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[qemu.git] / CODING_STYLE

Qemu Coding Style
=================

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.  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.

Typedefs are used to eliminate the redundant 'struct' keyword.  It is the
QEMU coding style.

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");
    }

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.