FreeBSD/Linux Kernel Cross Reference
sys/Documentation/CodingStyle

     
                     Linux kernel coding style 
     
     This is a short document describing the preferred coding style for the
     linux kernel.  Coding style is very personal, and I won't _force_ my
     views on anybody, but this is what goes for anything that I have to be
     able to maintain, and I'd prefer it for most other things too.  Please
     at least consider the points made here. 
     
    First off, I'd suggest printing out a copy of the GNU coding standards,
    and NOT read it.  Burn them, it's a great symbolic gesture. 
    
    Anyway, here goes:
    
    
                    Chapter 1: Indentation
    
    Tabs are 8 characters, and thus indentations are also 8 characters. 
    There are heretic movements that try to make indentations 4 (or even 2!)
    characters deep, and that is akin to trying to define the value of PI to
    be 3. 
    
    Rationale: The whole idea behind indentation is to clearly define where
    a block of control starts and ends.  Especially when you've been looking
    at your screen for 20 straight hours, you'll find it a lot easier to see
    how the indentation works if you have large indentations. 
    
    Now, some people will claim that having 8-character indentations makes
    the code move too far to the right, and makes it hard to read on a
    80-character terminal screen.  The answer to that is that if you need
    more than 3 levels of indentation, you're screwed anyway, and should fix
    your program. 
    
    In short, 8-char indents make things easier to read, and have the added
    benefit of warning you when you're nesting your functions too deep. 
    Heed that warning. 
    
    
                    Chapter 2: Placing Braces
    
    The other issue that always comes up in C styling is the placement of
    braces.  Unlike the indent size, there are few technical reasons to
    choose one placement strategy over the other, but the preferred way, as
    shown to us by the prophets Kernighan and Ritchie, is to put the opening
    brace last on the line, and put the closing brace first, thusly:
    
            if (x is true) {
                    we do y
            }
    
    However, there is one special case, namely functions: they have the
    opening brace at the beginning of the next line, thus:
    
            int function(int x)
            {
                    body of function
            }
    
    Heretic people all over the world have claimed that this inconsistency
    is ...  well ...  inconsistent, but all right-thinking people know that
    (a) K&R are _right_ and (b) K&R are right.  Besides, functions are
    special anyway (you can't nest them in C). 
    
    Note that the closing brace is empty on a line of its own, _except_ in
    the cases where it is followed by a continuation of the same statement,
    ie a "while" in a do-statement or an "else" in an if-statement, like
    this:
    
            do {
                    body of do-loop
            } while (condition);
    
    and
    
            if (x == y) {
                    ..
            } else if (x > y) {
                    ...
            } else {
                    ....
            }
                            
    Rationale: K&R. 
    
    Also, note that this brace-placement also minimizes the number of empty
    (or almost empty) lines, without any loss of readability.  Thus, as the
    supply of new-lines on your screen is not a renewable resource (think
    25-line terminal screens here), you have more empty lines to put
    comments on. 
    
    
                    Chapter 3: Naming
    
    C is a Spartan language, and so should your naming be.  Unlike Modula-2
    and Pascal programmers, C programmers do not use cute names like
    ThisVariableIsATemporaryCounter.  A C programmer would call that
    variable "tmp", which is much easier to write, and not the least more
    difficult to understand. 
    
   HOWEVER, while mixed-case names are frowned upon, descriptive names for
   global variables are a must.  To call a global function "foo" is a
   shooting offense. 
   
   GLOBAL variables (to be used only if you _really_ need them) need to
   have descriptive names, as do global functions.  If you have a function
   that counts the number of active users, you should call that
   "count_active_users()" or similar, you should _not_ call it "cntusr()". 
   
   Encoding the type of a function into the name (so-called Hungarian
   notation) is brain damaged - the compiler knows the types anyway and can
   check those, and it only confuses the programmer.  No wonder MicroSoft
   makes buggy programs. 
   
   LOCAL variable names should be short, and to the point.  If you have
   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
   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. 
   See next chapter. 
   
                   
                   Chapter 4: Functions
   
   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
   that 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. 
   
   
                   Chapter 5: Commenting
   
   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
   write the code so that the _working_ is obvious, and it's a waste of
   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,
   you should probably go back to chapter 4 for a while.  You can make
   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. 
   
   
                   Chapter 6: You've made a mess of it
   
   That's OK, we all do.  You've probably been told by your long-time Unix
   user helper that "GNU emacs" automatically formats the C sources for
   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 - a 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:
   
   (defun linux-c-mode ()
     "C mode with adjusted defaults for use with the Linux kernel."
     (interactive)
     (c-mode)
     (c-set-style "K&R")
     (setq c-basic-offset 8))
   
   This will define the M-x linux-c-mode command.  When hacking on a
   module, if you put the string -*- linux-c -*- somewhere on the first
   two lines, this mode will be automatically invoked. Also, you may want
   to add
   
   (setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode)
                          auto-mode-alist))
   
   to your .emacs file if you want to have linux-c-mode switched on
   automagically when you edit source files under /usr/src/linux.
   
   But even if you fail in getting emacs to do sane formatting, not
   everything is lost: use "indent".
   
   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
   options "-kr -i8" (stands for "K&R, 8 character indents"). 
   
   "indent" has a lot of options, and especially when it comes to comment
   re-formatting you may want to take a look at the manual page.  But
   remember: "indent" is not a fix for bad programming. 
   
   
                   Chapter 7: Configuration-files
   
   For configuration options (arch/xxx/config.in, and all the Config.in files),
   somewhat different indentation is used.
   
   An indention level of 3 is used in the code, while the text in the config-
   options should have an indention-level of 2 to indicate dependencies. The
   latter only applies to bool/tristate options. For other options, just use
   common sense. An example:
   
   if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then
      tristate 'Apply nitroglycerine inside the keyboard (DANGEROUS)' CONFIG_BOOM
      if [ "$CONFIG_BOOM" != "n" ]; then
         bool '  Output nice messages when you explode' CONFIG_CHEER
      fi
   fi
   
   Generally, CONFIG_EXPERIMENTAL should surround all options not considered
   stable. All options that are known to trash data (experimental write-
   support for file-systems, for instance) should be denoted (DANGEROUS), other
   Experimental options should be denoted (EXPERIMENTAL).
   
   
                   Chapter 8: Data structures
   
   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
   means that you absolutely _have_ to reference count all your uses. 
   
   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. 
   
   Note that locking is _not_ a replacement for reference counting. 
   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,
   when there are users of different "classes".  The subclass count counts
   the number of subclass users, and decrements the global count just once
   when the subclass count goes to zero.
   
   Examples of this kind of "multi-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).
   
   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.

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