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diff --git a/busybox/docs/style-guide.txt b/busybox/docs/style-guide.txt new file mode 100644 index 0000000..915d9b2 --- /dev/null +++ b/busybox/docs/style-guide.txt @@ -0,0 +1,680 @@ +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 program name and one-line description + - commented author name and email address(es) + - commented GPL boilerplate + - commented longer description / notes for the program (if needed) + - #includes of .h files with angle brackets (<>) around them + - #includes of .h files with quotes ("") around them + - #defines (if any, note the section below titled "Avoid the Preprocessor") + - const and global variables + - function declarations (if necessary) + - function implementations + + + +Whitespace and Formatting +------------------------- + +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 *whatever* 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 compound 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 bracket 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 statement. Examples: + + Don't do this: + + while (!done) + { + + do + { + + Don't do this either: + + while (!done){ + + do{ + + And for heaven's sake, don't do this: + + while (!done) + { + + do + { + + Do this instead: + + while (!done) { + + do { + + +Spacing around Parentheses +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +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++) { + + But do functions like this: + + static int my_func(int foo, char bar) + ... + baz = my_func(1, 2); + +Also, don't put a space between the left paren and the first term, nor between +the last arg and the right paren. + + Don't do this: + + if ( x < 1 ) + strcmp( thisstr, thatstr ) + + Do this instead: + + if (x < 1) + strcmp(thisstr, thatstr) + + +Cuddled Elses +~~~~~~~~~~~~~ + +Also, please "cuddle" your else statements by putting the else keyword on the +same line after the right bracket that closes an 'if' statement. + + Don't do this: + + if (foo) { + stmt; + } + else { + stmt; + } + + Do this instead: + + if (foo) { + stmt; + } else { + stmt; + } + +The exception to this rule is if you want to include a comment before the else +block. Example: + + if (foo) { + stmts... + } + /* otherwise, we're just kidding ourselves, so re-frob the input */ + else { + other_stmts... + } + + + +Variable and Function Names +--------------------------- + +Use the K&R style with names in all lower-case and underscores occasionally +used to separate 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. + + Frowned upon: + + hitList + TotalChars + szFileName + pf_Nfol_TriState + + Preferred: + + hit_list + total_chars + file_name + sensible_name + +Exceptions: + + - Enums, macros, and constant variables are occasionally written in all + upper-case with words optionally seperatedy by underscores (i.e. FIFOTYPE, + ISBLKDEV()). + + - Nobody is going to get mad at you for using 'pvar' as the name of a + variable that is a pointer to 'var'. + + +Converting to K&R +~~~~~~~~~~~~~~~~~ + +The Busybox codebase is very much a mixture of code gathered from a variety of +sources. This explains why the current codebase contains such a hodge-podge 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. + +If you want to do a search-and-replace of a single variable name in different +files, you can do the following in the busybox directory: + + $ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch] + +If you want to convert all the non-K&R vars in your file all at once, follow +these steps: + + - In the busybox directory type 'examples/mk2knr.pl files-to-convert'. This + does not do the actual conversion, rather, it generates a script called + 'convertme.pl' that shows what will be converted, giving you a chance to + review the changes beforehand. + + - Review the 'convertme.pl' script that gets generated in the busybox + directory and remove / edit any of the substitutions in there. Please + especially check for false positives (strings that should not be + converted). + + - Type './convertme.pl same-files-as-before' to perform the actual + conversion. + + - Compile and see if everything still works. + +Please be aware of changes that have cascading effects into other files. For +example, if you're changing the name of something in, say utility.c, you +should probably run 'examples/mk2knr.pl utility.c' at first, but when you run +the 'convertme.pl' script you should run it on _all_ files like so: +'./convertme.pl *.[ch]'. + + + +Avoid The Preprocessor +---------------------- + +At best, the preprocessor is a necessary evil, helping us account for platform +and architecture differences. Using the preprocessor unnecessarily is just +plain evil. + + +The Folly of #define +~~~~~~~~~~~~~~~~~~~~ + +Use 'const <type> var' for declaring constants. + + Don't do this: + + #define var 80 + + Do this instead, when the variable is in a header file and will be used in + several source files: + + const int var = 80; + + Or do this when the variable is used only in a single source file: + + static const int var = 80; + +Declaring variables as '[static] const' gives variables an actual type and +makes the compiler do type checking for you; the preprocessor does _no_ type +checking whatsoever, making it much more error prone. Declaring variables with +'[static] const' also makes debugging programs much easier since the value of +the variable can be easily queried and displayed. + + +The Folly of Macros +~~~~~~~~~~~~~~~~~~~ + +Use 'static inline' instead of a macro. + + Don't do this: + + #define mini_func(param1, param2) (param1 << param2) + + Do this instead: + + static inline int mini_func(int param1, param2) + { + return (param1 << param2); + } + +Static inline functions are greatly preferred over macros. They provide type +safety, have no length limitations, no formatting limitations, have an actual +return value, and under gcc they are as cheap as macros. Besides, really long +macros with backslashes at the end of each line are ugly as sin. + + +The Folly of #ifdef +~~~~~~~~~~~~~~~~~~~ + +Code cluttered with ifdefs is difficult to read and maintain. Don't do it. +Instead, put your ifdefs at the top of your .c file (or in a header), and +conditionally define 'static inline' functions, (or *maybe* macros), which are +used in the code. + + Don't do this: + + ret = my_func(bar, baz); + if (!ret) + return -1; + #ifdef CONFIG_FEATURE_FUNKY + maybe_do_funky_stuff(bar, baz); + #endif + + Do this instead: + + (in .h header file) + + #ifdef CONFIG_FEATURE_FUNKY + static inline void maybe_do_funky_stuff (int bar, int baz) + { + /* lotsa code in here */ + } + #else + static inline void maybe_do_funky_stuff (int bar, int baz) {} + #endif + + (in the .c source file) + + ret = my_func(bar, baz); + if (!ret) + return -1; + maybe_do_funky_stuff(bar, baz); + +The great thing about this approach is that the compiler will optimize away +the "no-op" case (the empty function) when the feature is turned off. + +Note also the use of the word 'maybe' in the function name to indicate +conditional execution. + + + +Notes on Strings +---------------- + +Strings in C can get a little thorny. Here's some guidelines for dealing with +strings in Busybox. (There is surely more that could be added to this +section.) + + +String Files +~~~~~~~~~~~~ + +Put all help/usage messages in usage.c. Put other strings in messages.c. +Putting these strings into their own file is a calculated decision designed to +confine spelling errors to a single place and aid internationalization +efforts, if needed. (Side Note: we might want to use a single file - maybe +called 'strings.c' - instead of two, food for thought). + + +Testing String Equivalence +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +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. In a more +perfect world, we would have a streq() function in the string library, but +that ain't the world we're living in. + + +Avoid Dangerous String Functions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Unfortunately, the way C handles strings makes them prone to overruns when +certain library functions are (mis)used. The following table offers a summary +of some of the more notorious troublemakers: + +function overflows preferred +---------------------------------------- +strcpy dest string strncpy +strcat dest string strncat +gets string it gets fgets +getwd buf string getcwd +[v]sprintf str buffer [v]snprintf +realpath path buffer use with pathconf +[vf]scanf its arguments just avoid it + + +The above is by no means a complete list. Be careful out there. + + + +Avoid Big Static Buffers +------------------------ + +First, some background to put this discussion in context: Static buffers look +like this in code: + + /* in a .c file outside any functions */ + static char *buffer[BUFSIZ]; /* happily used by any function in this file, + but ick! big! */ + +The problem with these is that any time any busybox app is run, you pay a +memory penalty for this buffer, even if the applet that uses said buffer is +not run. This can be fixed, thusly: + + static char *buffer; + ... + other_func() + { + strcpy(buffer, lotsa_chars); /* happily uses global *buffer */ + ... + foo_main() + { + buffer = xmalloc(sizeof(char)*BUFSIZ); + ... + +However, this approach trades bss segment for text segment. Rather than +mallocing the buffers (and thus growing the text size), buffers can be +declared on the stack in the *_main() function and made available globally by +assigning them to a global pointer thusly: + + static char *pbuffer; + ... + other_func() + { + strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */ + ... + foo_main() + { + char *buffer[BUFSIZ]; /* declared locally, on stack */ + pbuffer = buffer; /* but available globally */ + ... + +This last approach has some advantages (low code size, space not used until +it's needed), but can be a problem in some low resource machines that have +very limited stack space (e.g., uCLinux). + +A macro is declared in busybox.h that implements compile-time selection +between xmalloc() and stack creation, so you can code the line in question as + + RESERVE_CONFIG_BUFFER(buffer, BUFSIZ); + +and the right thing will happen, based on your configuration. + + + +Miscellaneous Coding Guidelines +------------------------------- + +The following are important items that don't fit into any of the above +sections. + + +Model Busybox Applets After GNU Counterparts +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When in doubt about the proper 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 it will make life easier on (nearly) +everyone. + +The only time we deviate from emulating the GNU behavior is when: + + - We are deliberately not supporting a feature (such as a command line + switch) + - Emulating the GNU behavior is prohibitively expensive (lots more code + would be required, lots more memory would be used, etc.) + - The difference is minor or cosmetic + +A note on the 'cosmetic' case: Output differences might be considered +cosmetic, but if the output is significant enough to break other scripts that +use the output, it should really be fixed. + + +Scope +~~~~~ + +If a const variable is used only in a single source file, put it in the source +file and not in a header file. Likewise, 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. Bottom line: Make a conscious effort to limit declarations +to the smallest scope possible. + +Inside applet files, all functions should be declared static so as to keep the +global name space 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. + + +Brackets Are Your Friends +~~~~~~~~~~~~~~~~~~~~~~~~~ + +Please use brackets on all if and else statements, even if it is only one +line. Example: + + Don't do this: + + if (foo) + stmt1; + stmt2 + stmt3; + + Do this instead: + + if (foo) { + stmt1; + } + stmt2 + stmt3; + +The "bracketless" approach is error prone because someday you might add a line +like this: + + if (foo) + stmt1; + new_line(); + stmt2 + stmt3; + +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. + + +Function Declarations +~~~~~~~~~~~~~~~~~~~~~ + +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) + { + .... + +The only time you would ever need to use the old declaration syntax is to +support ancient, antediluvian compilers. To our good fortune, we have access +to more modern compilers and the old declaration syntax is neither necessary +nor desired. + + +Emphasizing Logical Blocks +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Organization and readability are improved by putting extra newlines around +blocks of code that perform a single task. These are typically blocks that +begin with a C keyword, but not always. + +Furthermore, you should put a single comment (not necessarily one line, just +one comment) before the block, rather than commenting each and every line. +There is an optimal ammount of commenting that a program can have; you can +comment too much as well as too little. + +A picture is really worth a thousand words here, the following example +illustrates how to emphasize logical blocks: + + while (line = get_line_from_file(fp)) { + + /* eat the newline, if any */ + chomp(line); + + /* ignore blank lines */ + if (strlen(file_to_act_on) == 0) { + continue; + } + + /* if the search string is in this line, print it, + * unless we were told to be quiet */ + if (strstr(line, search) && !be_quiet) { + puts(line); + } + + /* clean up */ + free(line); + } + + +Processing Options with getopt +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If your applet needs to process command-line switches, please use getopt() to +do so. Numerous examples can be seen in many of the existing applets, but +basically it boils down to two things: at the top of the .c file, have this +line in the midst of your #includes: + + #include <getopt.h> + +And a code block similar to the following near the top of your applet_main() +routine: + + while ((opt = getopt(argc, argv, "abc")) > 0) { + switch (opt) { + case 'a': + do_a_opt = 1; + break; + case 'b': + do_b_opt = 1; + break; + case 'c': + do_c_opt = 1; + break; + default: + show_usage(); /* in utility.c */ + } + } + +If your applet takes no options (such as 'init'), there should be a line +somewhere in the file reads: + + /* no options, no getopt */ + +That way, when people go grepping to see which applets need to be converted to +use getopt, they won't get false positives. + +Additional Note: Do not use the getopt_long library function and do not try to +hand-roll your own long option parsing. Busybox applets should only support +short options. Explanations and examples of the short options should be +documented in usage.h. |