kernel guide to space

[Michael S. Tsirkin]

Hi!
I've been tasked with edicating some new hires on linux kernel coding style.
While we have Documentation/CodingStyle, it skips detail that is supposed to
be learned by example.

Since I've been burned by this a couple of times myself till I learned,
I've put together a short list of rules complementing Documentation/CodingStyle.
This list is attached, below.

Please cc me directly with comments, if any.

Thanks,
MST

---

kernel guide to space AKA a boring list of rules
http://www.mellanox.com/mst/boring.txt

This text deals mostly with whitespace issues, hence the name.

Whitespace -- In computer science, a whitespace (or a whitespace character) is
any character which does not display itself but does take up space.
	From Wikipedia, the free encyclopedia.

1. Read Documentation/CodingStyle. Yes, it applies to you.
   When working on a specific driver/subsystem, try to follow
   the style of the surrounding codebase.

2. The last character on a line is never a whitespace
		Get a decent editor and don't leave whitespace at the end of
		lines.
			Documentation/CodingStyle

Whitespace issues:

3. Space rules for C

3a. Binary operators
	+ - / * %
	== !=  > < >= <= && || 
	& | ^ << >> 
	= *= /= %= += -= <<= >>= &= ^= |=

	spaces around the operator
	a + b

3b. Unary operators
	! ~
	+ - *
	&

	no space between operator and operand
	*a

3c. * in types
	Leave space between name and * in types.
	Multiple * dont need additional space between them.

	struct foo **bar;

3d. Conditional
	?:
	spaces around both ? and :
	a ? b : c

3e. sizeof
	space after the operator
	sizeof a

3f. Braces etc
	() [] -> .

	no space around any of these (but see 3h)
	foo(bar)

3g. Comma
	,
	space after comma, no space before comma
	foo, bar

3h. Semicolon
	;
	no space before semicolon
	foo;

3i. if/else/do/while/for/switch
	space between if/else/do/while and following/preceeding
	statements/expressions, if any:

	if (a) {
	} else {
	}

	do {
	} while (b);

3j. Labels
	goto and case labels should have a line of their own (possibly
	with a comment).
	No space before colon in labels.

int foobar()
{
	...
foolabel: /* short comment */
	foo();
}

4. Indentation rules for C
	Use tabs, not spaces, for indentation. Tabs should be 8 characters wide.

4a. Labels
	case labels should be indented same as the switch statement.
	statements occurring after a case label are indented by one level.

	switch (foo) {
	case foo:
		bar();
	default:
		break;
	}

4b. Global scope
	Functions, type definitions/declarations, defines, global
	variables etc are global scope. Start them at the first
	character in a line (indent level 0).

static struct foo *foo_bar(struct foo *first, struct bar *second,
			   struct foobar* thirsd);

4c. Breaking long lines
		Descendants are always substantially shorter than the parent
		and are placed substantially to the right.
			Documentation/CodingStyle

	Descendant must be indented at least to the level of the innermost
	compound expression in the parent. All descendants at the same level
	are indented the same.
	if (foobar(.................................) + barbar * foobar(bar +
				     					foo *
									oof)) {
	}

5. Blank lines
	One blank line between functions.

void foo()
{
}

/* comment */
void bar()
{
}

	No more than one blank line in a row.
	Last (or first) line in a file is never blank.

Non-whitespace issues:

6. One-line statement does not need a {} block, so dont put it into one
	if (foo)
		bar;

7. Comments
	Dont use C99 // comments.

8. Return codes
	Functions that return success/failure status, should use 0 for success,
	a negative value for failure.
	Error codes are in linux/errno.h .

	if (do_something()) {
		handle_error();
		return -EINVAL;
	}

	Functions that test a condition return 1 if condition is satisfied,
	0 if its not.

	if (is_condition())
		condition_true();

9. Data types
	Standard linux types are in linux/types.h .
	See also Linux Device Drivers, Third Edition,
	Chapter 11: Data Types in the Kernel.  http://lwn.net/images/pdf/LDD3/

9a. Integer types
	int is the default integer type.
	Use unsigned type if you perform bit operations (<<,>>,&,|,~).
	Use unsigned long if you have to fit a pointer into integer.
	long long is at least 64 bit wide on all platforms.
	char is for ASCII characters and strings.
	Use u8,u16,u32,u64 if you need an integer of a specific size.

9b. typedef
	Using typedefs to hide the data type is generally discouraged.
	typedefs to function types are ok, since these can get very long.

typedef struct foo *(foo_bar_handler)(struct foo *first, struct bar *second,
				      struct foobar* thirsd);

Editor configuration:

9. The following is helpful with VIM
	set cinoptions=(0:0

Michael S. Tsirkin

-- 
MST
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