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:
112 And for heaven's sake, don't do this:
128 Put a space between C keywords and left parens, but not between
129 function names and the left paren that starts it's parameter list (whether it
130 is being declared or called). Examples:
135 for(i = 0; i < n; i++) {
140 for (i = 0; i < n; i++) {
142 But do functions like this:
144 static int my_func(int foo, char bar)
152 Also, please "cuddle" your else statements by putting the else keyword on the
153 same line after the right bracket that closes an 'if' statement.
172 The exception to this rule is if you want to include a comment before the else
178 /* otherwise, we're just kidding ourselves, so re-frob the input */
186 Variable and Function Names
187 ---------------------------
189 Use the K&R style with names in all lower-case and underscores occasionally
190 used to separate words (e.g., "variable_name" and "numchars" are both
191 acceptable). Using underscores makes variable and function names more readable
192 because it looks like whitespace; using lower-case is easy on the eyes.
210 - Enums, macros, and constant variables should all be in upper-case with
211 words optionally seperatedy by underscores (i.e. FIFOTYPE, ISBLKDEV()).
213 - Nobody is going to get mad at you for using 'pvar' as the name of a
214 variable that is a pointer to 'var'.
216 Note: The Busybox codebase is very much a mixture of code gathered from a
217 variety of sources. This explains why the current codebase contains such a
218 hodge-podge of different naming styles (Java, Pascal, K&R, just-plain-weird,
219 etc.). The K&R guideline explained above should therefore be used on new files
220 that are added to the repository. Furthermore, the maintainer of an existing
221 file that uses alternate naming conventions should -- at his own convenience
222 -- convert those names over to K&R style; converting variable names is a very
223 low priority task. Perhaps in the future we will include some magical Perl
224 script that can go through and convert variable names, left as an exercise for
227 For the time being, if you want to do a search-and-replace of a variable name
228 in different files, do the following in the busybox directory:
230 $ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch]
234 Avoid The Preprocessor
235 ----------------------
237 At best, the preprocessor is a necessary evil, helping us account for platform
238 and architecture differences. Using the preprocessor unnecessarily is just
245 Use 'const <type> var' for declaring constants.
251 Do this instead, when the variable is in a header file and will be used in
252 several source files:
256 Or do this when the variable is used only in a single source file:
258 static const int var = 80;
260 Declaring variables as '[static] const' gives variables an actual type and
261 makes the compiler do type checking for you; the preprocessor does _no_ type
262 checking whatsoever, making it much more error prone. Declaring variables with
263 '[static] const' also makes debugging programs much easier since the value of
264 the variable can be easily queried and displayed.
270 Use 'static inline' instead of a macro.
274 #define mini_func(param1, param2) (param1 << param2)
278 static inline int mini_func(int param1, param2)
280 return (param1 << param2);
283 Static inline functions are greatly preferred over macros. They provide type
284 safety, have no length limitations, no formatting limitations, and under gcc
285 they are as cheap as macros. Besides, really long macros with backslashes at
286 the end of each line are ugly as sin.
292 Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
293 Instead, put your ifdefs in a header, and conditionally define 'static inline'
294 functions, (or *maybe* macros), which are used in the code.
298 ret = my_func(bar, baz);
301 #ifdef BB_FEATURE_FUNKY
302 maybe_do_funky_stuff(bar, baz);
309 #ifndef BB_FEATURE_FUNKY
310 static inline void maybe_do_funky_stuff (int bar, int baz) {}
313 (in the .c source file)
315 ret = my_func(bar, baz);
318 maybe_do_funky_stuff(bar, baz);
320 The great thing about this approach is that the compiler will optimize away
321 the "no-op" case when the feature is turned off.
323 Note also the use of the word 'maybe' in the function name to indicate
324 conditional execution.
331 Strings in C can get a little thorny. Here's some guidelines for dealing with
332 strings in Busybox. (There is surely more that could be added to this
339 Put all help/usage messages in usage.c. Put other strings in messages.c.
340 Putting these strings into their own file is a calculated decision designed to
341 confine spelling errors to a single place and aid internationalization
342 efforts, if needed. (Side Note: we might want to use a single file - maybe
343 called 'strings.c' - instead of two, food for thought).
346 Testing String Equivalence
347 ~~~~~~~~~~~~~~~~~~~~~~~~~~
349 There's a right way and a wrong way to test for sting equivalence with
354 if (!strcmp(string, "foo")) {
359 if (strcmp(string, "foo") == 0){
362 The use of the "equals" (==) operator in the latter example makes it much more
363 obvious that you are testing for equivalence. The former example with the
364 "not" (!) operator makes it look like you are testing for an error. In a more
365 perfect world, we would have a streq() function in the string library, but
366 that ain't the world we're living in.
369 Avoid Dangerous String Functions
370 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
372 Unfortunately, the way C handles strings makes them prone to overruns when
373 certain library functions are (mis)used. The following table offers a summary
374 of some of the more notorious troublemakers:
376 function overflows preferred
377 ----------------------------------------
378 strcpy dest string strncpy
379 strcat dest string strncat
380 gets string it gets fgets
381 getwd buf string getcwd
382 [v]sprintf str buffer [v]snprintf
383 realpath path buffer use with pathconf
384 [vf]scanf its arguments just avoid it
387 The above is by no means a complete list. Be careful out there.
391 Avoid Big Static Buffers
392 ------------------------
394 First, some background to put this discussion in context: Static buffers look
397 /* in a .c file outside any functions */
398 static char *buffer[BUFSIZ]; /* happily used by any function in this file,
401 The problem with these is that any time any busybox app is run, you pay a
402 memory penalty for this buffer, even if the applet that uses said buffer is
403 not run. This can be fixed, thusly:
409 strcpy(buffer, lotsa_chars); /* happily uses global *buffer */
413 buffer = xmalloc(sizeof(char)*BUFSIZ);
416 However, this approach trades bss segment for text segment. Rather than
417 mallocing the buffers (and thus growing the text size), buffers can be
418 declared on the stack in the *_main() function and made available globally by
419 assigning them to a global pointer thusly:
421 static char *pbuffer;
425 strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */
429 char *buffer[BUFSIZ]; /* declared locally, on stack */
430 pbuffer = buffer; /* but available globally */
433 This last approach has some advantages (low code size, space not used until
434 it's needed), but can be a problem in some low resource machines that have
435 very limited stack space (e.g., uCLinux). busybox.h declares a macro that
436 implements compile-time selection between xmalloc() and stack creation, so
437 you can code the line in question as
438 RESERVE_BB_BUFFER(buffer, BUFSIZ);
439 and the right thing will happen, based on the customer's configuration.
443 Miscellaneous Coding Guidelines
444 -------------------------------
446 The following are important items that don't fit into any of the above
450 Model Busybox Applets After GNU Counterparts
451 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453 When in doubt about the proper behavior of a Busybox program (output,
454 formatting, options, etc.), model it after the equivalent GNU program.
455 Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
456 matter what the POSIX standard says or doesn't say, just model Busybox
457 programs after their GNU counterparts and nobody has to get hurt.
459 The only time we deviate from emulating the GNU behavior is when:
461 - We are deliberately not supporting a feature (such as a command line
463 - Emulating the GNU behavior is prohibitively expensive (lots more code
464 would be required, lots more memory would be used, etc.)
465 - The difference is minor or cosmetic
467 A note on the 'cosmetic' case: Output differences might be considered
468 cosmetic, but if the output is significant enough to break other scripts that
469 use the output, it should really be fixed.
475 If a const variable is used only in a single source file, put it in the source
476 file and not in a header file. Likewise, if a const variable is used in only
477 one function, do not make it global to the file. Instead, declare it inside
478 the function body. Bottom line: Make a conscious effort to limit declarations
479 to the smallest scope possible.
481 Inside applet files, all functions should be declared static so as to keep the
482 global name space clean. The only exception to this rule is the "applet_main"
483 function which must be declared extern.
485 If you write a function that performs a task that could be useful outside the
486 immediate file, turn it into a general-purpose function with no ties to any
487 applet and put it in the utility.c file instead.
490 Brackets Are Your Friends
491 ~~~~~~~~~~~~~~~~~~~~~~~~~
493 Please use brackets on all if and else statements, even if it is only one
511 The "bracketless" approach is error prone because someday you might add a line
520 And the resulting behavior of your program would totally bewilder you. (Don't
521 laugh, it happens to us all.) Remember folks, this is C, not Python.
524 Function Declarations
525 ~~~~~~~~~~~~~~~~~~~~~
527 Do not use old-style function declarations that declare variable types between
528 the parameter list and opening bracket. Example:
532 int foo(parm1, parm2)
540 int foo(char parm1, float parm2)
544 The only time you would ever need to use the old declaration syntax is to
545 support ancient, antediluvian compilers. To our good fortune, we have access
546 to more modern compilers and the old declaration syntax is neither necessary
550 Emphasizing Logical Blocks
551 ~~~~~~~~~~~~~~~~~~~~~~~~~~
553 Organization and readability are improved by putting extra newlines around
554 blocks of code that perform a single task. These are typically blocks that
555 begin with a C keyword, but not always.
557 Furthermore, you should put a single comment (not necessarily one line, just
558 one comment) before the block, rather than commenting each and every line.
559 There is an optimal ammount of commenting that a program can have; you can
560 comment too much as well as too little.
562 A picture is really worth a thousand words here, so here is an example that
563 illustrates emphasizing logical blocks:
565 while (line = get_line_from_file(fp)) {
567 /* eat the newline, if any */
568 if (line[strlen(line)-1] == '\n') {
569 line[strlen(line)-1] = '\0';
572 /* ignore blank lines */
573 if (strlen(file_to_act_on) == 0) {
577 /* if the search string is in this line, print it,
578 * unless we were told to be quiet */
579 if (strstr(line, search) && !be_quiet) {