Merge branch 'master' into gwin
This commit is contained in:
Joel Bodenmann
commit
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codingstyle.txt
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codingstyle.txt
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ChibiOS/GFX coding style
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ToDo
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To provide an easy-to-read code, we want to have a uniform
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coding style within ChibiOS/GFX.
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Because I personally like the widley used linux kernel coding style,
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I decided to use it for ChibiOS/GFX as well.
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Therefore, the coding style documentation is a 1:1 copy from the
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codingstyle.txt of the linux kernel source code.
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Please make sure you match these coding styles before you contribute
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any code. If you find any existing code which dosen't match these rules,
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please feel free to submit a patch.
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There are only two rules which are not similar to the following
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documentation:
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- Prefered tabsize is 4, not 8
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- We don't use 80 character columns
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Please read through the following carefully:
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Linux kernel coding style
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This is a short document describing the preferred coding style for the
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linux kernel. Coding style is very personal, and I won't _force_ my
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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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Chapter 1: Indentation
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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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Chapter 2: Placing Braces
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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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if (x is true) {
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we do y
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}
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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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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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(a) K&R are _right_ and (b) K&R are right. Besides, functions are
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special anyway (you can't nest them in C).
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Note that the closing brace is empty on a line of its own, _except_ in
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the cases where it is followed by a continuation of the same statement,
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ie a "while" in a do-statement or an "else" in an if-statement, like
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this:
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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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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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Chapter 3: Naming
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C is a Spartan language, and so should your naming be. Unlike Modula-2
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and Pascal programmers, C programmers do not use cute names like
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ThisVariableIsATemporaryCounter. A C programmer would call that
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variable "tmp", which is much easier to write, and not the least more
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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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global variables are a must. To call a global function "foo" is a
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shooting offense.
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GLOBAL variables (to be used only if you _really_ need them) need to
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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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"count_active_users()" or similar, you should _not_ call it "cntusr()".
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Encoding the type of a function into the name (so-called Hungarian
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notation) is brain damaged - the compiler knows the types anyway and can
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check those, and it only confuses the programmer. No wonder MicroSoft
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makes buggy programs.
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LOCAL variable names should be short, and to the point. If you have
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some random integer loop counter, it should probably be called "i".
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Calling it "loop_counter" is non-productive, if there is no chance of it
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being mis-understood. Similarly, "tmp" can be just about any type of
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variable that is used to hold a temporary value.
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If you are afraid to mix up your local variable names, you have another
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problem, which is called the function-growth-hormone-imbalance syndrome.
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See next chapter.
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Chapter 4: Functions
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Functions should be short and sweet, and do just one thing. They should
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fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
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as we all know), and do one thing and do that well.
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The maximum length of a function is inversely proportional to the
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complexity and indentation level of that function. So, if you have a
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conceptually simple function that is just one long (but simple)
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case-statement, where you have to do lots of small things for a lot of
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different cases, it's OK to have a longer function.
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However, if you have a complex function, and you suspect that a
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less-than-gifted first-year high-school student might not even
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understand what the function is all about, you should adhere to the
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maximum limits all the more closely. Use helper functions with
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descriptive names (you can ask the compiler to in-line them if you think
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it's performance-critical, and it will probably do a better job of it
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that you would have done).
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Another measure of the function is the number of local variables. They
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shouldn't exceed 5-10, or you're doing something wrong. Re-think the
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function, and split it into smaller pieces. A human brain can
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generally easily keep track of about 7 different things, anything more
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and it gets confused. You know you're brilliant, but maybe you'd like
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to understand what you did 2 weeks from now.
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Chapter 5: Commenting
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Comments are good, but there is also a danger of over-commenting. NEVER
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try to explain HOW your code works in a comment: it's much better to
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write the code so that the _working_ is obvious, and it's a waste of
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time to explain badly written code.
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Generally, you want your comments to tell WHAT your code does, not HOW.
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Also, try to avoid putting comments inside a function body: if the
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function is so complex that you need to separately comment parts of it,
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you should probably go back to chapter 4 for a while. You can make
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small comments to note or warn about something particularly clever (or
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ugly), but try to avoid excess. Instead, put the comments at the head
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of the function, telling people what it does, and possibly WHY it does
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it.
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Chapter 6: You've made a mess of it
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That's OK, we all do. You've probably been told by your long-time Unix
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user helper that "GNU emacs" automatically formats the C sources for
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you, and you've noticed that yes, it does do that, but the defaults it
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uses are less than desirable (in fact, they are worse than random
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typing - a infinite number of monkeys typing into GNU emacs would never
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make a good program).
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So, you can either get rid of GNU emacs, or change it to use saner
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values. To do the latter, you can stick the following in your .emacs file:
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(defun linux-c-mode ()
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"C mode with adjusted defaults for use with the Linux kernel."
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(interactive)
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(c-mode)
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(c-set-style "K&R")
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(setq c-basic-offset 8))
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This will define the M-x linux-c-mode command. When hacking on a
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module, if you put the string -*- linux-c -*- somewhere on the first
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two lines, this mode will be automatically invoked. Also, you may want
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to add
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(setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode)
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auto-mode-alist))
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to your .emacs file if you want to have linux-c-mode switched on
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automagically when you edit source files under /usr/src/linux.
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But even if you fail in getting emacs to do sane formatting, not
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everything is lost: use "indent".
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Now, again, GNU indent has the same brain dead settings that GNU emacs
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has, which is why you need to give it a few command line options.
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However, that's not too bad, because even the makers of GNU indent
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recognize the authority of K&R (the GNU people aren't evil, they are
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just severely misguided in this matter), so you just give indent the
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options "-kr -i8" (stands for "K&R, 8 character indents").
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"indent" has a lot of options, and especially when it comes to comment
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re-formatting you may want to take a look at the manual page. But
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remember: "indent" is not a fix for bad programming.
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Chapter 7: Configuration-files
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For configuration options (arch/xxx/config.in, and all the Config.in files),
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somewhat different indentation is used.
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An indention level of 3 is used in the code, while the text in the config-
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options should have an indention-level of 2 to indicate dependencies. The
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latter only applies to bool/tristate options. For other options, just use
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common sense. An example:
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if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then
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tristate 'Apply nitroglycerine inside the keyboard (DANGEROUS)' CONFIG_BOOM
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if [ "$CONFIG_BOOM" != "n" ]; then
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bool ' Output nice messages when you explode' CONFIG_CHEER
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fi
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fi
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Generally, CONFIG_EXPERIMENTAL should surround all options not considered
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stable. All options that are known to trash data (experimental write-
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support for file-systems, for instance) should be denoted (DANGEROUS), other
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Experimental options should be denoted (EXPERIMENTAL).
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Chapter 8: Data structures
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Data structures that have visibility outside the single-threaded
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environment they are created and destroyed in should always have
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reference counts. In the kernel, garbage collection doesn't exist (and
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outside the kernel garbage collection is slow and inefficient), which
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means that you absolutely _have_ to reference count all your uses.
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Reference counting means that you can avoid locking, and allows multiple
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users to have access to the data structure in parallel - and not having
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to worry about the structure suddenly going away from under them just
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because they slept or did something else for a while.
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Note that locking is _not_ a replacement for reference counting.
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Locking is used to keep data structures coherent, while reference
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counting is a memory management technique. Usually both are needed, and
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they are not to be confused with each other.
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Many data structures can indeed have two levels of reference counting,
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when there are users of different "classes". The subclass count counts
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the number of subclass users, and decrements the global count just once
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when the subclass count goes to zero.
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Examples of this kind of "multi-reference-counting" can be found in
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memory management ("struct mm_struct": mm_users and mm_count), and in
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filesystem code ("struct super_block": s_count and s_active).
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Remember: if another thread can find your data structure, and you don't
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have a reference count on it, you almost certainly have a bug.
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@ -681,8 +681,8 @@ void gdispGDrawBox(GDisplay *g, coord_t x, coord_t y, coord_t cx, coord_t cy, co
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*
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* @param[in] g The display to use
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* @param[in] x,y The position for the text
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* @param[in] font The font to use
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* @param[in] str The string to draw
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* @param[in] font The font to use
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* @param[in] color The color to use
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*
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* @api
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