4 This document describes the coding style conventions used in Busybox. If you
5 add a new file to Busybox or are editing an existing file, please format your
6 code according to this style. If you are the maintainer of a file that does
7 not follow these guidelines, please -- at your own convenience -- modify the
8 file(s) you maintain to bring them into conformance with this style guide.
9 Please note that this is a low priority task.
11 To help you format the whitespace of your programs, an ".indent.pro" file is
12 included in the main Busybox source directory that contains option flags to
13 format code as per this style guide. This way you can run GNU indent on your
14 files by typing 'indent myfile.c myfile.h' and it will magically apply all the
15 right formatting rules to your file. Please _do_not_ run this on all the files
16 in the directory, just your own.
23 Here is the order in which code should be laid out in a file:
25 - commented program name and one-line description
26 - commented author name and email address(es)
27 - commented GPL boilerplate
28 - commented longer description / notes for the program (if needed)
29 - #includes and #defines
30 - const and global variables
31 - function declarations (if necessary)
32 - function implementations
36 Whitespace and Formatting
37 -------------------------
39 This is everybody's favorite flame topic so let's get it out of the way right
43 Tabs vs. Spaces in Line Indentation
44 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
46 The preference in Busybox is to indent lines with tabs. Do not indent lines
47 with spaces and do not indents lines using a mixture of tabs and spaces. (The
48 indentation style in the Apache and Postfix source does this sort of thing:
49 \s\s\s\sif (expr) {\n\tstmt; --ick.) The only exception to this rule is
50 multi-line comments that use an asterisk at the beginning of each line, i.e.:
53 /t * This is a block comment.
54 /t * Note that it has multiple lines
55 /t * and that the beginning of each line has a tab plus a space
56 /t * except for the opening '/*' line where the slash
57 /t * is used instead of a space.
60 Furthermore, The preference is that tabs be set to display at four spaces
61 wide, but the beauty of using only tabs (and not spaces) at the beginning of
62 lines is that you can set your editor to display tabs at *whatever* number of
63 spaces is desired and the code will still look fine.
69 Put spaces between terms and operators. Example:
73 for(i=0;i<num_items;i++){
77 for (i = 0; i < num_items; i++) {
79 While it extends the line a bit longer, the spaced version is more
80 readable. An allowable exception to this rule is the situation where
81 excluding the spacing makes it more obvious that we are dealing with a
82 single term (even if it is a compound term) such as:
84 if (str[idx] == '/' && str[idx-1] != '\\')
88 if ((argc-1) - (optind+1) > 0)
94 If an opening bracket starts a function, it should be on the
95 next line with no spacing before it. However, if a bracket follows an opening
96 control block, it should be on the same line with a single space (not a tab)
97 between it and the opening control block statement. Examples:
107 Don't do this either:
113 And for heaven's sake, don't do this:
128 Spacing around Parentheses
129 ~~~~~~~~~~~~~~~~~~~~~~~~~~
131 Put a space between C keywords and left parens, but not between
132 function names and the left paren that starts it's parameter list (whether it
133 is being declared or called). Examples:
138 for(i = 0; i < n; i++) {
143 for (i = 0; i < n; i++) {
145 But do functions like this:
147 static int my_func(int foo, char bar)
151 Also, don't put a space between the left paren and the first term, nor between
152 the last arg and the right paren.
157 strcmp( thisstr, thatstr )
162 strcmp(thisstr, thatstr)
168 Also, please "cuddle" your else statements by putting the else keyword on the
169 same line after the right bracket that closes an 'if' statement.
188 The exception to this rule is if you want to include a comment before the else
194 /* otherwise, we're just kidding ourselves, so re-frob the input */
202 Variable and Function Names
203 ---------------------------
205 Use the K&R style with names in all lower-case and underscores occasionally
206 used to separate words (e.g., "variable_name" and "numchars" are both
207 acceptable). Using underscores makes variable and function names more readable
208 because it looks like whitespace; using lower-case is easy on the eyes.
226 - Enums, macros, and constant variables should all be in upper-case with
227 words optionally seperatedy by underscores (i.e. FIFOTYPE, ISBLKDEV()).
229 - Nobody is going to get mad at you for using 'pvar' as the name of a
230 variable that is a pointer to 'var'.
232 Note: The Busybox codebase is very much a mixture of code gathered from a
233 variety of sources. This explains why the current codebase contains such a
234 hodge-podge of different naming styles (Java, Pascal, K&R, just-plain-weird,
235 etc.). The K&R guideline explained above should therefore be used on new files
236 that are added to the repository. Furthermore, the maintainer of an existing
237 file that uses alternate naming conventions should -- at his own convenience
238 -- convert those names over to K&R style; converting variable names is a very
239 low priority task. Perhaps in the future we will include some magical Perl
240 script that can go through and convert variable names, left as an exercise for
243 For the time being, if you want to do a search-and-replace of a variable name
244 in different files, do the following in the busybox directory:
246 $ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch]
250 Avoid The Preprocessor
251 ----------------------
253 At best, the preprocessor is a necessary evil, helping us account for platform
254 and architecture differences. Using the preprocessor unnecessarily is just
261 Use 'const <type> var' for declaring constants.
267 Do this instead, when the variable is in a header file and will be used in
268 several source files:
272 Or do this when the variable is used only in a single source file:
274 static const int var = 80;
276 Declaring variables as '[static] const' gives variables an actual type and
277 makes the compiler do type checking for you; the preprocessor does _no_ type
278 checking whatsoever, making it much more error prone. Declaring variables with
279 '[static] const' also makes debugging programs much easier since the value of
280 the variable can be easily queried and displayed.
286 Use 'static inline' instead of a macro.
290 #define mini_func(param1, param2) (param1 << param2)
294 static inline int mini_func(int param1, param2)
296 return (param1 << param2);
299 Static inline functions are greatly preferred over macros. They provide type
300 safety, have no length limitations, no formatting limitations, and under gcc
301 they are as cheap as macros. Besides, really long macros with backslashes at
302 the end of each line are ugly as sin.
308 Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
309 Instead, put your ifdefs in a header, and conditionally define 'static inline'
310 functions, (or *maybe* macros), which are used in the code.
314 ret = my_func(bar, baz);
317 #ifdef BB_FEATURE_FUNKY
318 maybe_do_funky_stuff(bar, baz);
325 #ifdef BB_FEATURE_FUNKY
326 static inline void maybe_do_funky_stuff (int bar, int baz)
328 /* lotsa code in here */
331 static inline void maybe_do_funky_stuff (int bar, int baz) {}
334 (in the .c source file)
336 ret = my_func(bar, baz);
339 maybe_do_funky_stuff(bar, baz);
341 The great thing about this approach is that the compiler will optimize away
342 the "no-op" case (the empty function) when the feature is turned off.
344 Note also the use of the word 'maybe' in the function name to indicate
345 conditional execution.
352 Strings in C can get a little thorny. Here's some guidelines for dealing with
353 strings in Busybox. (There is surely more that could be added to this
360 Put all help/usage messages in usage.c. Put other strings in messages.c.
361 Putting these strings into their own file is a calculated decision designed to
362 confine spelling errors to a single place and aid internationalization
363 efforts, if needed. (Side Note: we might want to use a single file - maybe
364 called 'strings.c' - instead of two, food for thought).
367 Testing String Equivalence
368 ~~~~~~~~~~~~~~~~~~~~~~~~~~
370 There's a right way and a wrong way to test for sting equivalence with
375 if (!strcmp(string, "foo")) {
380 if (strcmp(string, "foo") == 0){
383 The use of the "equals" (==) operator in the latter example makes it much more
384 obvious that you are testing for equivalence. The former example with the
385 "not" (!) operator makes it look like you are testing for an error. In a more
386 perfect world, we would have a streq() function in the string library, but
387 that ain't the world we're living in.
390 Avoid Dangerous String Functions
391 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
393 Unfortunately, the way C handles strings makes them prone to overruns when
394 certain library functions are (mis)used. The following table offers a summary
395 of some of the more notorious troublemakers:
397 function overflows preferred
398 ----------------------------------------
399 strcpy dest string strncpy
400 strcat dest string strncat
401 gets string it gets fgets
402 getwd buf string getcwd
403 [v]sprintf str buffer [v]snprintf
404 realpath path buffer use with pathconf
405 [vf]scanf its arguments just avoid it
408 The above is by no means a complete list. Be careful out there.
412 Avoid Big Static Buffers
413 ------------------------
415 First, some background to put this discussion in context: Static buffers look
418 /* in a .c file outside any functions */
419 static char *buffer[BUFSIZ]; /* happily used by any function in this file,
422 The problem with these is that any time any busybox app is run, you pay a
423 memory penalty for this buffer, even if the applet that uses said buffer is
424 not run. This can be fixed, thusly:
430 strcpy(buffer, lotsa_chars); /* happily uses global *buffer */
434 buffer = xmalloc(sizeof(char)*BUFSIZ);
437 However, this approach trades bss segment for text segment. Rather than
438 mallocing the buffers (and thus growing the text size), buffers can be
439 declared on the stack in the *_main() function and made available globally by
440 assigning them to a global pointer thusly:
442 static char *pbuffer;
446 strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */
450 char *buffer[BUFSIZ]; /* declared locally, on stack */
451 pbuffer = buffer; /* but available globally */
454 This last approach has some advantages (low code size, space not used until
455 it's needed), but can be a problem in some low resource machines that have
456 very limited stack space (e.g., uCLinux).
458 A macro is declared in busybox.h that implements compile-time selection
459 between xmalloc() and stack creation, so you can code the line in question as
461 RESERVE_BB_BUFFER(buffer, BUFSIZ);
463 and the right thing will happen, based on your configuration.
467 Miscellaneous Coding Guidelines
468 -------------------------------
470 The following are important items that don't fit into any of the above
474 Model Busybox Applets After GNU Counterparts
475 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
477 When in doubt about the proper behavior of a Busybox program (output,
478 formatting, options, etc.), model it after the equivalent GNU program.
479 Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
480 matter what the POSIX standard says or doesn't say, just model Busybox
481 programs after their GNU counterparts and nobody has to get hurt.
483 The only time we deviate from emulating the GNU behavior is when:
485 - We are deliberately not supporting a feature (such as a command line
487 - Emulating the GNU behavior is prohibitively expensive (lots more code
488 would be required, lots more memory would be used, etc.)
489 - The difference is minor or cosmetic
491 A note on the 'cosmetic' case: Output differences might be considered
492 cosmetic, but if the output is significant enough to break other scripts that
493 use the output, it should really be fixed.
499 If a const variable is used only in a single source file, put it in the source
500 file and not in a header file. Likewise, if a const variable is used in only
501 one function, do not make it global to the file. Instead, declare it inside
502 the function body. Bottom line: Make a conscious effort to limit declarations
503 to the smallest scope possible.
505 Inside applet files, all functions should be declared static so as to keep the
506 global name space clean. The only exception to this rule is the "applet_main"
507 function which must be declared extern.
509 If you write a function that performs a task that could be useful outside the
510 immediate file, turn it into a general-purpose function with no ties to any
511 applet and put it in the utility.c file instead.
514 Brackets Are Your Friends
515 ~~~~~~~~~~~~~~~~~~~~~~~~~
517 Please use brackets on all if and else statements, even if it is only one
535 The "bracketless" approach is error prone because someday you might add a line
544 And the resulting behavior of your program would totally bewilder you. (Don't
545 laugh, it happens to us all.) Remember folks, this is C, not Python.
548 Function Declarations
549 ~~~~~~~~~~~~~~~~~~~~~
551 Do not use old-style function declarations that declare variable types between
552 the parameter list and opening bracket. Example:
556 int foo(parm1, parm2)
564 int foo(char parm1, float parm2)
568 The only time you would ever need to use the old declaration syntax is to
569 support ancient, antediluvian compilers. To our good fortune, we have access
570 to more modern compilers and the old declaration syntax is neither necessary
574 Emphasizing Logical Blocks
575 ~~~~~~~~~~~~~~~~~~~~~~~~~~
577 Organization and readability are improved by putting extra newlines around
578 blocks of code that perform a single task. These are typically blocks that
579 begin with a C keyword, but not always.
581 Furthermore, you should put a single comment (not necessarily one line, just
582 one comment) before the block, rather than commenting each and every line.
583 There is an optimal ammount of commenting that a program can have; you can
584 comment too much as well as too little.
586 A picture is really worth a thousand words here, so here is an example that
587 illustrates emphasizing logical blocks:
589 while (line = get_line_from_file(fp)) {
591 /* eat the newline, if any */
592 if (line[strlen(line)-1] == '\n') {
593 line[strlen(line)-1] = '\0';
596 /* ignore blank lines */
597 if (strlen(file_to_act_on) == 0) {
601 /* if the search string is in this line, print it,
602 * unless we were told to be quiet */
603 if (strstr(line, search) && !be_quiet) {
615 It's considered good form to test your new feature before you submit a patch
616 to the mailing list, and especially before you commit a change to CVS. Here
617 are some guildlines on testing your changes.
619 - Always test busybox applets against GNU counterparts and make sure the
620 behavior / output is identical between the two.
622 - Try several different permutations and combinations of the features you're
623 adding and make sure they all work. (Make sure one feature does not
624 interfere with another, etc.)
626 - Make sure you test compiling against the source both with the feature
627 turned on and turned off in Config.h and make sure busybox compiles cleanly