1QEMU Coding Style
   4Please use the script checkpatch.pl in the scripts directory to check
   5patches before submitting.
   71. Whitespace
   9Of course, the most important aspect in any coding style is whitespace.
  10Crusty old coders who have trouble spotting the glasses on their noses
  11can tell the difference between a tab and eight spaces from a distance
  12of approximately fifteen parsecs.  Many a flamewar have been fought and
  13lost on this issue.
  15QEMU indents are four spaces.  Tabs are never used, except in Makefiles
  16where they have been irreversibly coded into the syntax.
  17Spaces of course are superior to tabs because:
  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.
  30Do not leave whitespace dangling off the ends of lines.
  322. Line width
  34Lines are 80 characters; not longer.
  37 - Some people like to tile their 24" screens with a 6x4 matrix of 80x24
  38   xterms and use vi in all of them.  The best way to punish them is to
  39   let them keep doing it.
  40 - Code and especially patches is much more readable if limited to a sane
  41   line length.  Eighty is traditional.
  42 - It is the QEMU coding style.
  443. Naming
  46Variables are lower_case_with_underscores; easy to type and read.  Structured
  47type names are in CamelCase; harder to type but standing out.  Enum type
  48names and function type names should also be in CamelCase.  Scalar type
  49names are lower_case_with_underscores_ending_with_a_t, like the POSIX
  50uint64_t and family.  Note that this last convention contradicts POSIX
  51and is therefore likely to be changed.
  53When wrapping standard library functions, use the prefix qemu_ to alert
  54readers that they are seeing a wrapped version; otherwise avoid this prefix.
  564. Block structure
  58Every indented statement is braced; even if the block contains just one
  59statement.  The opening brace is on the line that contains the control
  60flow statement that introduces the new block; the closing brace is on the
  61same line as the else keyword, or on a line by itself if there is no else
  62keyword.  Example:
  64    if (a == 5) {
  65        printf("a was 5.\n");
  66    } else if (a == 6) {
  67        printf("a was 6.\n");
  68    } else {
  69        printf("a was something else entirely.\n");
  70    }
  72Note that 'else if' is considered a single statement; otherwise a long if/
  73else if/else if/.../else sequence would need an indent for every else
  76An exception is the opening brace for a function; for reasons of tradition
  77and clarity it comes on a line by itself:
  79    void a_function(void)
  80    {
  81        do_something();
  82    }
  84Rationale: a consistent (except for functions...) bracing style reduces
  85ambiguity and avoids needless churn when lines are added or removed.
  86Furthermore, it is the QEMU coding style.
  885. Declarations
  90Mixed declarations (interleaving statements and declarations within
  91blocks) are generally not allowed; declarations should be at the beginning
  92of blocks.
  94Every now and then, an exception is made for declarations inside a
  95#ifdef or #ifndef block: if the code looks nicer, such declarations can
  96be placed at the top of the block even if there are statements above.
  97On the other hand, however, it's often best to move that #ifdef/#ifndef
  98block to a separate function altogether.
 1006. Conditional statements
 102When comparing a variable for (in)equality with a constant, list the
 103constant on the right, as in:
 105if (a == 1) {
 106    /* Reads like: "If a equals 1" */
 107    do_something();
 110Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read.
 111Besides, good compilers already warn users when '==' is mis-typed as '=',
 112even when the constant is on the right.