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715 lines
19 KiB
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715 lines
19 KiB
Plaintext
Busybox Style Guide
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===================
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This document describes the coding style conventions used in Busybox. If you
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add a new file to Busybox or are editing an existing file, please format your
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code according to this style. If you are the maintainer of a file that does
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not follow these guidelines, please -- at your own convenience -- modify the
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file(s) you maintain to bring them into conformance with this style guide.
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Please note that this is a low priority task.
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To help you format the whitespace of your programs, an ".indent.pro" file is
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included in the main Busybox source directory that contains option flags to
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format code as per this style guide. This way you can run GNU indent on your
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files by typing 'indent myfile.c myfile.h' and it will magically apply all the
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right formatting rules to your file. Please _do_not_ run this on all the files
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in the directory, just your own.
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Declaration Order
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-----------------
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Here is the preferred order in which code should be laid out in a file:
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- commented program name and one-line description
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- commented author name and email address(es)
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- commented GPL boilerplate
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- commented longer description / notes for the program (if needed)
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- #includes of .h files with angle brackets (<>) around them
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- #includes of .h files with quotes ("") around them
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- #defines (if any, note the section below titled "Avoid the Preprocessor")
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- const and global variables
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- function declarations (if necessary)
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- function implementations
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Whitespace and Formatting
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-------------------------
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This is everybody's favorite flame topic so let's get it out of the way right
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up front.
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Tabs vs. Spaces in Line Indentation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The preference in Busybox is to indent lines with tabs. Do not indent lines
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with spaces and do not indents lines using a mixture of tabs and spaces. (The
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indentation style in the Apache and Postfix source does this sort of thing:
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\s\s\s\sif (expr) {\n\tstmt; --ick.) The only exception to this rule is
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multi-line comments that use an asterisk at the beginning of each line, i.e.:
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\t/*
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\t * This is a block comment.
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\t * Note that it has multiple lines
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\t * and that the beginning of each line has a tab plus a space
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\t * except for the opening '/*' line where the slash
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\t * is used instead of a space.
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\t */
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Furthermore, The preference is that tabs be set to display at four spaces
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wide, but the beauty of using only tabs (and not spaces) at the beginning of
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lines is that you can set your editor to display tabs at *whatever* number of
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spaces is desired and the code will still look fine.
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Operator Spacing
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~~~~~~~~~~~~~~~~
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Put spaces between terms and operators. Example:
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Don't do this:
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for(i=0;i<num_items;i++){
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Do this instead:
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for (i = 0; i < num_items; i++) {
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While it extends the line a bit longer, the spaced version is more
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readable. An allowable exception to this rule is the situation where
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excluding the spacing makes it more obvious that we are dealing with a
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single term (even if it is a compound term) such as:
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if (str[idx] == '/' && str[idx-1] != '\\')
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or
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if ((argc-1) - (optind+1) > 0)
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Bracket Spacing
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~~~~~~~~~~~~~~~
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If an opening bracket starts a function, it should be on the
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next line with no spacing before it. However, if a bracket follows an opening
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control block, it should be on the same line with a single space (not a tab)
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between it and the opening control block statement. Examples:
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Don't do this:
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while (!done)
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{
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do
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{
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Don't do this either:
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while (!done){
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do{
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And for heaven's sake, don't do this:
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while (!done)
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{
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do
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{
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Do this instead:
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while (!done) {
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do {
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If you have long logic statements that need to be wrapped, then uncuddling
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the bracket to improve readability is allowed. Generally, this style makes
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it easier for reader to notice that 2nd and following lines are still
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inside 'if':
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if (some_really_long_checks && some_other_really_long_checks
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&& some_more_really_long_checks
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&& even_more_of_long_checks
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) {
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do_foo_now;
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Spacing around Parentheses
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Put a space between C keywords and left parens, but not between function names
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and the left paren that starts it's parameter list (whether it is being
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declared or called). Examples:
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Don't do this:
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while(foo) {
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for(i = 0; i < n; i++) {
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Do this instead:
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while (foo) {
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for (i = 0; i < n; i++) {
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But do functions like this:
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static int my_func(int foo, char bar)
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...
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baz = my_func(1, 2);
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Also, don't put a space between the left paren and the first term, nor between
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the last arg and the right paren.
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Don't do this:
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if ( x < 1 )
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strcmp( thisstr, thatstr )
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Do this instead:
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if (x < 1)
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strcmp(thisstr, thatstr)
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Cuddled Elses
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~~~~~~~~~~~~~
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Also, please "cuddle" your else statements by putting the else keyword on the
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same line after the right bracket that closes an 'if' statement.
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Don't do this:
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if (foo) {
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stmt;
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}
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else {
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stmt;
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}
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Do this instead:
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if (foo) {
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stmt;
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} else {
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stmt;
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}
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The exception to this rule is if you want to include a comment before the else
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block. Example:
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if (foo) {
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stmts...
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}
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/* otherwise, we're just kidding ourselves, so re-frob the input */
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else {
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other_stmts...
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}
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Labels
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~~~~~~
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Labels should start at the beginning of the line, not indented to the block
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level (because they do not "belong" to block scope, only to whole function).
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if (foo) {
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stmt;
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label:
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stmt2;
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stmt;
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}
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(Putting label at position 1 prevents diff -p from confusing label for function
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name, but it's not a policy of busybox project to enforce such a minor detail).
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Variable and Function Names
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---------------------------
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Use the K&R style with names in all lower-case and underscores occasionally
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used to separate words (e.g., "variable_name" and "numchars" are both
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acceptable). Using underscores makes variable and function names more readable
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because it looks like whitespace; using lower-case is easy on the eyes.
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Frowned upon:
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hitList
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TotalChars
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szFileName
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pf_Nfol_TriState
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Preferred:
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hit_list
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total_chars
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file_name
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sensible_name
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Exceptions:
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- Enums, macros, and constant variables are occasionally written in all
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upper-case with words optionally separated by underscores (i.e. FIFO_TYPE,
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ISBLKDEV()).
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- Nobody is going to get mad at you for using 'pvar' as the name of a
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variable that is a pointer to 'var'.
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Converting to K&R
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~~~~~~~~~~~~~~~~~
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The Busybox codebase is very much a mixture of code gathered from a variety of
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sources. This explains why the current codebase contains such a hodge-podge of
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different naming styles (Java, Pascal, K&R, just-plain-weird, etc.). The K&R
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guideline explained above should therefore be used on new files that are added
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to the repository. Furthermore, the maintainer of an existing file that uses
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alternate naming conventions should, at his own convenience, convert those
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names over to K&R style. Converting variable names is a very low priority
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task.
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If you want to do a search-and-replace of a single variable name in different
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files, you can do the following in the busybox directory:
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$ perl -pi -e 's/\bOldVar\b/new_var/g' *.[ch]
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If you want to convert all the non-K&R vars in your file all at once, follow
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these steps:
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- In the busybox directory type 'examples/mk2knr.pl files-to-convert'. This
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does not do the actual conversion, rather, it generates a script called
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'convertme.pl' that shows what will be converted, giving you a chance to
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review the changes beforehand.
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- Review the 'convertme.pl' script that gets generated in the busybox
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directory and remove / edit any of the substitutions in there. Please
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especially check for false positives (strings that should not be
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converted).
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- Type './convertme.pl same-files-as-before' to perform the actual
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conversion.
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- Compile and see if everything still works.
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Please be aware of changes that have cascading effects into other files. For
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example, if you're changing the name of something in, say utility.c, you
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should probably run 'examples/mk2knr.pl utility.c' at first, but when you run
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the 'convertme.pl' script you should run it on _all_ files like so:
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'./convertme.pl *.[ch]'.
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Avoid The Preprocessor
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----------------------
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At best, the preprocessor is a necessary evil, helping us account for platform
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and architecture differences. Using the preprocessor unnecessarily is just
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plain evil.
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The Folly of #define
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~~~~~~~~~~~~~~~~~~~~
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Use 'const <type> var' for declaring constants.
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Don't do this:
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#define CONST 80
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Do this instead, when the variable is in a header file and will be used in
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several source files:
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enum { CONST = 80 };
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Although enum may look ugly to some people, it is better for code size.
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With "const int" compiler may fail to optimize it out and will reserve
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a real storage in rodata for it! (Hopefully, newer gcc will get better
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at it...). With "define", you have slight risk of polluting namespace
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(#define doesn't allow you to redefine the name in the inner scopes),
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and complex "define" are evaluated each time they uesd, not once
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at declarations like enums. Also, the preprocessor does _no_ type checking
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whatsoever, making it much more error prone.
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The Folly of Macros
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~~~~~~~~~~~~~~~~~~~
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Use 'static inline' instead of a macro.
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Don't do this:
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#define mini_func(param1, param2) (param1 << param2)
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Do this instead:
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static inline int mini_func(int param1, param2)
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{
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return (param1 << param2);
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}
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Static inline functions are greatly preferred over macros. They provide type
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safety, have no length limitations, no formatting limitations, have an actual
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return value, and under gcc they are as cheap as macros. Besides, really long
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macros with backslashes at the end of each line are ugly as sin.
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The Folly of #ifdef
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~~~~~~~~~~~~~~~~~~~
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Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
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Instead, put your ifdefs at the top of your .c file (or in a header), and
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conditionally define 'static inline' functions, (or *maybe* macros), which are
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used in the code.
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Don't do this:
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ret = my_func(bar, baz);
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if (!ret)
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return -1;
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#ifdef CONFIG_FEATURE_FUNKY
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maybe_do_funky_stuff(bar, baz);
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#endif
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Do this instead:
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(in .h header file)
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#if ENABLE_FEATURE_FUNKY
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static inline void maybe_do_funky_stuff(int bar, int baz)
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{
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/* lotsa code in here */
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}
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#else
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static inline void maybe_do_funky_stuff(int bar, int baz) {}
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#endif
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(in the .c source file)
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ret = my_func(bar, baz);
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if (!ret)
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return -1;
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maybe_do_funky_stuff(bar, baz);
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The great thing about this approach is that the compiler will optimize away
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the "no-op" case (the empty function) when the feature is turned off.
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Note also the use of the word 'maybe' in the function name to indicate
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conditional execution.
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Notes on Strings
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----------------
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Strings in C can get a little thorny. Here's some guidelines for dealing with
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strings in Busybox. (There is surely more that could be added to this
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section.)
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String Files
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~~~~~~~~~~~~
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Put all help/usage messages in usage.c. Put other strings in messages.c.
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Putting these strings into their own file is a calculated decision designed to
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confine spelling errors to a single place and aid internationalization
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efforts, if needed. (Side Note: we might want to use a single file - maybe
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called 'strings.c' - instead of two, food for thought).
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Testing String Equivalence
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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There's a right way and a wrong way to test for string equivalence with
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strcmp():
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The wrong way:
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if (!strcmp(string, "foo")) {
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...
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The right way:
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if (strcmp(string, "foo") == 0){
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...
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The use of the "equals" (==) operator in the latter example makes it much more
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obvious that you are testing for equivalence. The former example with the
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"not" (!) operator makes it look like you are testing for an error. In a more
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perfect world, we would have a streq() function in the string library, but
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that ain't the world we're living in.
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Avoid Dangerous String Functions
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Unfortunately, the way C handles strings makes them prone to overruns when
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certain library functions are (mis)used. The following table offers a summary
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of some of the more notorious troublemakers:
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function overflows preferred
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-------------------------------------------------
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strcpy dest string safe_strncpy
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strncpy may fail to 0-terminate dst safe_strncpy
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strcat dest string strncat
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gets string it gets fgets
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getwd buf string getcwd
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[v]sprintf str buffer [v]snprintf
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realpath path buffer use with pathconf
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[vf]scanf its arguments just avoid it
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The above is by no means a complete list. Be careful out there.
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Avoid Big Static Buffers
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------------------------
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First, some background to put this discussion in context: static buffers look
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like this in code:
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/* in a .c file outside any functions */
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static char buffer[BUFSIZ]; /* happily used by any function in this file,
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but ick! big! */
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The problem with these is that any time any busybox app is run, you pay a
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memory penalty for this buffer, even if the applet that uses said buffer is
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not run. This can be fixed, thusly:
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static char *buffer;
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...
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other_func()
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{
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strcpy(buffer, lotsa_chars); /* happily uses global *buffer */
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...
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foo_main()
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{
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buffer = xmalloc(sizeof(char)*BUFSIZ);
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...
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However, this approach trades bss segment for text segment. Rather than
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mallocing the buffers (and thus growing the text size), buffers can be
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declared on the stack in the *_main() function and made available globally by
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assigning them to a global pointer thusly:
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static char *pbuffer;
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...
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other_func()
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{
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strcpy(pbuffer, lotsa_chars); /* happily uses global *pbuffer */
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...
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foo_main()
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{
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char *buffer[BUFSIZ]; /* declared locally, on stack */
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pbuffer = buffer; /* but available globally */
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...
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This last approach has some advantages (low code size, space not used until
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it's needed), but can be a problem in some low resource machines that have
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very limited stack space (e.g., uCLinux).
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A macro is declared in busybox.h that implements compile-time selection
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between xmalloc() and stack creation, so you can code the line in question as
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RESERVE_CONFIG_BUFFER(buffer, BUFSIZ);
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and the right thing will happen, based on your configuration.
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Another relatively new trick of similar nature is explained
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in keep_data_small.txt.
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Miscellaneous Coding Guidelines
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-------------------------------
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The following are important items that don't fit into any of the above
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sections.
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Model Busybox Applets After GNU Counterparts
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When in doubt about the proper behavior of a Busybox program (output,
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formatting, options, etc.), model it after the equivalent GNU program.
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Doesn't matter how that program behaves on some other flavor of *NIX; doesn't
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matter what the POSIX standard says or doesn't say, just model Busybox
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programs after their GNU counterparts and it will make life easier on (nearly)
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everyone.
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The only time we deviate from emulating the GNU behavior is when:
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- We are deliberately not supporting a feature (such as a command line
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switch)
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- Emulating the GNU behavior is prohibitively expensive (lots more code
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would be required, lots more memory would be used, etc.)
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- The difference is minor or cosmetic
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A note on the 'cosmetic' case: output differences might be considered
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cosmetic, but if the output is significant enough to break other scripts that
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use the output, it should really be fixed.
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Scope
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~~~~~
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If a const variable is used only in a single source file, put it in the source
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file and not in a header file. Likewise, if a const variable is used in only
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one function, do not make it global to the file. Instead, declare it inside
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the function body. Bottom line: Make a conscious effort to limit declarations
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to the smallest scope possible.
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Inside applet files, all functions should be declared static so as to keep the
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global name space clean. The only exception to this rule is the "applet_main"
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function which must be declared extern.
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If you write a function that performs a task that could be useful outside the
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immediate file, turn it into a general-purpose function with no ties to any
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applet and put it in the utility.c file instead.
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Brackets Are Your Friends
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~~~~~~~~~~~~~~~~~~~~~~~~~
|
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Please use brackets on all if and else statements, even if it is only one
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line. Example:
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Don't do this:
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|
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if (foo)
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|
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 amount 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 = xmalloc_fgets(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 getopt32() 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, if you need to parse long options:
|
|
|
|
#include <getopt.h>
|
|
|
|
Then have long options defined:
|
|
|
|
static const struct option <applet>_long_options[] = {
|
|
{ "list", 0, NULL, 't' },
|
|
{ "extract", 0, NULL, 'x' },
|
|
{ NULL, 0, NULL, 0 }
|
|
};
|
|
|
|
And a code block similar to the following near the top of your applet_main()
|
|
routine:
|
|
|
|
char *str_b;
|
|
|
|
opt_complementary = "cryptic_string";
|
|
applet_long_options = <applet>_long_options; /* if you have them */
|
|
opt = getopt32(argc, argv, "ab:c", &str_b);
|
|
if (opt & 1) {
|
|
handle_option_a();
|
|
}
|
|
if (opt & 2) {
|
|
handle_option_b(str_b);
|
|
}
|
|
if (opt & 4) {
|
|
handle_option_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.
|
|
|
|
For more info and examples, examine getopt32.c, tar.c, wget.c etc.
|