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Stefan Weil6576b742012-04-07 09:23:37 +02001QEMU Coding Style
aliguorie68b98d2009-04-05 17:40:34 +00002=================
3
Blue Swirlb6469682011-01-20 20:58:56 +00004Please use the script checkpatch.pl in the scripts directory to check
5patches before submitting.
6
aliguorie68b98d2009-04-05 17:40:34 +000071. Whitespace
8
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.
14
15QEMU indents are four spaces. Tabs are never used, except in Makefiles
edgar_igl1cb499f2009-04-07 02:10:16 +000016where they have been irreversibly coded into the syntax.
aliguorie68b98d2009-04-05 17:40:34 +000017Spaces 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
30Do not leave whitespace dangling off the ends of lines.
31
322. Line width
33
34Lines are 80 characters; not longer.
35
36Rationale:
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.
43
443. Naming
45
Anthony Liguoric227f092009-10-01 16:12:16 -050046Variables are lower_case_with_underscores; easy to type and read. Structured
Peter Maydelle3c52bf2012-01-13 20:29:49 +000047type names are in CamelCase; harder to type but standing out. Enum type
48names and function type names should also be in CamelCase. Scalar type
Anthony Liguoric227f092009-10-01 16:12:16 -050049names 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.
52
Avi Kivity77ac4862010-03-11 16:48:43 +020053When wrapping standard library functions, use the prefix qemu_ to alert
54readers that they are seeing a wrapped version; otherwise avoid this prefix.
55
aliguorie68b98d2009-04-05 17:40:34 +0000564. Block structure
57
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:
63
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 }
71
Avi Kivity5f070c52011-07-25 18:55:53 +030072Note 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
74statement.
75
aliguorie68b98d2009-04-05 17:40:34 +000076An exception is the opening brace for a function; for reasons of tradition
77and clarity it comes on a line by itself:
78
79 void a_function(void)
80 {
81 do_something();
82 }
83
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.
Eduardo Habkoste939c6e2014-03-17 15:26:31 -030087
885. Declarations
89
Paolo Bonzini690a35e2015-06-19 09:28:13 +020090Mixed declarations (interleaving statements and declarations within
91blocks) are generally not allowed; declarations should be at the beginning
92of blocks.
93
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.
Gonglei2bb00202014-08-11 21:00:51 +080099
1006. Conditional statements
101
102When comparing a variable for (in)equality with a constant, list the
103constant on the right, as in:
104
105if (a == 1) {
106 /* Reads like: "If a equals 1" */
107 do_something();
108}
109
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.