|
translate specification usage of the terminology to the kernel coding
|
|
|
|
standard where possible.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
5) Typedefs
|
|
|
|
-----------
|
2006-06-23 17:05:58 +08:00
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
Please don't use things like ``vps_t``.
|
2016-09-19 19:07:46 +08:00
|
|
|
It's a **mistake** to use typedef for structures and pointers. When you see a
|
2006-06-23 17:05:58 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
|
|
|
|
2006-06-23 17:05:58 +08:00
|
|
|
vps_t a;
|
|
|
|
|
|
|
|
in the source, what does it mean?
|
|
|
|
In contrast, if it says
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2006-06-23 17:05:58 +08:00
|
|
|
struct virtual_container *a;
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
you can actually tell what ``a`` is.
|
2006-06-23 17:05:58 +08:00
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
Lots of people think that typedefs ``help readability``. Not so. They are
|
2006-06-23 17:05:58 +08:00
|
|
|
useful only for:
|
|
|
|
|
2016-09-19 19:07:46 +08:00
|
|
|
(a) totally opaque objects (where the typedef is actively used to **hide**
|
2006-06-23 17:05:58 +08:00
|
|
|
what the object is).
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
Example: ``pte_t`` etc. opaque objects that you can only access using
|
2006-06-23 17:05:58 +08:00
|
|
|
the proper accessor functions.
|
|
|
|
|
2016-09-19 19:07:47 +08:00
|
|
|
.. note::
|
|
|
|
|
|
|
|
Opaqueness and ``accessor functions`` are not good in themselves.
|
|
|
|
The reason we have them for things like pte_t etc. is that there
|
|
|
|
really is absolutely **zero** portably accessible information there.
|
2006-06-23 17:05:58 +08:00
|
|
|
|
2016-09-19 19:07:46 +08:00
|
|
|
(b) Clear integer types, where the abstraction **helps** avoid confusion
|
2016-09-19 19:07:45 +08:00
|
|
|
whether it is ``int`` or ``long``.
|
2006-06-23 17:05:58 +08:00
|
|
|
|
|
|
|
u8/u16/u32 are perfectly fine typedefs, although they fit into
|
|
|
|
category (d) better than here.
|
|
|
|
|
2016-09-19 19:07:47 +08:00
|
|
|
.. note::
|
|
|
|
|
|
|
|
Again - there needs to be a **reason** for this. If something is
|
|
|
|
``unsigned long``, then there's no reason to do
|
2006-06-23 17:05:58 +08:00
|
|
|
|
|
|
|
typedef unsigned long myflags_t;
|
|
|
|
|
|
|
|
but if there is a clear reason for why it under certain circumstances
|
2016-09-19 19:07:45 +08:00
|
|
|
might be an ``unsigned int`` and under other configurations might be
|
|
|
|
``unsigned long``, then by all means go ahead and use a typedef.
|
2006-06-23 17:05:58 +08:00
|
|
|
|
2016-09-19 19:07:46 +08:00
|
|
|
(c) when you use sparse to literally create a **new** type for
|
2006-06-23 17:05:58 +08:00
|
|
|
type-checking.
|
|
|
|
|
|
|
|
(d) New types which are identical to standard C99 types, in certain
|
|
|
|
exceptional circumstances.
|
|
|
|
|
|
|
|
Although it would only take a short amount of time for the eyes and
|
2016-09-19 19:07:45 +08:00
|
|
|
brain to become accustomed to the standard types like ``uint32_t``,
|
2006-06-23 17:05:58 +08:00
|
|
|
some people object to their use anyway.
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
|
2006-06-23 17:05:58 +08:00
|
|
|
signed equivalents which are identical to standard types are
|
|
|
|
permitted -- although they are not mandatory in new code of your
|
|
|
|
own.
|
|
|
|
|
|
|
|
When editing existing code which already uses one or the other set
|
|
|
|
of types, you should conform to the existing choices in that code.
|
|
|
|
|
|
|
|
(e) Types safe for use in userspace.
|
|
|
|
|
|
|
|
In certain structures which are visible to userspace, we cannot
|
2016-09-19 19:07:45 +08:00
|
|
|
require C99 types and cannot use the ``u32`` form above. Thus, we
|
2006-06-23 17:05:58 +08:00
|
|
|
use __u32 and similar types in all structures which are shared
|
|
|
|
with userspace.
|
|
|
|
|
|
|
|
Maybe there are other cases too, but the rule should basically be to NEVER
|
|
|
|
EVER use a typedef unless you can clearly match one of those rules.
|
|
|
|
|
|
|
|
In general, a pointer, or a struct that has elements that can reasonably
|
2016-09-19 19:07:46 +08:00
|
|
|
be directly accessed should **never** be a typedef.
|
2006-06-23 17:05:58 +08:00
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
6) Functions
|
|
|
|
------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Functions should be short and sweet, and do just one thing. They should
|
|
|
|
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
|
|
|
|
as we all know), and do one thing and do that well.
|
|
|
|
|
|
|
|
The maximum length of a function is inversely proportional to the
|
|
|
|
complexity and indentation level of that function. So, if you have a
|
|
|
|
conceptually simple function that is just one long (but simple)
|
|
|
|
case-statement, where you have to do lots of small things for a lot of
|
|
|
|
different cases, it's OK to have a longer function.
|
|
|
|
|
|
|
|
However, if you have a complex function, and you suspect that a
|
|
|
|
less-than-gifted first-year high-school student might not even
|
|
|
|
understand what the function is all about, you should adhere to the
|
|
|
|
maximum limits all the more closely. Use helper functions with
|
|
|
|
descriptive names (you can ask the compiler to in-line them if you think
|
|
|
|
it's performance-critical, and it will probably do a better job of it
|
|
|
|
than you would have done).
|
|
|
|
|
|
|
|
Another measure of the function is the number of local variables. They
|
|
|
|
shouldn't exceed 5-10, or you're doing something wrong. Re-think the
|
|
|
|
function, and split it into smaller pieces. A human brain can
|
|
|
|
generally easily keep track of about 7 different things, anything more
|
|
|
|
and it gets confused. You know you're brilliant, but maybe you'd like
|
|
|
|
to understand what you did 2 weeks from now.
|
|
|
|
|
2006-12-10 18:18:56 +08:00
|
|
|
In source files, separate functions with one blank line. If the function is
|
2016-09-19 19:07:44 +08:00
|
|
|
exported, the **EXPORT** macro for it should follow immediately after the
|
|
|
|
closing function brace line. E.g.:
|
|
|
|
|
|
|
|
.. code-block:: c
|
2006-12-10 18:18:56 +08:00
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
int system_is_up(void)
|
|
|
|
{
|
|
|
|
return system_state == SYSTEM_RUNNING;
|
|
|
|
}
|
|
|
|
EXPORT_SYMBOL(system_is_up);
|
2006-12-10 18:18:56 +08:00
|
|
|
|
|
|
|
In function prototypes, include parameter names with their data types.
|
|
|
|
Although this is not required by the C language, it is preferred in Linux
|
|
|
|
because it is a simple way to add valuable information for the reader.
|
|
|
|
|
2019-01-14 11:28:58 +08:00
|
|
|
Do not use the ``extern`` keyword with function prototypes as this makes
|
2019-01-04 07:26:16 +08:00
|
|
|
lines longer and isn't strictly necessary.
|
|
|
|
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
7) Centralized exiting of functions
|
|
|
|
-----------------------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Albeit deprecated by some people, the equivalent of the goto statement is
|
|
|
|
used frequently by compilers in form of the unconditional jump instruction.
|
|
|
|
|
|
|
|
The goto statement comes in handy when a function exits from multiple
|
2013-07-04 06:08:08 +08:00
|
|
|
locations and some common work such as cleanup has to be done. If there is no
|
|
|
|
cleanup needed then just return directly.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2014-12-02 16:59:50 +08:00
|
|
|
Choose label names which say what the goto does or why the goto exists. An
|
2016-09-19 19:07:45 +08:00
|
|
|
example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
|
|
|
|
Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
|
2016-07-25 20:29:06 +08:00
|
|
|
renumber them if you ever add or remove exit paths, and they make correctness
|
|
|
|
difficult to verify anyway.
|
|
|
|
|
2014-12-02 16:59:50 +08:00
|
|
|
The rationale for using gotos is:
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
- unconditional statements are easier to understand and follow
|
|
|
|
- nesting is reduced
|
|
|
|
- errors by not updating individual exit points when making
|
2016-09-19 19:07:44 +08:00
|
|
|
modifications are prevented
|
2005-04-17 06:20:36 +08:00
|
|
|
- saves the compiler work to optimize redundant code away ;)
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
int fun(int a)
|
|
|
|
{
|
|
|
|
int result = 0;
|
|
|
|
char *buffer;
|
|
|
|
|
|
|
|
buffer = kmalloc(SIZE, GFP_KERNEL);
|
|
|
|
if (!buffer)
|
|
|
|
return -ENOMEM;
|
|
|
|
|
|
|
|
if (condition1) {
|
|
|
|
while (loop1) {
|
|
|
|
...
|
|
|
|
}
|
|
|
|
result = 1;
|
2016-11-03 00:57:34 +08:00
|
|
|
goto out_free_buffer;
|
2005-04-17 06:20:36 +08:00
|
|
|
}
|
2015-02-17 01:26:18 +08:00
|
|
|
...
|
2016-09-22 05:46:18 +08:00
|
|
|
out_free_buffer:
|
2015-02-17 01:26:18 +08:00
|
|
|
kfree(buffer);
|
|
|
|
return result;
|
2005-04-17 06:20:36 +08:00
|
|
|
}
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
A common type of bug to be aware of is ``one err bugs`` which look like this:
|
2014-12-02 16:59:50 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2016-09-22 05:46:18 +08:00
|
|
|
err:
|
2015-02-17 01:26:18 +08:00
|
|
|
kfree(foo->bar);
|
|
|
|
kfree(foo);
|
|
|
|
return ret;
|
2014-12-02 16:59:50 +08:00
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
The bug in this code is that on some exit paths ``foo`` is NULL. Normally the
|
|
|
|
fix for this is to split it up into two error labels ``err_free_bar:`` and
|
|
|
|
``err_free_foo:``:
|
2016-07-25 20:29:06 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2016-07-25 20:29:06 +08:00
|
|
|
err_free_bar:
|
|
|
|
kfree(foo->bar);
|
|
|
|
err_free_foo:
|
|
|
|
kfree(foo);
|
|
|
|
return ret;
|
|
|
|
|
|
|
|
Ideally you should simulate errors to test all exit paths.
|
2014-12-02 16:59:50 +08:00
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
8) Commenting
|
|
|
|
-------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Comments are good, but there is also a danger of over-commenting. NEVER
|
|
|
|
try to explain HOW your code works in a comment: it's much better to
|
2016-09-19 19:07:46 +08:00
|
|
|
write the code so that the **working** is obvious, and it's a waste of
|
2005-04-17 06:20:36 +08:00
|
|
|
time to explain badly written code.
|
|
|
|
|
|
|
|
Generally, you want your comments to tell WHAT your code does, not HOW.
|
|
|
|
Also, try to avoid putting comments inside a function body: if the
|
|
|
|
function is so complex that you need to separately comment parts of it,
|
2006-12-10 18:18:56 +08:00
|
|
|
you should probably go back to chapter 6 for a while. You can make
|
2005-04-17 06:20:36 +08:00
|
|
|
small comments to note or warn about something particularly clever (or
|
|
|
|
ugly), but try to avoid excess. Instead, put the comments at the head
|
|
|
|
of the function, telling people what it does, and possibly WHY it does
|
|
|
|
it.
|
|
|
|
|
2006-12-10 18:18:56 +08:00
|
|
|
When commenting the kernel API functions, please use the kernel-doc format.
|
2016-11-17 18:32:33 +08:00
|
|
|
See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
|
|
|
|
``scripts/kernel-doc`` for details.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2006-12-10 18:18:56 +08:00
|
|
|
The preferred style for long (multi-line) comments is:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2006-12-10 18:18:56 +08:00
|
|
|
/*
|
|
|
|
* This is the preferred style for multi-line
|
|
|
|
* comments in the Linux kernel source code.
|
|
|
|
* Please use it consistently.
|
|
|
|
*
|
|
|
|
* Description: A column of asterisks on the left side,
|
|
|
|
* with beginning and ending almost-blank lines.
|
|
|
|
*/
|
|
|
|
|
2012-10-05 08:13:36 +08:00
|
|
|
For files in net/ and drivers/net/ the preferred style for long (multi-line)
|
|
|
|
comments is a little different.
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2012-10-05 08:13:36 +08:00
|
|
|
/* The preferred comment style for files in net/ and drivers/net
|
|
|
|
* looks like this.
|
|
|
|
*
|
|
|
|
* It is nearly the same as the generally preferred comment style,
|
|
|
|
* but there is no initial almost-blank line.
|
|
|
|
*/
|
|
|
|
|
2006-12-10 18:18:56 +08:00
|
|
|
It's also important to comment data, whether they are basic types or derived
|
|
|
|
types. To this end, use just one data declaration per line (no commas for
|
|
|
|
multiple data declarations). This leaves you room for a small comment on each
|
|
|
|
item, explaining its use.
|
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
9) You've made a mess of it
|
|
|
|
---------------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
That's OK, we all do. You've probably been told by your long-time Unix
|
2016-09-19 19:07:45 +08:00
|
|
|
user helper that ``GNU emacs`` automatically formats the C sources for
|
2005-04-17 06:20:36 +08:00
|
|
|
you, and you've noticed that yes, it does do that, but the defaults it
|
|
|
|
uses are less than desirable (in fact, they are worse than random
|
|
|
|
typing - an infinite number of monkeys typing into GNU emacs would never
|
|
|
|
make a good program).
|
|
|
|
|
|
|
|
So, you can either get rid of GNU emacs, or change it to use saner
|
|
|
|
values. To do the latter, you can stick the following in your .emacs file:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: none
|
|
|
|
|
|
|
|
(defun c-lineup-arglist-tabs-only (ignored)
|
|
|
|
"Line up argument lists by tabs, not spaces"
|
|
|
|
(let* ((anchor (c-langelem-pos c-syntactic-element))
|
|
|
|
(column (c-langelem-2nd-pos c-syntactic-element))
|
|
|
|
(offset (- (1+ column) anchor))
|
|
|
|
(steps (floor offset c-basic-offset)))
|
|
|
|
(* (max steps 1)
|
|
|
|
c-basic-offset)))
|
|
|
|
|
2019-01-08 02:20:19 +08:00
|
|
|
(dir-locals-set-class-variables
|
|
|
|
'linux-kernel
|
|
|
|
'((c-mode . (
|
|
|
|
(c-basic-offset . 8)
|
|
|
|
(c-label-minimum-indentation . 0)
|
|
|
|
(c-offsets-alist . (
|
|
|
|
(arglist-close . c-lineup-arglist-tabs-only)
|
|
|
|
(arglist-cont-nonempty .
|
|
|
|
(c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
|
|
|
|
(arglist-intro . +)
|
|
|
|
(brace-list-intro . +)
|
|
|
|
(c . c-lineup-C-comments)
|
|
|
|
(case-label . 0)
|
|
|
|
(comment-intro . c-lineup-comment)
|
|
|
|
(cpp-define-intro . +)
|
|
|
|
(cpp-macro . -1000)
|
|
|
|
(cpp-macro-cont . +)
|
|
|
|
(defun-block-intro . +)
|
|
|
|
(else-clause . 0)
|
|
|
|
(func-decl-cont . +)
|
|
|
|
(inclass . +)
|
|
|
|
(inher-cont . c-lineup-multi-inher)
|
|
|
|
(knr-argdecl-intro . 0)
|
|
|
|
(label . -1000)
|
|
|
|
(statement . 0)
|
|
|
|
(statement-block-intro . +)
|
|
|
|
(statement-case-intro . +)
|
|
|
|
(statement-cont . +)
|
|
|
|
(substatement . +)
|
|
|
|
))
|
|
|
|
(indent-tabs-mode . t)
|
|
|
|
(show-trailing-whitespace . t)
|
|
|
|
))))
|
|
|
|
|
|
|
|
(dir-locals-set-directory-class
|
|
|
|
(expand-file-name "~/src/linux-trees")
|
|
|
|
'linux-kernel)
|
2008-07-25 16:45:51 +08:00
|
|
|
|
|
|
|
This will make emacs go better with the kernel coding style for C
|
2016-09-19 19:07:44 +08:00
|
|
|
files below ``~/src/linux-trees``.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
But even if you fail in getting emacs to do sane formatting, not
|
2016-09-19 19:07:45 +08:00
|
|
|
everything is lost: use ``indent``.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Now, again, GNU indent has the same brain-dead settings that GNU emacs
|
|
|
|
has, which is why you need to give it a few command line options.
|
|
|
|
However, that's not too bad, because even the makers of GNU indent
|
|
|
|
recognize the authority of K&R (the GNU people aren't evil, they are
|
|
|
|
just severely misguided in this matter), so you just give indent the
|
2016-09-19 19:07:45 +08:00
|
|
|
options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
|
|
|
|
``scripts/Lindent``, which indents in the latest style.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
``indent`` has a lot of options, and especially when it comes to comment
|
2005-04-17 06:20:36 +08:00
|
|
|
re-formatting you may want to take a look at the man page. But
|
2016-09-19 19:07:45 +08:00
|
|
|
remember: ``indent`` is not a fix for bad programming.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
clang-format: add configuration file
clang-format is a tool to format C/C++/... code according to a set of
rules and heuristics. Like most tools, it is not perfect nor covers
every single case, but it is good enough to be helpful.
In particular, it is useful for quickly re-formatting blocks of code
automatically, for reviewing full files in order to spot coding style
mistakes, typos and possible improvements. It is also handy for sorting
``#includes``, for aligning variables and macros, for reflowing text and
other similar tasks. It also serves as a teaching tool/guide for
newcomers.
The tool itself has been already included in the repositories of popular
Linux distributions for a long time. The rules in this file are
intended for clang-format >= 4, which is easily available in most
distributions.
This commit adds the configuration file that contains the rules that the
tool uses to know how to format the code according to the kernel coding
style. This gives us several advantages:
* clang-format works out of the box with reasonable defaults;
avoiding that everyone has to re-do the configuration.
* Everyone agrees (eventually) on what is the most useful default
configuration for most of the kernel.
* If it becomes commonplace among kernel developers, clang-format
may feel compelled to support us better. They already recognize
the Linux kernel and its style in their documentation and in one
of the style sub-options.
Some of clang-format's features relevant for the kernel are:
* Uses clang's tooling support behind the scenes to parse and rewrite
the code. It is not based on ad-hoc regexps.
* Supports reasonably well the Linux kernel coding style.
* Fast enough to be used at the press of a key.
* There are already integrations (either built-in or third-party)
for many common editors used by kernel developers (e.g. vim,
emacs, Sublime, Atom...) that allow you to format an entire file
or, more usefully, just your selection.
* Able to parse unified diffs -- you can, for instance, reformat
only the lines changed by a git commit.
* Able to reflow text comments as well.
* Widely supported and used by hundreds of developers in highly
complex projects and organizations (e.g. the LLVM project itself,
Chromium, WebKit, Google, Mozilla...). Therefore, it will be
supported for a long time.
See more information about the tool at:
https://clang.llvm.org/docs/ClangFormat.html
https://clang.llvm.org/docs/ClangFormatStyleOptions.html
Link: http://lkml.kernel.org/r/20180318171632.qfkemw3mwbcukth6@gmail.com
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2018-04-11 07:32:40 +08:00
|
|
|
Note that you can also use the ``clang-format`` tool to help you with
|
|
|
|
these rules, to quickly re-format parts of your code automatically,
|
|
|
|
and to review full files in order to spot coding style mistakes,
|
|
|
|
typos and possible improvements. It is also handy for sorting ``#includes``,
|
|
|
|
for aligning variables/macros, for reflowing text and other similar tasks.
|
|
|
|
See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
|
|
|
|
for more details.
|
|
|
|
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
10) Kconfig configuration files
|
|
|
|
-------------------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2007-05-24 04:57:42 +08:00
|
|
|
For all of the Kconfig* configuration files throughout the source tree,
|
2016-09-19 19:07:45 +08:00
|
|
|
the indentation is somewhat different. Lines under a ``config`` definition
|
2007-05-24 04:57:42 +08:00
|
|
|
are indented with one tab, while help text is indented an additional two
|
2016-09-19 19:07:44 +08:00
|
|
|
spaces. Example::
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
config AUDIT
|
2007-05-24 04:57:42 +08:00
|
|
|
bool "Auditing support"
|
|
|
|
depends on NET
|
2005-04-17 06:20:36 +08:00
|
|
|
help
|
2007-05-24 04:57:42 +08:00
|
|
|
Enable auditing infrastructure that can be used with another
|
|
|
|
kernel subsystem, such as SELinux (which requires this for
|
|
|
|
logging of avc messages output). Does not do system-call
|
|
|
|
auditing without CONFIG_AUDITSYSCALL.
|
|
|
|
|
2012-10-03 02:16:15 +08:00
|
|
|
Seriously dangerous features (such as write support for certain
|
2016-09-19 19:07:44 +08:00
|
|
|
filesystems) should advertise this prominently in their prompt string::
|
2007-05-24 04:57:42 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
config ADFS_FS_RW
|
2007-05-24 04:57:42 +08:00
|
|
|
bool "ADFS write support (DANGEROUS)"
|
|
|
|
depends on ADFS_FS
|
|
|
|
...
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2007-05-24 04:57:42 +08:00
|
|
|
For full documentation on the configuration files, see the file
|
2019-06-13 01:52:48 +08:00
|
|
|
Documentation/kbuild/kconfig-language.rst.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
11) Data structures
|
|
|
|
-------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Data structures that have visibility outside the single-threaded
|
|
|
|
environment they are created and destroyed in should always have
|
|
|
|
reference counts. In the kernel, garbage collection doesn't exist (and
|
|
|
|
outside the kernel garbage collection is slow and inefficient), which
|
2016-09-19 19:07:46 +08:00
|
|
|
means that you absolutely **have** to reference count all your uses.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Reference counting means that you can avoid locking, and allows multiple
|
|
|
|
users to have access to the data structure in parallel - and not having
|
|
|
|
to worry about the structure suddenly going away from under them just
|
|
|
|
because they slept or did something else for a while.
|
|
|
|
|
2016-09-19 19:07:46 +08:00
|
|
|
Note that locking is **not** a replacement for reference counting.
|
2005-04-17 06:20:36 +08:00
|
|
|
Locking is used to keep data structures coherent, while reference
|
|
|
|
counting is a memory management technique. Usually both are needed, and
|
|
|
|
they are not to be confused with each other.
|
|
|
|
|
|
|
|
Many data structures can indeed have two levels of reference counting,
|
2016-09-19 19:07:45 +08:00
|
|
|
when there are users of different ``classes``. The subclass count counts
|
2005-04-17 06:20:36 +08:00
|
|
|
the number of subclass users, and decrements the global count just once
|
|
|
|
when the subclass count goes to zero.
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
Examples of this kind of ``multi-level-reference-counting`` can be found in
|
|
|
|
memory management (``struct mm_struct``: mm_users and mm_count), and in
|
|
|
|
filesystem code (``struct super_block``: s_count and s_active).
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Remember: if another thread can find your data structure, and you don't
|
|
|
|
have a reference count on it, you almost certainly have a bug.
|
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
12) Macros, Enums and RTL
|
|
|
|
-------------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Names of macros defining constants and labels in enums are capitalized.
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#define CONSTANT 0x12345
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Enums are preferred when defining several related constants.
|
|
|
|
|
|
|
|
CAPITALIZED macro names are appreciated but macros resembling functions
|
|
|
|
may be named in lower case.
|
|
|
|
|
|
|
|
Generally, inline functions are preferable to macros resembling functions.
|
|
|
|
|
|
|
|
Macros with multiple statements should be enclosed in a do - while block:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
|
|
|
#define macrofun(a, b, c) \
|
2015-02-17 01:26:18 +08:00
|
|
|
do { \
|
|
|
|
if (a == 5) \
|
|
|
|
do_this(b, c); \
|
|
|
|
} while (0)
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Things to avoid when using macros:
|
|
|
|
|
|
|
|
1) macros that affect control flow:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#define FOO(x) \
|
|
|
|
do { \
|
|
|
|
if (blah(x) < 0) \
|
|
|
|
return -EBUGGERED; \
|
2016-01-25 13:54:39 +08:00
|
|
|
} while (0)
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:46 +08:00
|
|
|
is a **very** bad idea. It looks like a function call but exits the ``calling``
|
2005-04-17 06:20:36 +08:00
|
|
|
function; don't break the internal parsers of those who will read the code.
|
|
|
|
|
|
|
|
2) macros that depend on having a local variable with a magic name:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#define FOO(val) bar(index, val)
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
might look like a good thing, but it's confusing as hell when one reads the
|
|
|
|
code and it's prone to breakage from seemingly innocent changes.
|
|
|
|
|
|
|
|
3) macros with arguments that are used as l-values: FOO(x) = y; will
|
|
|
|
bite you if somebody e.g. turns FOO into an inline function.
|
|
|
|
|
|
|
|
4) forgetting about precedence: macros defining constants using expressions
|
|
|
|
must enclose the expression in parentheses. Beware of similar issues with
|
|
|
|
macros using parameters.
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#define CONSTANT 0x4000
|
|
|
|
#define CONSTEXP (CONSTANT | 3)
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2015-04-17 03:43:31 +08:00
|
|
|
5) namespace collisions when defining local variables in macros resembling
|
|
|
|
functions:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
|
|
|
#define FOO(x) \
|
|
|
|
({ \
|
|
|
|
typeof(x) ret; \
|
|
|
|
ret = calc_ret(x); \
|
|
|
|
(ret); \
|
|
|
|
})
|
2015-04-17 03:43:31 +08:00
|
|
|
|
|
|
|
ret is a common name for a local variable - __foo_ret is less likely
|
|
|
|
to collide with an existing variable.
|
|
|
|
|
2005-04-17 06:20:36 +08:00
|
|
|
The cpp manual deals with macros exhaustively. The gcc internals manual also
|
|
|
|
covers RTL which is used frequently with assembly language in the kernel.
|
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
13) Printing kernel messages
|
|
|
|
----------------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Kernel developers like to be seen as literate. Do mind the spelling
|
2020-02-17 08:08:26 +08:00
|
|
|
of kernel messages to make a good impression. Do not use incorrect
|
|
|
|
contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
|
|
|
|
messages concise, clear, and unambiguous.
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
Kernel messages do not have to be terminated with a period.
|
|
|
|
|
|
|
|
Printing numbers in parentheses (%d) adds no value and should be avoided.
|
|
|
|
|
2007-07-14 07:32:09 +08:00
|
|
|
There are a number of driver model diagnostic macros in <linux/device.h>
|
|
|
|
which you should use to make sure messages are matched to the right device
|
|
|
|
and driver, and are tagged with the right level: dev_err(), dev_warn(),
|
|
|
|
dev_info(), and so forth. For messages that aren't associated with a
|
2014-06-05 07:11:44 +08:00
|
|
|
particular device, <linux/printk.h> defines pr_notice(), pr_info(),
|
|
|
|
pr_warn(), pr_err(), etc.
|
2007-07-14 07:32:09 +08:00
|
|
|
|
|
|
|
Coming up with good debugging messages can be quite a challenge; and once
|
2014-06-05 07:11:44 +08:00
|
|
|
you have them, they can be a huge help for remote troubleshooting. However
|
|
|
|
debug message printing is handled differently than printing other non-debug
|
|
|
|
messages. While the other pr_XXX() functions print unconditionally,
|
|
|
|
pr_debug() does not; it is compiled out by default, unless either DEBUG is
|
|
|
|
defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
|
|
|
|
and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
|
|
|
|
the ones already enabled by DEBUG.
|
|
|
|
|
|
|
|
Many subsystems have Kconfig debug options to turn on -DDEBUG in the
|
|
|
|
corresponding Makefile; in other cases specific files #define DEBUG. And
|
|
|
|
when a debug message should be unconditionally printed, such as if it is
|
2014-07-24 17:39:44 +08:00
|
|
|
already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
|
2014-06-05 07:11:44 +08:00
|
|
|
used.
|
2007-07-14 07:32:09 +08:00
|
|
|
|
2005-04-17 06:20:36 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
14) Allocating memory
|
|
|
|
---------------------
|
2005-09-17 10:28:11 +08:00
|
|
|
|
|
|
|
The kernel provides the following general purpose memory allocators:
|
2012-06-01 07:26:04 +08:00
|
|
|
kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
|
|
|
|
vzalloc(). Please refer to the API documentation for further information
|
2019-03-31 01:25:03 +08:00
|
|
|
about them. :ref:`Documentation/core-api/memory-allocation.rst
|
|
|
|
<memory_allocation>`
|
2005-09-17 10:28:11 +08:00
|
|
|
|
|
|
|
The preferred form for passing a size of a struct is the following:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2005-09-17 10:28:11 +08:00
|
|
|
p = kmalloc(sizeof(*p), ...);
|
|
|
|
|
|
|
|
The alternative form where struct name is spelled out hurts readability and
|
|
|
|
introduces an opportunity for a bug when the pointer variable type is changed
|
|
|
|
but the corresponding sizeof that is passed to a memory allocator is not.
|
|
|
|
|
|
|
|
Casting the return value which is a void pointer is redundant. The conversion
|
|
|
|
from void pointer to any other pointer type is guaranteed by the C programming
|
|
|
|
language.
|
|
|
|
|
2012-06-01 07:26:04 +08:00
|
|
|
The preferred form for allocating an array is the following:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2012-06-01 07:26:04 +08:00
|
|
|
p = kmalloc_array(n, sizeof(...), ...);
|
|
|
|
|
|
|
|
The preferred form for allocating a zeroed array is the following:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2012-06-01 07:26:04 +08:00
|
|
|
p = kcalloc(n, sizeof(...), ...);
|
|
|
|
|
|
|
|
Both forms check for overflow on the allocation size n * sizeof(...),
|
|
|
|
and return NULL if that occurred.
|
|
|
|
|
2019-03-31 01:25:03 +08:00
|
|
|
These generic allocation functions all emit a stack dump on failure when used
|
|
|
|
without __GFP_NOWARN so there is no use in emitting an additional failure
|
|
|
|
message when NULL is returned.
|
2005-09-17 10:28:11 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
15) The inline disease
|
|
|
|
----------------------
|
2006-01-08 17:05:04 +08:00
|
|
|
|
|
|
|
There appears to be a common misperception that gcc has a magic "make me
|
2016-09-19 19:07:45 +08:00
|
|
|
faster" speedup option called ``inline``. While the use of inlines can be
|
2007-05-08 15:31:06 +08:00
|
|
|
appropriate (for example as a means of replacing macros, see Chapter 12), it
|
2006-01-08 17:05:04 +08:00
|
|
|
very often is not. Abundant use of the inline keyword leads to a much bigger
|
|
|
|
kernel, which in turn slows the system as a whole down, due to a bigger
|
|
|
|
icache footprint for the CPU and simply because there is less memory
|
|
|
|
available for the pagecache. Just think about it; a pagecache miss causes a
|
2009-04-23 17:37:37 +08:00
|
|
|
disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
|
|
|
|
that can go into these 5 milliseconds.
|
2006-01-08 17:05:04 +08:00
|
|
|
|
|
|
|
A reasonable rule of thumb is to not put inline at functions that have more
|
|
|
|
than 3 lines of code in them. An exception to this rule are the cases where
|
|
|
|
a parameter is known to be a compiletime constant, and as a result of this
|
|
|
|
constantness you *know* the compiler will be able to optimize most of your
|
|
|
|
function away at compile time. For a good example of this later case, see
|
|
|
|
the kmalloc() inline function.
|
|
|
|
|
|
|
|
Often people argue that adding inline to functions that are static and used
|
|
|
|
only once is always a win since there is no space tradeoff. While this is
|
|
|
|
technically correct, gcc is capable of inlining these automatically without
|
|
|
|
help, and the maintenance issue of removing the inline when a second user
|
|
|
|
appears outweighs the potential value of the hint that tells gcc to do
|
|
|
|
something it would have done anyway.
|
|
|
|
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
16) Function return values and names
|
|
|
|
------------------------------------
|
2006-09-29 17:01:21 +08:00
|
|
|
|
|
|
|
Functions can return values of many different kinds, and one of the
|
|
|
|
most common is a value indicating whether the function succeeded or
|
|
|
|
failed. Such a value can be represented as an error-code integer
|
2016-09-19 19:07:45 +08:00
|
|
|
(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
|
2006-09-29 17:01:21 +08:00
|
|
|
non-zero = success).
|
|
|
|
|
|
|
|
Mixing up these two sorts of representations is a fertile source of
|
|
|
|
difficult-to-find bugs. If the C language included a strong distinction
|
|
|
|
between integers and booleans then the compiler would find these mistakes
|
|
|
|
for us... but it doesn't. To help prevent such bugs, always follow this
|
2016-09-19 19:07:44 +08:00
|
|
|
convention::
|
2006-09-29 17:01:21 +08:00
|
|
|
|
|
|
|
If the name of a function is an action or an imperative command,
|
|
|
|
the function should return an error-code integer. If the name
|
|
|
|
is a predicate, the function should return a "succeeded" boolean.
|
|
|
|
|
2016-09-19 19:07:45 +08:00
|
|
|
For example, ``add work`` is a command, and the add_work() function returns 0
|
|
|
|
for success or -EBUSY for failure. In the same way, ``PCI device present`` is
|
2006-09-29 17:01:21 +08:00
|
|
|
a predicate, and the pci_dev_present() function returns 1 if it succeeds in
|
|
|
|
finding a matching device or 0 if it doesn't.
|
|
|
|
|
|
|
|
All EXPORTed functions must respect this convention, and so should all
|
|
|
|
public functions. Private (static) functions need not, but it is
|
|
|
|
recommended that they do.
|
|
|
|
|
|
|
|
Functions whose return value is the actual result of a computation, rather
|
|
|
|
than an indication of whether the computation succeeded, are not subject to
|
|
|
|
this rule. Generally they indicate failure by returning some out-of-range
|
|
|
|
result. Typical examples would be functions that return pointers; they use
|
|
|
|
NULL or the ERR_PTR mechanism to report failure.
|
|
|
|
|
|
|
|
|
2019-01-19 06:50:47 +08:00
|
|
|
17) Using bool
|
|
|
|
--------------
|
|
|
|
|
|
|
|
The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
|
|
|
|
only evaluate to 0 or 1, and implicit or explicit conversion to bool
|
|
|
|
automatically converts the value to true or false. When using bool types the
|
|
|
|
!! construction is not needed, which eliminates a class of bugs.
|
|
|
|
|
|
|
|
When working with bool values the true and false definitions should be used
|
|
|
|
instead of 1 and 0.
|
|
|
|
|
|
|
|
bool function return types and stack variables are always fine to use whenever
|
|
|
|
appropriate. Use of bool is encouraged to improve readability and is often a
|
|
|
|
better option than 'int' for storing boolean values.
|
|
|
|
|
|
|
|
Do not use bool if cache line layout or size of the value matters, as its size
|
|
|
|
and alignment varies based on the compiled architecture. Structures that are
|
|
|
|
optimized for alignment and size should not use bool.
|
|
|
|
|
|
|
|
If a structure has many true/false values, consider consolidating them into a
|
|
|
|
bitfield with 1 bit members, or using an appropriate fixed width type, such as
|
|
|
|
u8.
|
|
|
|
|
|
|
|
Similarly for function arguments, many true/false values can be consolidated
|
|
|
|
into a single bitwise 'flags' argument and 'flags' can often be a more
|
|
|
|
readable alternative if the call-sites have naked true/false constants.
|
|
|
|
|
|
|
|
Otherwise limited use of bool in structures and arguments can improve
|
|
|
|
readability.
|
|
|
|
|
|
|
|
18) Don't re-invent the kernel macros
|
2016-09-19 19:07:44 +08:00
|
|
|
-------------------------------------
|
2006-12-22 17:09:11 +08:00
|
|
|
|
|
|
|
The header file include/linux/kernel.h contains a number of macros that
|
|
|
|
you should use, rather than explicitly coding some variant of them yourself.
|
|
|
|
For example, if you need to calculate the length of an array, take advantage
|
|
|
|
of the macro
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
|
2006-12-22 17:09:11 +08:00
|
|
|
|
|
|
|
Similarly, if you need to calculate the size of some structure member, use
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2019-12-10 02:31:43 +08:00
|
|
|
#define sizeof_field(t, f) (sizeof(((t*)0)->f))
|
2006-12-22 17:09:11 +08:00
|
|
|
|
|
|
|
There are also min() and max() macros that do strict type checking if you
|
|
|
|
need them. Feel free to peruse that header file to see what else is already
|
|
|
|
defined that you shouldn't reproduce in your code.
|
|
|
|
|
|
|
|
|
2019-01-19 06:50:47 +08:00
|
|
|
19) Editor modelines and other cruft
|
2016-09-19 19:07:44 +08:00
|
|
|
------------------------------------
|
2007-07-16 14:41:37 +08:00
|
|
|
|
|
|
|
Some editors can interpret configuration information embedded in source files,
|
|
|
|
indicated with special markers. For example, emacs interprets lines marked
|
|
|
|
like this:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
-*- mode: c -*-
|
2007-07-16 14:41:37 +08:00
|
|
|
|
|
|
|
Or like this:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
/*
|
|
|
|
Local Variables:
|
|
|
|
compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
|
|
|
|
End:
|
|
|
|
*/
|
2007-07-16 14:41:37 +08:00
|
|
|
|
|
|
|
Vim interprets markers that look like this:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
/* vim:set sw=8 noet */
|
2007-07-16 14:41:37 +08:00
|
|
|
|
|
|
|
Do not include any of these in source files. People have their own personal
|
|
|
|
editor configurations, and your source files should not override them. This
|
|
|
|
includes markers for indentation and mode configuration. People may use their
|
|
|
|
own custom mode, or may have some other magic method for making indentation
|
|
|
|
work correctly.
|
|
|
|
|
|
|
|
|
2019-01-19 06:50:47 +08:00
|
|
|
20) Inline assembly
|
2016-09-19 19:07:44 +08:00
|
|
|
-------------------
|
2012-03-31 04:37:10 +08:00
|
|
|
|
|
|
|
In architecture-specific code, you may need to use inline assembly to interface
|
|
|
|
with CPU or platform functionality. Don't hesitate to do so when necessary.
|
|
|
|
However, don't use inline assembly gratuitously when C can do the job. You can
|
|
|
|
and should poke hardware from C when possible.
|
|
|
|
|
|
|
|
Consider writing simple helper functions that wrap common bits of inline
|
|
|
|
assembly, rather than repeatedly writing them with slight variations. Remember
|
|
|
|
that inline assembly can use C parameters.
|
|
|
|
|
|
|
|
Large, non-trivial assembly functions should go in .S files, with corresponding
|
|
|
|
C prototypes defined in C header files. The C prototypes for assembly
|
2016-09-19 19:07:45 +08:00
|
|
|
functions should use ``asmlinkage``.
|
2012-03-31 04:37:10 +08:00
|
|
|
|
|
|
|
You may need to mark your asm statement as volatile, to prevent GCC from
|
|
|
|
removing it if GCC doesn't notice any side effects. You don't always need to
|
|
|
|
do so, though, and doing so unnecessarily can limit optimization.
|
|
|
|
|
|
|
|
When writing a single inline assembly statement containing multiple
|
|
|
|
instructions, put each instruction on a separate line in a separate quoted
|
2017-05-19 02:32:57 +08:00
|
|
|
string, and end each string except the last with ``\n\t`` to properly indent
|
|
|
|
the next instruction in the assembly output:
|
2012-03-31 04:37:10 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2012-03-31 04:37:10 +08:00
|
|
|
asm ("magic %reg1, #42\n\t"
|
|
|
|
"more_magic %reg2, %reg3"
|
|
|
|
: /* outputs */ : /* inputs */ : /* clobbers */);
|
|
|
|
|
|
|
|
|
2019-01-19 06:50:47 +08:00
|
|
|
21) Conditional Compilation
|
2016-09-19 19:07:44 +08:00
|
|
|
---------------------------
|
2014-10-30 02:15:17 +08:00
|
|
|
|
|
|
|
Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
|
|
|
|
files; doing so makes code harder to read and logic harder to follow. Instead,
|
|
|
|
use such conditionals in a header file defining functions for use in those .c
|
|
|
|
files, providing no-op stub versions in the #else case, and then call those
|
|
|
|
functions unconditionally from .c files. The compiler will avoid generating
|
|
|
|
any code for the stub calls, producing identical results, but the logic will
|
|
|
|
remain easy to follow.
|
|
|
|
|
|
|
|
Prefer to compile out entire functions, rather than portions of functions or
|
|
|
|
portions of expressions. Rather than putting an ifdef in an expression, factor
|
|
|
|
out part or all of the expression into a separate helper function and apply the
|
|
|
|
conditional to that function.
|
|
|
|
|
|
|
|
If you have a function or variable which may potentially go unused in a
|
|
|
|
particular configuration, and the compiler would warn about its definition
|
|
|
|
going unused, mark the definition as __maybe_unused rather than wrapping it in
|
|
|
|
a preprocessor conditional. (However, if a function or variable *always* goes
|
|
|
|
unused, delete it.)
|
|
|
|
|
|
|
|
Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
|
|
|
|
symbol into a C boolean expression, and use it in a normal C conditional:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2014-10-30 02:15:17 +08:00
|
|
|
if (IS_ENABLED(CONFIG_SOMETHING)) {
|
|
|
|
...
|
|
|
|
}
|
|
|
|
|
|
|
|
The compiler will constant-fold the conditional away, and include or exclude
|
|
|
|
the block of code just as with an #ifdef, so this will not add any runtime
|
|
|
|
overhead. However, this approach still allows the C compiler to see the code
|
|
|
|
inside the block, and check it for correctness (syntax, types, symbol
|
|
|
|
references, etc). Thus, you still have to use an #ifdef if the code inside the
|
|
|
|
block references symbols that will not exist if the condition is not met.
|
|
|
|
|
|
|
|
At the end of any non-trivial #if or #ifdef block (more than a few lines),
|
|
|
|
place a comment after the #endif on the same line, noting the conditional
|
|
|
|
expression used. For instance:
|
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
.. code-block:: c
|
|
|
|
|
2015-02-17 01:26:18 +08:00
|
|
|
#ifdef CONFIG_SOMETHING
|
|
|
|
...
|
|
|
|
#endif /* CONFIG_SOMETHING */
|
2014-10-30 02:15:17 +08:00
|
|
|
|
2006-01-08 17:05:04 +08:00
|
|
|
|
2016-09-19 19:07:44 +08:00
|
|
|
Appendix I) References
|
|
|
|
----------------------
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
The C Programming Language, Second Edition
|
|
|
|
by Brian W. Kernighan and Dennis M. Ritchie.
|
|
|
|
Prentice Hall, Inc., 1988.
|
|
|
|
ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).
|
|
|
|
|
|
|
|
The Practice of Programming
|
|
|
|
by Brian W. Kernighan and Rob Pike.
|
|
|
|
Addison-Wesley, Inc., 1999.
|
|
|
|
ISBN 0-201-61586-X.
|
|
|
|
|
|
|
|
GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
|
2020-06-21 21:36:30 +08:00
|
|
|
gcc internals and indent, all available from https://www.gnu.org/manual/
|
2005-04-17 06:20:36 +08:00
|
|
|
|
|
|
|
WG14 is the international standardization working group for the programming
|
2006-01-08 17:02:49 +08:00
|
|
|
language C, URL: http://www.open-std.org/JTC1/SC22/WG14/
|
|
|
|
|
2018-11-21 08:35:19 +08:00
|
|
|
Kernel :ref:`process/coding-style.rst <codingstyle>`, by greg@kroah.com at OLS 2002:
|
2006-01-08 17:02:49 +08:00
|
|
|
http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
|