docs/style-guide.txt - busybox - Git at Google

 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 author name and email address(es)
  - commented GPL boilerplate
  - commented description of program
  - #includes and #defines
  - const and globals 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 *watever* 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 compund 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 bracet 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 statment. Examples:

 	Don't do this:

 		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++) {

 	Do functions like this:

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


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

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

 	Don't do this:

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

 	Do this instead:

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


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

 Use the K&R style with names in all lower-case and underscores occasionally
 used to seperate 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 locations. This explains why the current codebase contains such a
 plethora 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 exersize to the
 reader.


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

 The following are simple coding guidelines that should be followed:

  - When in doubt about the propper 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 typechecking 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 queried.

  - 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 namespace 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
    (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.

  - 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.
	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 author name and email address(es)
	- commented GPL boilerplate
	- commented description of program
	- #includes and #defines
	- const and globals 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 watever 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 compund 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 bracet 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 statment. Examples:

	Don't do this:

	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++) {

	Do functions like this:

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


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

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

	Don't do this:

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

	Do this instead:

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


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

	Use the K&R style with names in all lower-case and underscores occasionally
	used to seperate 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 locations. This explains why the current codebase contains such a
	plethora 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 exersize to the
	reader.


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

	The following are simple coding guidelines that should be followed:

	- When in doubt about the propper 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 typechecking 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 queried.

	- 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 namespace 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
	(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.

	- 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.