busybox/docs/style-guide.txt

Busybox Style Guide
===================

This document describes the coding style conventions used in Busybox. If you
add a new file to Busybox or are editing an existing file, please format your
code according to this style. If you are the maintainer of a file that does
not follow these guidelines, please -- at your own convenience -- modify the
file(s) you maintain to bring them into conformance with this style guide.
Please note that this is a low priority task.

To help you format the whitespace of your programs, an ".indent.pro" file is
included in the main Busybox source directory that contains option flags to
format code as per this style guide. This way you can run GNU indent on your
files by typing 'indent myfile.c myfile.h' and it will magically apply all the
right formatting rules to your file. Please _do_not_ run this on all the files
in the directory, just your own.


Declaration Order
-----------------

Here is the order in which code should be laid out in a file:

 - commented program name and one-line description
 - commented author name and email address(es)
 - commented GPL boilerplate
 - commented longer description / notes for the program (if needed)
 - #includes and #defines
 - const and global variables
 - function declarations (if necessary)
 - function implementations


Whitespace
----------

This is everybody's favorite flame topic so let's get it out of the way right
up front.


Tabs vs. Spaces in Line Indentation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The preference in Busybox is to indent lines with tabs. Do not indent lines
with spaces and do not indents lines using a mixture of tabs and spaces. (The
indentation style in the Apache and Postfix source does this sort of thing:
\s\s\s\sif (expr) {\n\tstmt; --ick.) The only exception to this rule is
multi-line comments that use an asterisk at the beginning of each line, i.e.:

	/t/*
	/t * This is a block comment.
	/t * Note that it has multiple lines
	/t * and that the beginning of each line has a tab plus a space
	/t * except for the opening '/*' line where the slash
	/t * is used instead of a space.
	/t */

Furthermore, The preference is that tabs be set to display at four spaces
wide, but the beauty of using only tabs (and not spaces) at the beginning of
lines is that you can set your editor to display tabs at *whatever* number of
spaces is desired and the code will still look fine.


Operator Spacing
~~~~~~~~~~~~~~~~

Put spaces between terms and operators. Example:

	Don't do this:

		for(i=0;i<num_items;i++){

	Do this instead:

		for (i = 0; i < num_items; i++) {

	While it extends the line a bit longer, the spaced version is more
	readable. An allowable exception to this rule is the situation where
	excluding the spacing makes it more obvious that we are dealing with a
	single term (even if it is a compound term) such as:

		if (str[idx] == '/' && str[idx-1] != '\\')

	or

		if ((argc-1) - (optind+1) > 0)


Bracket Spacing
~~~~~~~~~~~~~~~

If an opening bracket starts a function, it should be on the
next line with no spacing before it. However, if a bracket follows an opening
control block, it should be on the same line with a single space (not a tab)
between it and the opening control block statement. Examples:

	Don't do this:

		while (!done)
		{

		do
		{

	Don't do this either:

		while (!done){
		do{

	Do this instead:

		while (!done) {
		do {


Paren Spacing
~~~~~~~~~~~~~

Put a space between C keywords and left parens, but not between
function names and the left paren that starts it's parameter list (whether it
is being declared or called). Examples:

	Don't do this:

		while(foo) {
		for(i = 0; i < n; i++) {

	Do this instead:

		while (foo) {
		for (i = 0; i < n; i++) {

	But do functions like this:

		static int my_func(int foo, char bar)
		...
		baz = my_func(1, 2);


Cuddled Elses
~~~~~~~~~~~~~

Also, please "cuddle" your else statements by putting the else keyword on the
same line after the right bracket that closes an 'if' statement.

	Don't do this:

	if (foo) {
		stmt;
	}
	else {
		stmt;
	}

	Do this instead:

	if (foo) {
		stmt;
	} else {
		stmt;
	}

The exception to this rule is if you want to include a comment before the else
block. Example:

	if (foo) {
		stmts...
	}
	/* otherwise, we're just kidding ourselves, so re-frob the input */
	else {
		other_stmts...
	}


Variable and Function Names
---------------------------

Use the K&R style with names in all lower-case and underscores occasionally
used to separate words (e.g., "variable_name" and "numchars" are both
acceptable). Using underscores makes variable and function names more readable
because it looks like whitespace; using lower-case is easy on the eyes.

Note: The Busybox codebase is very much a mixture of code gathered from a
variety of sources. This explains why the current codebase contains such a
hodge-podge of different naming styles (Java, Pascal, K&R, just-plain-weird,
etc.). The K&R guideline explained above should therefore be used on new files
that are added to the repository. Furthermore, the maintainer of an existing
file that uses alternate naming conventions should -- at his own convenience --
convert those names over to K&R style; converting variable names is a very low
priority task. Perhaps in the future we will include some magical Perl script
that can go through and convert files -- left as an exercise to the reader for
now.


Tip and Pointers
----------------

The following are simple coding guidelines that should be followed:

 - When in doubt about the proper behavior of a Busybox program (output,
   formatting, options, etc.), model it after the equivalent GNU program.
   Doesn't matter how that program behaves on some other flavor of *NIX;
   doesn't matter what the POSIX standard says or doesn't say, just model
   Busybox programs after their GNU counterparts and nobody has to get hurt.

 - Don't use a '#define var 80' when you can use 'static const int var 80'
   instead. This makes the compiler do type checking for you (rather than
   relying on the more error-prone preprocessor) and it makes debugging
   programs much easier since the value of the variable can be easily
   displayed.

 - If a const variable is used in only one function, do not make it global to
   the file. Instead, declare it inside the function body.

 - Inside applet files, all functions should be declared static so as to keep
   the global name space clean. The only exception to this rule is the
   "applet_main" function which must be declared extern.

 - If you write a function that performs a task that could be useful outside
   the immediate file, turn it into a general-purpose function with no ties to
   any applet and put it in the utility.c file instead.

 - Put all help/usage messages in usage.c. Put other strings in messages.c.
   Putting these strings into their own file is a calculated decision designed
   to confine spelling errors to a single place and aid internationalization
   efforts, if needed.  (Side Note: we might want to use a single file instead
   of two, food for thought).

 - There's a right way and a wrong way to test for sting equivalence with
   strcmp:

	The wrong way:

	if (!strcmp(string, "foo")) {
		...

	The right way:

	if (strcmp(string, "foo") == 0){
		...

	The use of the "equals" (==) operator in the latter example makes it much
	more obvious that you are testing for equivalence. The former example with
	the "not" (!) operator makes it look like you are testing for an error. In
	a more perfect world, we would have a streq() function in the string
	library, but that ain't the world we're living in.

 - Do not use old-style function declarations that declare variable types
   between the parameter list and opening bracket. Example:

	Don't do this:

		int foo(parm1, parm2)
			char parm1;
			float parm2;
		{
			....

	Do this instead:

		int foo(char parm1, float parm2)
		{
			....

 - Please use brackets on all if and else statements, even if it is only one
   line. Example:

	Don't do this:

		if (foo)
			stmt;
		else
			stmt;

	Do this instead:

		if (foo) {
			stmt;
		} else {
			stmt;
		}

	The "bracketless" approach is error prone because someday you might add a
	line like this:

		if (foo)
			stmt;
			new_line();
		else
			stmt;

	And the resulting behavior of your program would totally bewilder you.
	(Don't laugh, it happens to us all.) Remember folks, this is C, not
	Python.