source: MondoRescue/branches/stable/mindi-busybox/docs/style-guide.txt @ 821

Last change on this file since 821 was 821, checked in by Bruno Cornec, 14 years ago

Addition of busybox 1.2.1 as a mindi-busybox new package
This should avoid delivering binary files in mindi not built there (Fedora and Debian are quite serious about that)

File size: 18.7 KB
Line 
1Busybox Style Guide
2===================
3
4This document describes the coding style conventions used in Busybox. If you
5add a new file to Busybox or are editing an existing file, please format your
6code according to this style. If you are the maintainer of a file that does
7not follow these guidelines, please -- at your own convenience -- modify the
8file(s) you maintain to bring them into conformance with this style guide.
9Please note that this is a low priority task.
10
11To help you format the whitespace of your programs, an ".indent.pro" file is
12included in the main Busybox source directory that contains option flags to
13format code as per this style guide. This way you can run GNU indent on your
14files by typing 'indent myfile.c myfile.h' and it will magically apply all the
15right formatting rules to your file. Please _do_not_ run this on all the files
16in the directory, just your own.
17
18
19
20Declaration Order
21-----------------
22
23Here is the order in which code should be laid out in a file:
24
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
35
36
37
38Whitespace and Formatting
39-------------------------
40
41This is everybody's favorite flame topic so let's get it out of the way right
42up front.
43
44
45Tabs vs. Spaces in Line Indentation
46~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
47
48The preference in Busybox is to indent lines with tabs. Do not indent lines
49with spaces and do not indents lines using a mixture of tabs and spaces. (The
50indentation 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
52multi-line comments that use an asterisk at the beginning of each line, i.e.:
53
54    /t/*
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.
60    /t */
61
62Furthermore, The preference is that tabs be set to display at four spaces
63wide, but the beauty of using only tabs (and not spaces) at the beginning of
64lines is that you can set your editor to display tabs at *whatever* number of
65spaces is desired and the code will still look fine.
66
67
68Operator Spacing
69~~~~~~~~~~~~~~~~
70
71Put spaces between terms and operators. Example:
72
73    Don't do this:
74
75        for(i=0;i<num_items;i++){
76
77    Do this instead:
78
79        for (i = 0; i < num_items; i++) {
80
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:
85
86        if (str[idx] == '/' && str[idx-1] != '\\')
87
88    or
89
90        if ((argc-1) - (optind+1) > 0)
91
92
93Bracket Spacing
94~~~~~~~~~~~~~~~
95
96If an opening bracket starts a function, it should be on the
97next line with no spacing before it. However, if a bracket follows an opening
98control block, it should be on the same line with a single space (not a tab)
99between it and the opening control block statement. Examples:
100
101    Don't do this:
102
103        while (!done)
104        {
105
106        do
107        {
108
109    Don't do this either:
110
111        while (!done){
112
113        do{
114
115    And for heaven's sake, don't do this:
116
117        while (!done)
118          {
119
120        do
121          {
122
123    Do this instead:
124
125        while (!done) {
126
127        do {
128
129Exceptions:
130
131 - if you have long logic statements that need to be wrapped, then uncuddling
132   the bracket to improve readability is allowed:
133
134        if (some_really_long_checks && some_other_really_long_checks \
135            && some_more_really_long_checks)
136        {
137            do_foo_now;
138
139Spacing around Parentheses
140~~~~~~~~~~~~~~~~~~~~~~~~~~
141
142Put a space between C keywords and left parens, but not between function names
143and the left paren that starts it's parameter list (whether it is being
144declared or called). Examples:
145
146    Don't do this:
147
148        while(foo) {
149        for(i = 0; i < n; i++) {
150
151    Do this instead:
152
153        while (foo) {
154        for (i = 0; i < n; i++) {
155
156    But do functions like this:
157
158        static int my_func(int foo, char bar)
159        ...
160        baz = my_func(1, 2);
161
162Also, don't put a space between the left paren and the first term, nor between
163the last arg and the right paren.
164
165    Don't do this:
166
167        if ( x < 1 )
168        strcmp( thisstr, thatstr )
169
170    Do this instead:
171
172        if (x < 1)
173        strcmp(thisstr, thatstr)
174
175
176Cuddled Elses
177~~~~~~~~~~~~~
178
179Also, please "cuddle" your else statements by putting the else keyword on the
180same line after the right bracket that closes an 'if' statement.
181
182    Don't do this:
183
184    if (foo) {
185        stmt;
186    }
187    else {
188        stmt;
189    }
190
191    Do this instead:
192
193    if (foo) {
194        stmt;
195    } else {
196        stmt;
197    }
198
199The exception to this rule is if you want to include a comment before the else
200block. Example:
201
202    if (foo) {
203        stmts...
204    }
205    /* otherwise, we're just kidding ourselves, so re-frob the input */
206    else {
207        other_stmts...
208    }
209
210
211
212Variable and Function Names
213---------------------------
214
215Use the K&R style with names in all lower-case and underscores occasionally
216used to separate words (e.g., "variable_name" and "numchars" are both
217acceptable). Using underscores makes variable and function names more readable
218because it looks like whitespace; using lower-case is easy on the eyes.
219
220    Frowned upon:
221
222        hitList
223        TotalChars
224        szFileName
225        pf_Nfol_TriState
226
227    Preferred:
228
229        hit_list
230        total_chars
231        file_name
232        sensible_name
233
234Exceptions:
235
236 - Enums, macros, and constant variables are occasionally written in all
237   upper-case with words optionally seperatedy by underscores (i.e. FIFOTYPE,
238   ISBLKDEV()).
239
240 - Nobody is going to get mad at you for using 'pvar' as the name of a
241   variable that is a pointer to 'var'.
242
243
244Converting to K&R
245~~~~~~~~~~~~~~~~~
246
247The Busybox codebase is very much a mixture of code gathered from a variety of
248sources. This explains why the current codebase contains such a hodge-podge of
249different naming styles (Java, Pascal, K&R, just-plain-weird, etc.). The K&R
250guideline explained above should therefore be used on new files that are added
251to the repository. Furthermore, the maintainer of an existing file that uses
252alternate naming conventions should, at his own convenience, convert those
253names over to K&R style. Converting variable names is a very low priority
254task.
255
256If you want to do a search-and-replace of a single variable name in different
257files, you can do the following in the busybox directory:
258
259    $ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch]
260
261If you want to convert all the non-K&R vars in your file all at once, follow
262these steps:
263
264 - In the busybox directory type 'examples/mk2knr.pl files-to-convert'. This
265   does not do the actual conversion, rather, it generates a script called
266   'convertme.pl' that shows what will be converted, giving you a chance to
267   review the changes beforehand.
268
269 - Review the 'convertme.pl' script that gets generated in the busybox
270   directory and remove / edit any of the substitutions in there. Please
271   especially check for false positives (strings that should not be
272   converted).
273
274 - Type './convertme.pl same-files-as-before' to perform the actual
275   conversion.
276
277 - Compile and see if everything still works.
278
279Please be aware of changes that have cascading effects into other files. For
280example, if you're changing the name of something in, say utility.c, you
281should probably run 'examples/mk2knr.pl utility.c' at first, but when you run
282the 'convertme.pl' script you should run it on _all_ files like so:
283'./convertme.pl *.[ch]'.
284
285
286
287Avoid The Preprocessor
288----------------------
289
290At best, the preprocessor is a necessary evil, helping us account for platform
291and architecture differences. Using the preprocessor unnecessarily is just
292plain evil.
293
294
295The Folly of #define
296~~~~~~~~~~~~~~~~~~~~
297
298Use 'const <type> var' for declaring constants.
299
300    Don't do this:
301
302        #define var 80
303
304    Do this instead, when the variable is in a header file and will be used in
305    several source files:
306
307        const int var = 80;
308
309    Or do this when the variable is used only in a single source file:
310
311        static const int var = 80;
312
313Declaring variables as '[static] const' gives variables an actual type and
314makes the compiler do type checking for you; the preprocessor does _no_ type
315checking whatsoever, making it much more error prone. Declaring variables with
316'[static] const' also makes debugging programs much easier since the value of
317the variable can be easily queried and displayed.
318
319
320The Folly of Macros
321~~~~~~~~~~~~~~~~~~~
322
323Use 'static inline' instead of a macro.
324
325    Don't do this:
326
327        #define mini_func(param1, param2) (param1 << param2)
328
329    Do this instead:
330
331        static inline int mini_func(int param1, param2)
332        {
333            return (param1 << param2);
334        }
335
336Static inline functions are greatly preferred over macros. They provide type
337safety, have no length limitations, no formatting limitations, have an actual
338return value, and under gcc they are as cheap as macros. Besides, really long
339macros with backslashes at the end of each line are ugly as sin.
340
341
342The Folly of #ifdef
343~~~~~~~~~~~~~~~~~~~
344
345Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
346Instead, put your ifdefs at the top of your .c file (or in a header), and
347conditionally define 'static inline' functions, (or *maybe* macros), which are
348used in the code.
349
350    Don't do this:
351
352        ret = my_func(bar, baz);
353        if (!ret)
354            return -1;
355        #ifdef CONFIG_FEATURE_FUNKY
356            maybe_do_funky_stuff(bar, baz);
357        #endif
358
359    Do this instead:
360
361    (in .h header file)
362
363        #ifdef CONFIG_FEATURE_FUNKY
364        static inline void maybe_do_funky_stuff (int bar, int baz)
365        {
366            /* lotsa code in here */
367        }
368        #else
369        static inline void maybe_do_funky_stuff (int bar, int baz) {}
370        #endif
371
372    (in the .c source file)
373
374        ret = my_func(bar, baz);
375        if (!ret)
376            return -1;
377        maybe_do_funky_stuff(bar, baz);
378
379The great thing about this approach is that the compiler will optimize away
380the "no-op" case (the empty function) when the feature is turned off.
381
382Note also the use of the word 'maybe' in the function name to indicate
383conditional execution.
384
385
386
387Notes on Strings
388----------------
389
390Strings in C can get a little thorny. Here's some guidelines for dealing with
391strings in Busybox. (There is surely more that could be added to this
392section.)
393
394
395String Files
396~~~~~~~~~~~~
397
398Put all help/usage messages in usage.c. Put other strings in messages.c.
399Putting these strings into their own file is a calculated decision designed to
400confine spelling errors to a single place and aid internationalization
401efforts, if needed. (Side Note: we might want to use a single file - maybe
402called 'strings.c' - instead of two, food for thought).
403
404
405Testing String Equivalence
406~~~~~~~~~~~~~~~~~~~~~~~~~~
407
408There's a right way and a wrong way to test for sting equivalence with
409strcmp():
410
411    The wrong way:
412
413        if (!strcmp(string, "foo")) {
414            ...
415
416    The right way:
417
418        if (strcmp(string, "foo") == 0){
419            ...
420
421The use of the "equals" (==) operator in the latter example makes it much more
422obvious that you are testing for equivalence. The former example with the
423"not" (!) operator makes it look like you are testing for an error. In a more
424perfect world, we would have a streq() function in the string library, but
425that ain't the world we're living in.
426
427
428Avoid Dangerous String Functions
429~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
430
431Unfortunately, the way C handles strings makes them prone to overruns when
432certain library functions are (mis)used. The following table  offers a summary
433of some of the more notorious troublemakers:
434
435function     overflows         preferred
436----------------------------------------
437strcpy       dest string       strncpy
438strcat       dest string       strncat
439gets         string it gets    fgets
440getwd        buf string        getcwd
441[v]sprintf   str buffer        [v]snprintf
442realpath     path buffer       use with pathconf
443[vf]scanf    its arguments     just avoid it
444
445
446The above is by no means a complete list. Be careful out there.
447
448
449
450Avoid Big Static Buffers
451------------------------
452
453First, some background to put this discussion in context: Static buffers look
454like this in code:
455
456    /* in a .c file outside any functions */
457    static char buffer[BUFSIZ]; /* happily used by any function in this file,
458                                    but ick! big! */
459
460The problem with these is that any time any busybox app is run, you pay a
461memory penalty for this buffer, even if the applet that uses said buffer is
462not run. This can be fixed, thusly:
463
464    static char *buffer;
465    ...
466    other_func()
467    {
468        strcpy(buffer, lotsa_chars); /* happily uses global *buffer */
469    ...
470    foo_main()
471    {
472        buffer = xmalloc(sizeof(char)*BUFSIZ);
473    ...
474
475However, this approach trades bss segment for text segment. Rather than
476mallocing the buffers (and thus growing the text size), buffers can be
477declared on the stack in the *_main() function and made available globally by
478assigning them to a global pointer thusly:
479
480    static char *pbuffer;
481    ...
482    other_func()
483    {
484        strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */
485    ...
486    foo_main()
487    {
488        char *buffer[BUFSIZ]; /* declared locally, on stack */
489        pbuffer = buffer;     /* but available globally */
490    ...
491
492This last approach has some advantages (low code size, space not used until
493it's needed), but can be a problem in some low resource machines that have
494very limited stack space (e.g., uCLinux).
495
496A macro is declared in busybox.h that implements compile-time selection
497between xmalloc() and stack creation, so you can code the line in question as
498
499        RESERVE_CONFIG_BUFFER(buffer, BUFSIZ);
500
501and the right thing will happen, based on your configuration.
502
503
504
505Miscellaneous Coding Guidelines
506-------------------------------
507
508The following are important items that don't fit into any of the above
509sections.
510
511
512Model Busybox Applets After GNU Counterparts
513~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
514
515When in doubt about the proper behavior of a Busybox program (output,
516formatting, options, etc.), model it after the equivalent GNU program.
517Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
518matter what the POSIX standard says or doesn't say, just model Busybox
519programs after their GNU counterparts and it will make life easier on (nearly)
520everyone.
521
522The only time we deviate from emulating the GNU behavior is when:
523
524    - We are deliberately not supporting a feature (such as a command line
525      switch)
526    - Emulating the GNU behavior is prohibitively expensive (lots more code
527      would be required, lots more memory would be used, etc.)
528    - The difference is minor or cosmetic
529
530A note on the 'cosmetic' case: Output differences might be considered
531cosmetic, but if the output is significant enough to break other scripts that
532use the output, it should really be fixed.
533
534
535Scope
536~~~~~
537
538If a const variable is used only in a single source file, put it in the source
539file and not in a header file. Likewise, if a const variable is used in only
540one function, do not make it global to the file. Instead, declare it inside
541the function body. Bottom line: Make a conscious effort to limit declarations
542to the smallest scope possible.
543
544Inside applet files, all functions should be declared static so as to keep the
545global name space clean. The only exception to this rule is the "applet_main"
546function which must be declared extern.
547
548If you write a function that performs a task that could be useful outside the
549immediate file, turn it into a general-purpose function with no ties to any
550applet and put it in the utility.c file instead.
551
552
553Brackets Are Your Friends
554~~~~~~~~~~~~~~~~~~~~~~~~~
555
556Please use brackets on all if and else statements, even if it is only one
557line. Example:
558
559    Don't do this:
560
561        if (foo)
562            stmt1;
563        stmt2
564        stmt3;
565
566    Do this instead:
567
568        if (foo) {
569            stmt1;
570        }
571        stmt2
572        stmt3;
573
574The "bracketless" approach is error prone because someday you might add a line
575like this:
576
577        if (foo)
578            stmt1;
579            new_line();
580        stmt2
581        stmt3;
582
583And the resulting behavior of your program would totally bewilder you. (Don't
584laugh, it happens to us all.) Remember folks, this is C, not Python.
585
586
587Function Declarations
588~~~~~~~~~~~~~~~~~~~~~
589
590Do not use old-style function declarations that declare variable types between
591the parameter list and opening bracket. Example:
592
593    Don't do this:
594
595        int foo(parm1, parm2)
596            char parm1;
597            float parm2;
598        {
599            ....
600
601    Do this instead:
602
603        int foo(char parm1, float parm2)
604        {
605            ....
606
607The only time you would ever need to use the old declaration syntax is to
608support ancient, antediluvian compilers. To our good fortune, we have access
609to more modern compilers and the old declaration syntax is neither necessary
610nor desired.
611
612
613Emphasizing Logical Blocks
614~~~~~~~~~~~~~~~~~~~~~~~~~~
615
616Organization and readability are improved by putting extra newlines around
617blocks of code that perform a single task. These are typically blocks that
618begin with a C keyword, but not always.
619
620Furthermore, you should put a single comment (not necessarily one line, just
621one comment) before the block, rather than commenting each and every line.
622There is an optimal ammount of commenting that a program can have; you can
623comment too much as well as too little.
624
625A picture is really worth a thousand words here, the following example
626illustrates how to emphasize logical blocks:
627
628    while (line = get_line_from_file(fp)) {
629
630        /* eat the newline, if any */
631        chomp(line);
632
633        /* ignore blank lines */
634        if (strlen(file_to_act_on) == 0) {
635            continue;
636        }
637
638        /* if the search string is in this line, print it,
639         * unless we were told to be quiet */
640        if (strstr(line, search) && !be_quiet) {
641            puts(line);
642        }
643
644        /* clean up */
645        free(line);
646    }
647
648
649Processing Options with getopt
650~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
651
652If your applet needs to process  command-line switches, please use getopt() to
653do so. Numerous examples can be seen in many of the existing applets, but
654basically it boils down to two things: at the top of the .c file, have this
655line in the midst of your #includes:
656
657    #include <getopt.h>
658
659And a code block similar to the following near the top of your applet_main()
660routine:
661
662    while ((opt = getopt(argc, argv, "abc")) > 0) {
663            switch (opt) {
664            case 'a':
665                do_a_opt = 1;
666                break;
667            case 'b':
668                do_b_opt = 1;
669                break;
670            case 'c':
671                do_c_opt = 1;
672                break;
673            default:
674                show_usage();    /* in utility.c */
675            }
676    }
677
678If your applet takes no options (such as 'init'), there should be a line
679somewhere in the file reads:
680
681    /* no options, no getopt */
682
683That way, when people go grepping to see which applets need to be converted to
684use getopt, they won't get false positives.
685
686Additional Note: Do not use the getopt_long library function and do not try to
687hand-roll your own long option parsing. Busybox applets should only support
688short options. Explanations and examples of the short options should be
689documented in usage.h.
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