CODING_STYLE

   1 QEMU Coding Style
   2 =================
   3
   4 Please use the script checkpatch.pl in the scripts directory to check
   5 patches before submitting.
   6
   7 1. Whitespace
   8
   9 Of course, the most important aspect in any coding style is whitespace.
  10 Crusty old coders who have trouble spotting the glasses on their noses
  11 can tell the difference between a tab and eight spaces from a distance
  12 of approximately fifteen parsecs.  Many a flamewar has been fought and
  13 lost on this issue.
  14
  15 QEMU indents are four spaces.  Tabs are never used, except in Makefiles
  16 where they have been irreversibly coded into the syntax.
  17 Spaces of course are superior to tabs because:
  18
  19  - You have just one way to specify whitespace, not two.  Ambiguity breeds
  20    mistakes.
  21  - The confusion surrounding 'use tabs to indent, spaces to justify' is gone.
  22  - Tab indents push your code to the right, making your screen seriously
  23    unbalanced.
  24  - Tabs will be rendered incorrectly on editors who are misconfigured not
  25    to use tab stops of eight positions.
  26  - Tabs are rendered badly in patches, causing off-by-one errors in almost
  27    every line.
  28  - It is the QEMU coding style.
  29
  30 Do not leave whitespace dangling off the ends of lines.
  31
  32 1.1 Multiline Indent
  33
  34 There are several places where indent is necessary:
  35
  36  - if/else
  37  - while/for
  38  - function definition & call
  39
  40 When breaking up a long line to fit within line width, we need a proper indent
  41 for the following lines.
  42
  43 In case of if/else, while/for, align the secondary lines just after the
  44 opening parenthesis of the first.
  45
  46 For example:
  47
  48     if (a == 1 &&
  49         b == 2) {
  50
  51     while (a == 1 &&
  52            b == 2) {
  53
  54 In case of function, there are several variants:
  55
  56     * 4 spaces indent from the beginning
  57     * align the secondary lines just after the opening parenthesis of the
  58       first
  59
  60 For example:
  61
  62     do_something(x, y,
  63         z);
  64
  65     do_something(x, y,
  66                  z);
  67
  68     do_something(x, do_another(y,
  69                                z));
  70
  71 2. Line width
  72
  73 Lines should be 80 characters; try not to make them longer.
  74
  75 Sometimes it is hard to do, especially when dealing with QEMU subsystems
  76 that use long function or symbol names.  Even in that case, do not make
  77 lines much longer than 80 characters.
  78
  79 Rationale:
  80  - Some people like to tile their 24" screens with a 6x4 matrix of 80x24
  81    xterms and use vi in all of them.  The best way to punish them is to
  82    let them keep doing it.
  83  - Code and especially patches is much more readable if limited to a sane
  84    line length.  Eighty is traditional.
  85  - The four-space indentation makes the most common excuse ("But look
  86    at all that white space on the left!") moot.
  87  - It is the QEMU coding style.
  88
  89 3. Naming
  90
  91 Variables are lower_case_with_underscores; easy to type and read.  Structured
  92 type names are in CamelCase; harder to type but standing out.  Enum type
  93 names and function type names should also be in CamelCase.  Scalar type
  94 names are lower_case_with_underscores_ending_with_a_t, like the POSIX
  95 uint64_t and family.  Note that this last convention contradicts POSIX
  96 and is therefore likely to be changed.
  97
  98 When wrapping standard library functions, use the prefix qemu_ to alert
  99 readers that they are seeing a wrapped version; otherwise avoid this prefix.
 100
 101 4. Block structure
 102
 103 Every indented statement is braced; even if the block contains just one
 104 statement.  The opening brace is on the line that contains the control
 105 flow statement that introduces the new block; the closing brace is on the
 106 same line as the else keyword, or on a line by itself if there is no else
 107 keyword.  Example:
 108
 109     if (a == 5) {
 110         printf("a was 5.\n");
 111     } else if (a == 6) {
 112         printf("a was 6.\n");
 113     } else {
 114         printf("a was something else entirely.\n");
 115     }
 116
 117 Note that 'else if' is considered a single statement; otherwise a long if/
 118 else if/else if/.../else sequence would need an indent for every else
 119 statement.
 120
 121 An exception is the opening brace for a function; for reasons of tradition
 122 and clarity it comes on a line by itself:
 123
 124     void a_function(void)
 125     {
 126         do_something();
 127     }
 128
 129 Rationale: a consistent (except for functions...) bracing style reduces
 130 ambiguity and avoids needless churn when lines are added or removed.
 131 Furthermore, it is the QEMU coding style.
 132
 133 5. Declarations
 134
 135 Mixed declarations (interleaving statements and declarations within
 136 blocks) are generally not allowed; declarations should be at the beginning
 137 of blocks.
 138
 139 Every now and then, an exception is made for declarations inside a
 140 #ifdef or #ifndef block: if the code looks nicer, such declarations can
 141 be placed at the top of the block even if there are statements above.
 142 On the other hand, however, it's often best to move that #ifdef/#ifndef
 143 block to a separate function altogether.
 144
 145 6. Conditional statements
 146
 147 When comparing a variable for (in)equality with a constant, list the
 148 constant on the right, as in:
 149
 150     if (a == 1) {
 151         /* Reads like: "If a equals 1" */
 152         do_something();
 153     }
 154
 155 Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read.
 156 Besides, good compilers already warn users when '==' is mis-typed as '=',
 157 even when the constant is on the right.
 158
 159 7. Comment style
 160
 161 We use traditional C-style /* */ comments and avoid // comments.
 162
 163 Rationale: The // form is valid in C99, so this is purely a matter of
 164 consistency of style. The checkpatch script will warn you about this.
 165
 166 Multiline comment blocks should have a row of stars on the left,
 167 and the initial /* and terminating */ both on their own lines:
 168     /*
 169      * like
 170      * this
 171      */
 172 This is the same format required by the Linux kernel coding style.
 173
 174 (Some of the existing comments in the codebase use the GNU Coding
 175 Standards form which does not have stars on the left, or other
 176 variations; avoid these when writing new comments, but don't worry
 177 about converting to the preferred form unless you're editing that
 178 comment anyway.)
 179
 180 Rationale: Consistency, and ease of visually picking out a multiline
 181 comment from the surrounding code.
 182
 183 8. trace-events style
 184
 185 8.1 0x prefix
 186
 187 In trace-events files, use a '0x' prefix to specify hex numbers, as in:
 188
 189 some_trace(unsigned x, uint64_t y) "x 0x%x y 0x" PRIx64
 190
 191 An exception is made for groups of numbers that are hexadecimal by
 192 convention and separated by the symbols '.', '/', ':', or ' ' (such as
 193 PCI bus id):
 194
 195 another_trace(int cssid, int ssid, int dev_num) "bus id: %x.%x.%04x"
 196
 197 However, you can use '0x' for such groups if you want. Anyway, be sure that
 198 it is obvious that numbers are in hex, ex.:
 199
 200 data_dump(uint8_t c1, uint8_t c2, uint8_t c3) "bytes (in hex): %02x %02x %02x"
 201
 202 Rationale: hex numbers are hard to read in logs when there is no 0x prefix,
 203 especially when (occasionally) the representation doesn't contain any letters
 204 and especially in one line with other decimal numbers. Number groups are allowed
 205 to not use '0x' because for some things notations like %x.%x.%x are used not
 206 only in Qemu. Also dumping raw data bytes with '0x' is less readable.
 207
 208 8.2 '#' printf flag
 209
 210 Do not use printf flag '#', like '%#x'.
 211
 212 Rationale: there are two ways to add a '0x' prefix to printed number: '0x%...'
 213 and '%#...'. For consistency the only one way should be used. Arguments for
 214 '0x%' are:
 215  - it is more popular
 216  - '%#' omits the 0x for the value 0 which makes output inconsistent