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