1QEMU Coding Style 2================= 3 4Please use the script checkpatch.pl in the scripts directory to check 5patches before submitting. 6 71. 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 has been fought and 13lost on this issue. 14 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: 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. 31 321.1 Multiline Indent 33 34There are several places where indent is necessary: 35 36 - if/else 37 - while/for 38 - function definition & call 39 40When breaking up a long line to fit within line width, we need a proper indent 41for the following lines. 42 43In case of if/else, while/for, align the secondary lines just after the 44opening parenthesis of the first. 45 46For example: 47 48 if (a == 1 && 49 b == 2) { 50 51 while (a == 1 && 52 b == 2) { 53 54In case of function, there are several variants: 55 56 * 4 spaces indent from the beginning 57 * align the secondary lines just after the opening parenthesis of the 58 first 59 60For example: 61 62 do_something(x, y, 63 z); 64 65 do_something(x, y, 66 z); 67 68 do_something(x, do_another(y, 69 z)); 70 712. Line width 72 73Lines should be 80 characters; try not to make them longer. 74 75Sometimes it is hard to do, especially when dealing with QEMU subsystems 76that use long function or symbol names. Even in that case, do not make 77lines much longer than 80 characters. 78 79Rationale: 80 - Some people like to tile their 24" screens with a 6x4 matrix of 80x24 81 xterms and use vi in all of them. The best way to punish them is to 82 let them keep doing it. 83 - Code and especially patches is much more readable if limited to a sane 84 line length. Eighty is traditional. 85 - The four-space indentation makes the most common excuse ("But look 86 at all that white space on the left!") moot. 87 - It is the QEMU coding style. 88 893. Naming 90 91Variables are lower_case_with_underscores; easy to type and read. Structured 92type names are in CamelCase; harder to type but standing out. Enum type 93names and function type names should also be in CamelCase. Scalar type 94names are lower_case_with_underscores_ending_with_a_t, like the POSIX 95uint64_t and family. Note that this last convention contradicts POSIX 96and is therefore likely to be changed. 97 98When wrapping standard library functions, use the prefix qemu_ to alert 99readers that they are seeing a wrapped version; otherwise avoid this prefix. 100 1014. Block structure 102 103Every indented statement is braced; even if the block contains just one 104statement. The opening brace is on the line that contains the control 105flow statement that introduces the new block; the closing brace is on the 106same line as the else keyword, or on a line by itself if there is no else 107keyword. Example: 108 109 if (a == 5) { 110 printf("a was 5.\n"); 111 } else if (a == 6) { 112 printf("a was 6.\n"); 113 } else { 114 printf("a was something else entirely.\n"); 115 } 116 117Note that 'else if' is considered a single statement; otherwise a long if/ 118else if/else if/.../else sequence would need an indent for every else 119statement. 120 121An exception is the opening brace for a function; for reasons of tradition 122and clarity it comes on a line by itself: 123 124 void a_function(void) 125 { 126 do_something(); 127 } 128 129Rationale: a consistent (except for functions...) bracing style reduces 130ambiguity and avoids needless churn when lines are added or removed. 131Furthermore, it is the QEMU coding style. 132 1335. Declarations 134 135Mixed declarations (interleaving statements and declarations within 136blocks) are generally not allowed; declarations should be at the beginning 137of blocks. 138 139Every now and then, an exception is made for declarations inside a 140#ifdef or #ifndef block: if the code looks nicer, such declarations can 141be placed at the top of the block even if there are statements above. 142On the other hand, however, it's often best to move that #ifdef/#ifndef 143block to a separate function altogether. 144 1456. Conditional statements 146 147When comparing a variable for (in)equality with a constant, list the 148constant on the right, as in: 149 150 if (a == 1) { 151 /* Reads like: "If a equals 1" */ 152 do_something(); 153 } 154 155Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read. 156Besides, good compilers already warn users when '==' is mis-typed as '=', 157even when the constant is on the right. 158 1597. Comment style 160 161We use traditional C-style /* */ comments and avoid // comments. 162 163Rationale: The // form is valid in C99, so this is purely a matter of 164consistency of style. The checkpatch script will warn you about this. 165 166Multiline comment blocks should have a row of stars on the left, 167and the initial /* and terminating */ both on their own lines: 168 /* 169 * like 170 * this 171 */ 172This is the same format required by the Linux kernel coding style. 173 174(Some of the existing comments in the codebase use the GNU Coding 175Standards form which does not have stars on the left, or other 176variations; avoid these when writing new comments, but don't worry 177about converting to the preferred form unless you're editing that 178comment anyway.) 179 180Rationale: Consistency, and ease of visually picking out a multiline 181comment from the surrounding code. 182 1838. trace-events style 184 1858.1 0x prefix 186 187In trace-events files, use a '0x' prefix to specify hex numbers, as in: 188 189some_trace(unsigned x, uint64_t y) "x 0x%x y 0x" PRIx64 190 191An exception is made for groups of numbers that are hexadecimal by 192convention and separated by the symbols '.', '/', ':', or ' ' (such as 193PCI bus id): 194 195another_trace(int cssid, int ssid, int dev_num) "bus id: %x.%x.%04x" 196 197However, you can use '0x' for such groups if you want. Anyway, be sure that 198it is obvious that numbers are in hex, ex.: 199 200data_dump(uint8_t c1, uint8_t c2, uint8_t c3) "bytes (in hex): %02x %02x %02x" 201 202Rationale: hex numbers are hard to read in logs when there is no 0x prefix, 203especially when (occasionally) the representation doesn't contain any letters 204and especially in one line with other decimal numbers. Number groups are allowed 205to not use '0x' because for some things notations like %x.%x.%x are used not 206only in Qemu. Also dumping raw data bytes with '0x' is less readable. 207 2088.2 '#' printf flag 209 210Do not use printf flag '#', like '%#x'. 211 212Rationale: there are two ways to add a '0x' prefix to printed number: '0x%...' 213and '%#...'. For consistency the only one way should be used. Arguments for 214'0x%' are: 215 - it is more popular 216 - '%#' omits the 0x for the value 0 which makes output inconsistent 217