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 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 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 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.