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 lost on this issue. QEMU indents are four spaces. Tabs are never used, except in Makefiles where they have been irreversibly coded into the syntax. Spaces of course are superior to tabs because: - You have just one way to specify whitespace, not two. Ambiguity breeds mistakes. - The confusion surrounding 'use tabs to indent, spaces to justify' is gone. - Tab indents push your code to the right, making your screen seriously unbalanced. - Tabs will be rendered incorrectly on editors who are misconfigured not to use tab stops of eight positions. - Tabs are rendered badly in patches, causing off-by-one errors in almost every line. - It is the QEMU coding style. Do not leave whitespace dangling off the ends of lines. 2. Line width Lines are 80 characters; not longer. Rationale: - Some people like to tile their 24" screens with a 6x4 matrix of 80x24 xterms and use vi in all of them. The best way to punish them is to let them keep doing it. - Code and especially patches is much more readable if limited to a sane line length. Eighty is traditional. - 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. 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 Every indented statement is braced; even if the block contains just one statement. The opening brace is on the line that contains the control flow statement that introduces the new block; the closing brace is on the same line as the else keyword, or on a line by itself if there is no else keyword. Example: if (a == 5) { printf("a was 5.\n"); } else if (a == 6) { printf("a was 6.\n"); } else { 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: void a_function(void) { do_something(); } 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.