-Qemu Coding Style
+QEMU Coding Style
=================
Please use the script checkpatch.pl in the scripts directory to check
Of course, the most important aspect in any coding style is whitespace.
Crusty old coders who have trouble spotting the glasses on their noses
can tell the difference between a tab and eight spaces from a distance
-of approximately fifteen parsecs. Many a flamewar have been fought and
+of approximately fifteen parsecs. Many a flamewar has been fought and
lost on this issue.
QEMU indents are four spaces. Tabs are never used, except in Makefiles
Do not leave whitespace dangling off the ends of lines.
+1.1 Multiline Indent
+
+There are several places where indent is necessary:
+
+ - if/else
+ - while/for
+ - function definition & call
+
+When breaking up a long line to fit within line width, we need a proper indent
+for the following lines.
+
+In case of if/else, while/for, align the secondary lines just after the
+opening parenthesis of the first.
+
+For example:
+
+ if (a == 1 &&
+ b == 2) {
+
+ while (a == 1 &&
+ b == 2) {
+
+In case of function, there are several variants:
+
+ * 4 spaces indent from the beginning
+ * align the secondary lines just after the opening parenthesis of the
+ first
+
+For example:
+
+ do_something(x, y,
+ z);
+
+ do_something(x, y,
+ z);
+
+ do_something(x, do_another(y,
+ z));
+
2. Line width
-Lines are 80 characters; not longer.
+Lines should be 80 characters; try not to make them longer.
+
+Sometimes it is hard to do, especially when dealing with QEMU subsystems
+that use long function or symbol names. Even in that case, do not make
+lines much longer than 80 characters.
Rationale:
- Some people like to tile their 24" screens with a 6x4 matrix of 80x24
let them keep doing it.
- Code and especially patches is much more readable if limited to a sane
line length. Eighty is traditional.
+ - The four-space indentation makes the most common excuse ("But look
+ at all that white space on the left!") moot.
- It is the QEMU coding style.
3. Naming
Variables are lower_case_with_underscores; easy to type and read. Structured
-type names are in CamelCase; harder to type but standing out. Scalar type
+type names are in CamelCase; harder to type but standing out. Enum type
+names and function type names should also be in CamelCase. Scalar type
names are lower_case_with_underscores_ending_with_a_t, like the POSIX
uint64_t and family. Note that this last convention contradicts POSIX
and is therefore likely to be changed.
printf("a was something else entirely.\n");
}
+Note that 'else if' is considered a single statement; otherwise a long if/
+else if/else if/.../else sequence would need an indent for every else
+statement.
+
An exception is the opening brace for a function; for reasons of tradition
and clarity it comes on a line by itself:
Rationale: a consistent (except for functions...) bracing style reduces
ambiguity and avoids needless churn when lines are added or removed.
Furthermore, it is the QEMU coding style.
+
+5. Declarations
+
+Mixed declarations (interleaving statements and declarations within
+blocks) are generally not allowed; declarations should be at the beginning
+of blocks.
+
+Every now and then, an exception is made for declarations inside a
+#ifdef or #ifndef block: if the code looks nicer, such declarations can
+be placed at the top of the block even if there are statements above.
+On the other hand, however, it's often best to move that #ifdef/#ifndef
+block to a separate function altogether.
+
+6. Conditional statements
+
+When comparing a variable for (in)equality with a constant, list the
+constant on the right, as in:
+
+ if (a == 1) {
+ /* Reads like: "If a equals 1" */
+ do_something();
+ }
+
+Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read.
+Besides, good compilers already warn users when '==' is mis-typed as '=',
+even when the constant is on the right.
+
+7. Comment style
+
+We use traditional C-style /* */ comments and avoid // comments.
+
+Rationale: The // form is valid in C99, so this is purely a matter of
+consistency of style. The checkpatch script will warn you about this.
+
+Multiline comment blocks should have a row of stars on the left,
+and the initial /* and terminating */ both on their own lines:
+ /*
+ * like
+ * this
+ */
+This is the same format required by the Linux kernel coding style.
+
+(Some of the existing comments in the codebase use the GNU Coding
+Standards form which does not have stars on the left, or other
+variations; avoid these when writing new comments, but don't worry
+about converting to the preferred form unless you're editing that
+comment anyway.)
+
+Rationale: Consistency, and ease of visually picking out a multiline
+comment from the surrounding code.
+
+8. trace-events style
+
+8.1 0x prefix
+
+In trace-events files, use a '0x' prefix to specify hex numbers, as in:
+
+some_trace(unsigned x, uint64_t y) "x 0x%x y 0x" PRIx64
+
+An exception is made for groups of numbers that are hexadecimal by
+convention and separated by the symbols '.', '/', ':', or ' ' (such as
+PCI bus id):
+
+another_trace(int cssid, int ssid, int dev_num) "bus id: %x.%x.%04x"
+
+However, you can use '0x' for such groups if you want. Anyway, be sure that
+it is obvious that numbers are in hex, ex.:
+
+data_dump(uint8_t c1, uint8_t c2, uint8_t c3) "bytes (in hex): %02x %02x %02x"
+
+Rationale: hex numbers are hard to read in logs when there is no 0x prefix,
+especially when (occasionally) the representation doesn't contain any letters
+and especially in one line with other decimal numbers. Number groups are allowed
+to not use '0x' because for some things notations like %x.%x.%x are used not
+only in Qemu. Also dumping raw data bytes with '0x' is less readable.
+
+8.2 '#' printf flag
+
+Do not use printf flag '#', like '%#x'.
+
+Rationale: there are two ways to add a '0x' prefix to printed number: '0x%...'
+and '%#...'. For consistency the only one way should be used. Arguments for
+'0x%' are:
+ - it is more popular
+ - '%#' omits the 0x for the value 0 which makes output inconsistent
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