-Qemu Coding Style
+QEMU Coding Style
=================
+Please use the script checkpatch.pl in the scripts directory to check
+patches before submitting.
+
1. Whitespace
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
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 names are a_lower_case_beginning_with_an a or an.
-Do not use _t suffix if you are including any headers.
+Variables are lower_case_with_underscores; easy to type and read. Structured
+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.
+
+When wrapping standard library functions, use the prefix qemu_ to alert
+readers that they are seeing a wrapped version; otherwise avoid this prefix.
4. Block structure
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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