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 of .h files with angle brackets (<>) around them
30 - #includes of .h files with quotes ("") around them
31 - #defines (if any, note the section below titled "Avoid the Preprocessor")
32 - const and global variables
33 - function declarations (if necessary)
34 - function implementations
38 Whitespace and Formatting
39 -------------------------
41 This is everybody's favorite flame topic so let's get it out of the way right
45 Tabs vs. Spaces in Line Indentation
46 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
48 The preference in Busybox is to indent lines with tabs. Do not indent lines
49 with spaces and do not indents lines using a mixture of tabs and spaces. (The
50 indentation style in the Apache and Postfix source does this sort of thing:
51 \s\s\s\sif (expr) {\n\tstmt; --ick.) The only exception to this rule is
52 multi-line comments that use an asterisk at the beginning of each line, i.e.:
55 /t * This is a block comment.
56 /t * Note that it has multiple lines
57 /t * and that the beginning of each line has a tab plus a space
58 /t * except for the opening '/*' line where the slash
59 /t * is used instead of a space.
62 Furthermore, The preference is that tabs be set to display at four spaces
63 wide, but the beauty of using only tabs (and not spaces) at the beginning of
64 lines is that you can set your editor to display tabs at *whatever* number of
65 spaces is desired and the code will still look fine.
71 Put spaces between terms and operators. Example:
75 for(i=0;i<num_items;i++){
79 for (i = 0; i < num_items; i++) {
81 While it extends the line a bit longer, the spaced version is more
82 readable. An allowable exception to this rule is the situation where
83 excluding the spacing makes it more obvious that we are dealing with a
84 single term (even if it is a compound term) such as:
86 if (str[idx] == '/' && str[idx-1] != '\\')
90 if ((argc-1) - (optind+1) > 0)
96 If an opening bracket starts a function, it should be on the
97 next line with no spacing before it. However, if a bracket follows an opening
98 control block, it should be on the same line with a single space (not a tab)
99 between it and the opening control block statement. Examples:
109 Don't do this either:
115 And for heaven's sake, don't do this:
130 Spacing around Parentheses
131 ~~~~~~~~~~~~~~~~~~~~~~~~~~
133 Put a space between C keywords and left parens, but not between function names
134 and the left paren that starts it's parameter list (whether it is being
135 declared or called). Examples:
140 for(i = 0; i < n; i++) {
145 for (i = 0; i < n; i++) {
147 But do functions like this:
149 static int my_func(int foo, char bar)
153 Also, don't put a space between the left paren and the first term, nor between
154 the last arg and the right paren.
159 strcmp( thisstr, thatstr )
164 strcmp(thisstr, thatstr)
170 Also, please "cuddle" your else statements by putting the else keyword on the
171 same line after the right bracket that closes an 'if' statement.
190 The exception to this rule is if you want to include a comment before the else
196 /* otherwise, we're just kidding ourselves, so re-frob the input */
203 Variable and Function Names
204 ---------------------------
206 Use the K&R style with names in all lower-case and underscores occasionally
207 used to separate words (e.g., "variable_name" and "numchars" are both
208 acceptable). Using underscores makes variable and function names more readable
209 because it looks like whitespace; using lower-case is easy on the eyes.
227 - Enums, macros, and constant variables are occasionally written in all
228 upper-case with words optionally seperatedy by underscores (i.e. FIFOTYPE,
231 - Nobody is going to get mad at you for using 'pvar' as the name of a
232 variable that is a pointer to 'var'.
238 The Busybox codebase is very much a mixture of code gathered from a variety of
239 sources. This explains why the current codebase contains such a hodge-podge of
240 different naming styles (Java, Pascal, K&R, just-plain-weird, etc.). The K&R
241 guideline explained above should therefore be used on new files that are added
242 to the repository. Furthermore, the maintainer of an existing file that uses
243 alternate naming conventions should, at his own convenience, convert those
244 names over to K&R style. Converting variable names is a very low priority
247 If you want to do a search-and-replace of a single variable name in different
248 files, you can do the following in the busybox directory:
250 $ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch]
252 If you want to convert all the non-K&R vars in your file all at once, follow
255 - In the busybox directory type 'examples/mk2knr.pl files-to-convert'. This
256 does not do the actual conversion, rather, it generates a script called
257 'convertme.pl' that shows what will be converted, giving you a chance to
258 review the changes beforehand.
260 - Review the 'convertme.pl' script that gets generated in the busybox
261 directory and remove / edit any of the substitutions in there. Please
262 especially check for false positives (strings that should not be
265 - Type './convertme.pl same-files-as-before' to perform the actual
268 - Compile and see if everything still works.
270 Please be aware of changes that have cascading effects into other files. For
271 example, if you're changing the name of something in, say utility.c, you
272 should probably run 'examples/mk2knr.pl utility.c' at first, but when you run
273 the 'convertme.pl' script you should run it on _all_ files like so:
274 './convertme.pl *.[ch]'.
278 Avoid The Preprocessor
279 ----------------------
281 At best, the preprocessor is a necessary evil, helping us account for platform
282 and architecture differences. Using the preprocessor unnecessarily is just
289 Use 'const <type> var' for declaring constants.
295 Do this instead, when the variable is in a header file and will be used in
296 several source files:
300 Or do this when the variable is used only in a single source file:
302 static const int var = 80;
304 Declaring variables as '[static] const' gives variables an actual type and
305 makes the compiler do type checking for you; the preprocessor does _no_ type
306 checking whatsoever, making it much more error prone. Declaring variables with
307 '[static] const' also makes debugging programs much easier since the value of
308 the variable can be easily queried and displayed.
314 Use 'static inline' instead of a macro.
318 #define mini_func(param1, param2) (param1 << param2)
322 static inline int mini_func(int param1, param2)
324 return (param1 << param2);
327 Static inline functions are greatly preferred over macros. They provide type
328 safety, have no length limitations, no formatting limitations, have an actual
329 return value, and under gcc they are as cheap as macros. Besides, really long
330 macros with backslashes at the end of each line are ugly as sin.
336 Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
337 Instead, put your ifdefs at the top of your .c file (or in a header), and
338 conditionally define 'static inline' functions, (or *maybe* macros), which are
343 ret = my_func(bar, baz);
346 #ifdef CONFIG_FEATURE_FUNKY
347 maybe_do_funky_stuff(bar, baz);
354 #ifdef CONFIG_FEATURE_FUNKY
355 static inline void maybe_do_funky_stuff (int bar, int baz)
357 /* lotsa code in here */
360 static inline void maybe_do_funky_stuff (int bar, int baz) {}
363 (in the .c source file)
365 ret = my_func(bar, baz);
368 maybe_do_funky_stuff(bar, baz);
370 The great thing about this approach is that the compiler will optimize away
371 the "no-op" case (the empty function) when the feature is turned off.
373 Note also the use of the word 'maybe' in the function name to indicate
374 conditional execution.
381 Strings in C can get a little thorny. Here's some guidelines for dealing with
382 strings in Busybox. (There is surely more that could be added to this
389 Put all help/usage messages in usage.c. Put other strings in messages.c.
390 Putting these strings into their own file is a calculated decision designed to
391 confine spelling errors to a single place and aid internationalization
392 efforts, if needed. (Side Note: we might want to use a single file - maybe
393 called 'strings.c' - instead of two, food for thought).
396 Testing String Equivalence
397 ~~~~~~~~~~~~~~~~~~~~~~~~~~
399 There's a right way and a wrong way to test for sting equivalence with
404 if (!strcmp(string, "foo")) {
409 if (strcmp(string, "foo") == 0){
412 The use of the "equals" (==) operator in the latter example makes it much more
413 obvious that you are testing for equivalence. The former example with the
414 "not" (!) operator makes it look like you are testing for an error. In a more
415 perfect world, we would have a streq() function in the string library, but
416 that ain't the world we're living in.
419 Avoid Dangerous String Functions
420 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
422 Unfortunately, the way C handles strings makes them prone to overruns when
423 certain library functions are (mis)used. The following table offers a summary
424 of some of the more notorious troublemakers:
426 function overflows preferred
427 ----------------------------------------
428 strcpy dest string strncpy
429 strcat dest string strncat
430 gets string it gets fgets
431 getwd buf string getcwd
432 [v]sprintf str buffer [v]snprintf
433 realpath path buffer use with pathconf
434 [vf]scanf its arguments just avoid it
437 The above is by no means a complete list. Be careful out there.
441 Avoid Big Static Buffers
442 ------------------------
444 First, some background to put this discussion in context: Static buffers look
447 /* in a .c file outside any functions */
448 static char *buffer[BUFSIZ]; /* happily used by any function in this file,
451 The problem with these is that any time any busybox app is run, you pay a
452 memory penalty for this buffer, even if the applet that uses said buffer is
453 not run. This can be fixed, thusly:
459 strcpy(buffer, lotsa_chars); /* happily uses global *buffer */
463 buffer = xmalloc(sizeof(char)*BUFSIZ);
466 However, this approach trades bss segment for text segment. Rather than
467 mallocing the buffers (and thus growing the text size), buffers can be
468 declared on the stack in the *_main() function and made available globally by
469 assigning them to a global pointer thusly:
471 static char *pbuffer;
475 strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */
479 char *buffer[BUFSIZ]; /* declared locally, on stack */
480 pbuffer = buffer; /* but available globally */
483 This last approach has some advantages (low code size, space not used until
484 it's needed), but can be a problem in some low resource machines that have
485 very limited stack space (e.g., uCLinux).
487 A macro is declared in busybox.h that implements compile-time selection
488 between xmalloc() and stack creation, so you can code the line in question as
490 RESERVE_CONFIG_BUFFER(buffer, BUFSIZ);
492 and the right thing will happen, based on your configuration.
496 Miscellaneous Coding Guidelines
497 -------------------------------
499 The following are important items that don't fit into any of the above
503 Model Busybox Applets After GNU Counterparts
504 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
506 When in doubt about the proper behavior of a Busybox program (output,
507 formatting, options, etc.), model it after the equivalent GNU program.
508 Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
509 matter what the POSIX standard says or doesn't say, just model Busybox
510 programs after their GNU counterparts and it will make life easier on (nearly)
513 The only time we deviate from emulating the GNU behavior is when:
515 - We are deliberately not supporting a feature (such as a command line
517 - Emulating the GNU behavior is prohibitively expensive (lots more code
518 would be required, lots more memory would be used, etc.)
519 - The difference is minor or cosmetic
521 A note on the 'cosmetic' case: Output differences might be considered
522 cosmetic, but if the output is significant enough to break other scripts that
523 use the output, it should really be fixed.
529 If a const variable is used only in a single source file, put it in the source
530 file and not in a header file. Likewise, if a const variable is used in only
531 one function, do not make it global to the file. Instead, declare it inside
532 the function body. Bottom line: Make a conscious effort to limit declarations
533 to the smallest scope possible.
535 Inside applet files, all functions should be declared static so as to keep the
536 global name space clean. The only exception to this rule is the "applet_main"
537 function which must be declared extern.
539 If you write a function that performs a task that could be useful outside the
540 immediate file, turn it into a general-purpose function with no ties to any
541 applet and put it in the utility.c file instead.
544 Brackets Are Your Friends
545 ~~~~~~~~~~~~~~~~~~~~~~~~~
547 Please use brackets on all if and else statements, even if it is only one
565 The "bracketless" approach is error prone because someday you might add a line
574 And the resulting behavior of your program would totally bewilder you. (Don't
575 laugh, it happens to us all.) Remember folks, this is C, not Python.
578 Function Declarations
579 ~~~~~~~~~~~~~~~~~~~~~
581 Do not use old-style function declarations that declare variable types between
582 the parameter list and opening bracket. Example:
586 int foo(parm1, parm2)
594 int foo(char parm1, float parm2)
598 The only time you would ever need to use the old declaration syntax is to
599 support ancient, antediluvian compilers. To our good fortune, we have access
600 to more modern compilers and the old declaration syntax is neither necessary
604 Emphasizing Logical Blocks
605 ~~~~~~~~~~~~~~~~~~~~~~~~~~
607 Organization and readability are improved by putting extra newlines around
608 blocks of code that perform a single task. These are typically blocks that
609 begin with a C keyword, but not always.
611 Furthermore, you should put a single comment (not necessarily one line, just
612 one comment) before the block, rather than commenting each and every line.
613 There is an optimal ammount of commenting that a program can have; you can
614 comment too much as well as too little.
616 A picture is really worth a thousand words here, the following example
617 illustrates how to emphasize logical blocks:
619 while (line = get_line_from_file(fp)) {
621 /* eat the newline, if any */
624 /* ignore blank lines */
625 if (strlen(file_to_act_on) == 0) {
629 /* if the search string is in this line, print it,
630 * unless we were told to be quiet */
631 if (strstr(line, search) && !be_quiet) {
640 Processing Options with getopt
641 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
643 If your applet needs to process command-line switches, please use getopt() to
644 do so. Numerous examples can be seen in many of the existing applets, but
645 basically it boils down to two things: at the top of the .c file, have this
646 line in the midst of your #includes:
650 And a code block similar to the following near the top of your applet_main()
653 while ((opt = getopt(argc, argv, "abc")) > 0) {
665 show_usage(); /* in utility.c */
669 If your applet takes no options (such as 'init'), there should be a line
670 somewhere in the file reads:
672 /* no options, no getopt */
674 That way, when people go grepping to see which applets need to be converted to
675 use getopt, they won't get false positives.
677 Additional Note: Do not use the getopt_long library function and do not try to
678 hand-roll your own long option parsing. Busybox applets should only support
679 short options. Explanations and examples of the short options should be
680 documented in usage.h.