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 has 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 should be 80 characters; try not to make them longer.
  36Sometimes it is hard to do, especially when dealing with QEMU subsystems
  37that use long function or symbol names.  Even in that case, do not make
  38lines much longer than 80 characters.
  41 - Some people like to tile their 24" screens with a 6x4 matrix of 80x24
  42   xterms and use vi in all of them.  The best way to punish them is to
  43   let them keep doing it.
  44 - Code and especially patches is much more readable if limited to a sane
  45   line length.  Eighty is traditional.
  46 - The four-space indentation makes the most common excuse ("But look
  47   at all that white space on the left!") moot.
  48 - It is the QEMU coding style.
  503. Naming
  52Variables are lower_case_with_underscores; easy to type and read.  Structured
  53type names are in CamelCase; harder to type but standing out.  Enum type
  54names and function type names should also be in CamelCase.  Scalar type
  55names are lower_case_with_underscores_ending_with_a_t, like the POSIX
  56uint64_t and family.  Note that this last convention contradicts POSIX
  57and is therefore likely to be changed.
  59When wrapping standard library functions, use the prefix qemu_ to alert
  60readers that they are seeing a wrapped version; otherwise avoid this prefix.
  624. Block structure
  64Every indented statement is braced; even if the block contains just one
  65statement.  The opening brace is on the line that contains the control
  66flow statement that introduces the new block; the closing brace is on the
  67same line as the else keyword, or on a line by itself if there is no else
  68keyword.  Example:
  70    if (a == 5) {
  71        printf("a was 5.\n");
  72    } else if (a == 6) {
  73        printf("a was 6.\n");
  74    } else {
  75        printf("a was something else entirely.\n");
  76    }
  78Note that 'else if' is considered a single statement; otherwise a long if/
  79else if/else if/.../else sequence would need an indent for every else
  82An exception is the opening brace for a function; for reasons of tradition
  83and clarity it comes on a line by itself:
  85    void a_function(void)
  86    {
  87        do_something();
  88    }
  90Rationale: a consistent (except for functions...) bracing style reduces
  91ambiguity and avoids needless churn when lines are added or removed.
  92Furthermore, it is the QEMU coding style.
  945. Declarations
  96Mixed declarations (interleaving statements and declarations within
  97blocks) are generally not allowed; declarations should be at the beginning
  98of blocks.
 100Every now and then, an exception is made for declarations inside a
 101#ifdef or #ifndef block: if the code looks nicer, such declarations can
 102be placed at the top of the block even if there are statements above.
 103On the other hand, however, it's often best to move that #ifdef/#ifndef
 104block to a separate function altogether.
 1066. Conditional statements
 108When comparing a variable for (in)equality with a constant, list the
 109constant on the right, as in:
 111if (a == 1) {
 112    /* Reads like: "If a equals 1" */
 113    do_something();
 116Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read.
 117Besides, good compilers already warn users when '==' is mis-typed as '=',
 118even when the constant is on the right.
 1207. Comment style
 122We use traditional C-style /* */ comments and avoid // comments.
 124Rationale: The // form is valid in C99, so this is purely a matter of
 125consistency of style. The checkpatch script will warn you about this.
 127Multiline comment blocks should have a row of stars on the left,
 128and the initial /* and terminating */ both on their own lines:
 129    /*
 130     * like
 131     * this
 132     */
 133This is the same format required by the Linux kernel coding style.
 135(Some of the existing comments in the codebase use the GNU Coding
 136Standards form which does not have stars on the left, or other
 137variations; avoid these when writing new comments, but don't worry
 138about converting to the preferred form unless you're editing that
 139comment anyway.)
 141Rationale: Consistency, and ease of visually picking out a multiline
 142comment from the surrounding code.
 1448. trace-events style
 1468.1 0x prefix
 148In trace-events files, use a '0x' prefix to specify hex numbers, as in:
 150some_trace(unsigned x, uint64_t y) "x 0x%x y 0x" PRIx64
 152An exception is made for groups of numbers that are hexadecimal by
 153convention and separated by the symbols '.', '/', ':', or ' ' (such as
 154PCI bus id):
 156another_trace(int cssid, int ssid, int dev_num) "bus id: %x.%x.%04x"
 158However, you can use '0x' for such groups if you want. Anyway, be sure that
 159it is obvious that numbers are in hex, ex.:
 161data_dump(uint8_t c1, uint8_t c2, uint8_t c3) "bytes (in hex): %02x %02x %02x"
 163Rationale: hex numbers are hard to read in logs when there is no 0x prefix,
 164especially when (occasionally) the representation doesn't contain any letters
 165and especially in one line with other decimal numbers. Number groups are allowed
 166to not use '0x' because for some things notations like %x.%x.%x are used not
 167only in Qemu. Also dumping raw data bytes with '0x' is less readable.
 1698.2 '#' printf flag
 171Do not use printf flag '#', like '%#x'.
 173Rationale: there are two ways to add a '0x' prefix to printed number: '0x%...'
 174and '%#...'. For consistency the only one way should be used. Arguments for
 175'0x%' are:
 176 - it is more popular
 177 - '%#' omits the 0x for the value 0 which makes output inconsistent