SXEmacs Policies & Procedures Manual: General C Style

SXEmacs C code follows, to a large degree, the coding style of the Linux Kernel source. Much of this section is a verbatim copy of ./Documentation/CodingStyle from the Linux kernel sources.

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.

Don’t put multiple statements on a single line unless you have something to hide:

	if (condition) do_this;
	  do_something_everytime;

Outside of comments and documentation, spaces are never used for indentation, and the above example is deliberately broken.

Don’t leave whitespace at the end of lines. There is a whitespace.el which you can get from http://www.dsmit.com/lisp/. Use it.

Breaking long lines and strings

Coding style is all about readability and maintainability using commonly available tools.

Statements longer than 80 columns will be broken into sensible chunks. Descendants are always substantially shorter than the parent and are placed substantially to the right. The same applies to function headers with a long argument list. Long strings are as well broken into shorter strings.

void
fun(int a, int b, int c)
{
	if (condition)
		printf("Warning this is a very very very long printf with "
						"3 parameters a: %u b: %u "
						"c: %u \n", a, b, c);
	else
		next_statement;
}

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);

	if (x == y) {
		..
	} else if (x > y) {
		...
	} else {
		....
	}

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 hidden buffers, you should call that count_hidden_buffers() or similar, you should not call it cntbuf().

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.

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.

int
main(int argc, char **argv)
{
	...
	...
}

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.

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 section on Functions 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.

A comment in C looks like /* a comment */. A comment in C++ looks like // a comment. Don’t get them confused and don’t ever use C++ style comments.

/*
 * A comment style in C that is quite often used
 * for multi-line comments.
 */

Macros

#define CONSTANT 0x12345

CAPITALISED macro names are appreciated but macros resembling functions may be named in lower case.

#define macrofun(a,b,c) 			\
	do {					\
		if (a == 5)			\
			do_this(b,c);		\
	} while (0)

Things to avoid when using macros:

10 General C Style