641 lines
21 KiB
ReStructuredText
641 lines
21 KiB
ReStructuredText
=================
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QEMU Coding Style
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=================
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.. contents:: Table of Contents
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Please use the script checkpatch.pl in the scripts directory to check
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patches before submitting.
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Formatting and style
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********************
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Whitespace
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==========
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Of course, the most important aspect in any coding style is whitespace.
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Crusty old coders who have trouble spotting the glasses on their noses
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can tell the difference between a tab and eight spaces from a distance
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of approximately fifteen parsecs. Many a flamewar has been fought and
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lost on this issue.
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QEMU indents are four spaces. Tabs are never used, except in Makefiles
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where they have been irreversibly coded into the syntax.
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Spaces of course are superior to tabs because:
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* You have just one way to specify whitespace, not two. Ambiguity breeds
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mistakes.
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* The confusion surrounding 'use tabs to indent, spaces to justify' is gone.
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* Tab indents push your code to the right, making your screen seriously
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unbalanced.
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* Tabs will be rendered incorrectly on editors who are misconfigured not
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to use tab stops of eight positions.
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* Tabs are rendered badly in patches, causing off-by-one errors in almost
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every line.
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* It is the QEMU coding style.
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Do not leave whitespace dangling off the ends of lines.
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Multiline Indent
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----------------
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There are several places where indent is necessary:
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* if/else
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* while/for
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* function definition & call
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When breaking up a long line to fit within line width, we need a proper indent
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for the following lines.
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In case of if/else, while/for, align the secondary lines just after the
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opening parenthesis of the first.
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For example:
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.. code-block:: c
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if (a == 1 &&
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b == 2) {
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while (a == 1 &&
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b == 2) {
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In case of function, there are several variants:
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* 4 spaces indent from the beginning
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* align the secondary lines just after the opening parenthesis of the first
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For example:
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.. code-block:: c
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do_something(x, y,
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z);
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do_something(x, y,
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z);
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do_something(x, do_another(y,
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z));
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Line width
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==========
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Lines should be 80 characters; try not to make them longer.
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Sometimes it is hard to do, especially when dealing with QEMU subsystems
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that use long function or symbol names. Even in that case, do not make
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lines much longer than 80 characters.
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Rationale:
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* Some people like to tile their 24" screens with a 6x4 matrix of 80x24
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xterms and use vi in all of them. The best way to punish them is to
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let them keep doing it.
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* Code and especially patches is much more readable if limited to a sane
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line length. Eighty is traditional.
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* The four-space indentation makes the most common excuse ("But look
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at all that white space on the left!") moot.
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* It is the QEMU coding style.
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Naming
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======
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Variables are lower_case_with_underscores; easy to type and read. Structured
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type names are in CamelCase; harder to type but standing out. Enum type
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names and function type names should also be in CamelCase. Scalar type
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names are lower_case_with_underscores_ending_with_a_t, like the POSIX
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uint64_t and family. Note that this last convention contradicts POSIX
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and is therefore likely to be changed.
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When wrapping standard library functions, use the prefix ``qemu_`` to alert
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readers that they are seeing a wrapped version; otherwise avoid this prefix.
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Block structure
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===============
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Every indented statement is braced; even if the block contains just one
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statement. The opening brace is on the line that contains the control
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flow statement that introduces the new block; the closing brace is on the
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same line as the else keyword, or on a line by itself if there is no else
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keyword. Example:
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.. code-block:: c
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if (a == 5) {
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printf("a was 5.\n");
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} else if (a == 6) {
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printf("a was 6.\n");
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} else {
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printf("a was something else entirely.\n");
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}
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Note that 'else if' is considered a single statement; otherwise a long if/
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else if/else if/.../else sequence would need an indent for every else
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statement.
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An exception is the opening brace for a function; for reasons of tradition
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and clarity it comes on a line by itself:
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.. code-block:: c
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void a_function(void)
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{
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do_something();
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}
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Rationale: a consistent (except for functions...) bracing style reduces
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ambiguity and avoids needless churn when lines are added or removed.
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Furthermore, it is the QEMU coding style.
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Declarations
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============
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Mixed declarations (interleaving statements and declarations within
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blocks) are generally not allowed; declarations should be at the beginning
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of blocks.
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Every now and then, an exception is made for declarations inside a
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#ifdef or #ifndef block: if the code looks nicer, such declarations can
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be placed at the top of the block even if there are statements above.
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On the other hand, however, it's often best to move that #ifdef/#ifndef
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block to a separate function altogether.
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Conditional statements
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======================
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When comparing a variable for (in)equality with a constant, list the
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constant on the right, as in:
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.. code-block:: c
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if (a == 1) {
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/* Reads like: "If a equals 1" */
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do_something();
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}
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Rationale: Yoda conditions (as in 'if (1 == a)') are awkward to read.
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Besides, good compilers already warn users when '==' is mis-typed as '=',
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even when the constant is on the right.
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Comment style
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=============
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We use traditional C-style /``*`` ``*``/ comments and avoid // comments.
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Rationale: The // form is valid in C99, so this is purely a matter of
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consistency of style. The checkpatch script will warn you about this.
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Multiline comment blocks should have a row of stars on the left,
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and the initial /``*`` and terminating ``*``/ both on their own lines:
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.. code-block:: c
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/*
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* like
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* this
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*/
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This is the same format required by the Linux kernel coding style.
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(Some of the existing comments in the codebase use the GNU Coding
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Standards form which does not have stars on the left, or other
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variations; avoid these when writing new comments, but don't worry
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about converting to the preferred form unless you're editing that
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comment anyway.)
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Rationale: Consistency, and ease of visually picking out a multiline
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comment from the surrounding code.
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Language usage
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**************
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Preprocessor
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============
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Variadic macros
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---------------
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For variadic macros, stick with this C99-like syntax:
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.. code-block:: c
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#define DPRINTF(fmt, ...) \
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do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
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Include directives
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------------------
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Order include directives as follows:
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.. code-block:: c
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#include "qemu/osdep.h" /* Always first... */
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#include <...> /* then system headers... */
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#include "..." /* and finally QEMU headers. */
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The "qemu/osdep.h" header contains preprocessor macros that affect the behavior
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of core system headers like <stdint.h>. It must be the first include so that
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core system headers included by external libraries get the preprocessor macros
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that QEMU depends on.
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Do not include "qemu/osdep.h" from header files since the .c file will have
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already included it.
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C types
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=======
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It should be common sense to use the right type, but we have collected
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a few useful guidelines here.
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Scalars
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-------
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If you're using "int" or "long", odds are good that there's a better type.
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If a variable is counting something, it should be declared with an
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unsigned type.
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If it's host memory-size related, size_t should be a good choice (use
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ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
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but only for RAM, it may not cover whole guest address space.
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If it's file-size related, use off_t.
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If it's file-offset related (i.e., signed), use off_t.
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If it's just counting small numbers use "unsigned int";
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(on all but oddball embedded systems, you can assume that that
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type is at least four bytes wide).
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In the event that you require a specific width, use a standard type
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like int32_t, uint32_t, uint64_t, etc. The specific types are
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mandatory for VMState fields.
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Don't use Linux kernel internal types like u32, __u32 or __le32.
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Use hwaddr for guest physical addresses except pcibus_t
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for PCI addresses. In addition, ram_addr_t is a QEMU internal address
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space that maps guest RAM physical addresses into an intermediate
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address space that can map to host virtual address spaces. Generally
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speaking, the size of guest memory can always fit into ram_addr_t but
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it would not be correct to store an actual guest physical address in a
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ram_addr_t.
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For CPU virtual addresses there are several possible types.
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vaddr is the best type to use to hold a CPU virtual address in
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target-independent code. It is guaranteed to be large enough to hold a
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virtual address for any target, and it does not change size from target
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to target. It is always unsigned.
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target_ulong is a type the size of a virtual address on the CPU; this means
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it may be 32 or 64 bits depending on which target is being built. It should
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therefore be used only in target-specific code, and in some
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performance-critical built-per-target core code such as the TLB code.
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There is also a signed version, target_long.
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abi_ulong is for the ``*``-user targets, and represents a type the size of
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'void ``*``' in that target's ABI. (This may not be the same as the size of a
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full CPU virtual address in the case of target ABIs which use 32 bit pointers
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on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
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the target's ABI must use this type for anything that on the target is defined
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to be an 'unsigned long' or a pointer type.
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There is also a signed version, abi_long.
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Of course, take all of the above with a grain of salt. If you're about
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to use some system interface that requires a type like size_t, pid_t or
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off_t, use matching types for any corresponding variables.
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Also, if you try to use e.g., "unsigned int" as a type, and that
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conflicts with the signedness of a related variable, sometimes
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it's best just to use the *wrong* type, if "pulling the thread"
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and fixing all related variables would be too invasive.
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Finally, while using descriptive types is important, be careful not to
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go overboard. If whatever you're doing causes warnings, or requires
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casts, then reconsider or ask for help.
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Pointers
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--------
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Ensure that all of your pointers are "const-correct".
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Unless a pointer is used to modify the pointed-to storage,
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give it the "const" attribute. That way, the reader knows
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up-front that this is a read-only pointer. Perhaps more
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importantly, if we're diligent about this, when you see a non-const
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pointer, you're guaranteed that it is used to modify the storage
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it points to, or it is aliased to another pointer that is.
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Typedefs
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--------
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Typedefs are used to eliminate the redundant 'struct' keyword, since type
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names have a different style than other identifiers ("CamelCase" versus
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"snake_case"). Each named struct type should have a CamelCase name and a
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corresponding typedef.
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Since certain C compilers choke on duplicated typedefs, you should avoid
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them and declare a typedef only in one header file. For common types,
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you can use "include/qemu/typedefs.h" for example. However, as a matter
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of convenience it is also perfectly fine to use forward struct
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definitions instead of typedefs in headers and function prototypes; this
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avoids problems with duplicated typedefs and reduces the need to include
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headers from other headers.
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Reserved namespaces in C and POSIX
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----------------------------------
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Underscore capital, double underscore, and underscore 't' suffixes should be
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avoided.
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Low level memory management
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===========================
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Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
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APIs is not allowed in the QEMU codebase. Instead of these routines,
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use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
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g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree
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APIs.
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Please note that g_malloc will exit on allocation failure, so there
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is no need to test for failure (as you would have to with malloc).
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Calling g_malloc with a zero size is valid and will return NULL.
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Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following
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reasons:
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* It catches multiplication overflowing size_t;
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* It returns T ``*`` instead of void ``*``, letting compiler catch more type errors.
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Declarations like
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.. code-block:: c
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T *v = g_malloc(sizeof(*v))
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are acceptable, though.
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Memory allocated by qemu_memalign or qemu_blockalign must be freed with
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qemu_vfree, since breaking this will cause problems on Win32.
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String manipulation
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===================
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Do not use the strncpy function. As mentioned in the man page, it does *not*
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guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
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It also zeros trailing destination bytes out to the specified length. Instead,
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use this similar function when possible, but note its different signature:
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.. code-block:: c
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void pstrcpy(char *dest, int dest_buf_size, const char *src)
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Don't use strcat because it can't check for buffer overflows, but:
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.. code-block:: c
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char *pstrcat(char *buf, int buf_size, const char *s)
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The same limitation exists with sprintf and vsprintf, so use snprintf and
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vsnprintf.
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QEMU provides other useful string functions:
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.. code-block:: c
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int strstart(const char *str, const char *val, const char **ptr)
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int stristart(const char *str, const char *val, const char **ptr)
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int qemu_strnlen(const char *s, int max_len)
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There are also replacement character processing macros for isxyz and toxyz,
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so instead of e.g. isalnum you should use qemu_isalnum.
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Because of the memory management rules, you must use g_strdup/g_strndup
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instead of plain strdup/strndup.
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Printf-style functions
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======================
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Whenever you add a new printf-style function, i.e., one with a format
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string argument and following "..." in its prototype, be sure to use
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gcc's printf attribute directive in the prototype.
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This makes it so gcc's -Wformat and -Wformat-security options can do
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their jobs and cross-check format strings with the number and types
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of arguments.
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C standard, implementation defined and undefined behaviors
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==========================================================
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C code in QEMU should be written to the C99 language specification. A copy
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of the final version of the C99 standard with corrigenda TC1, TC2, and TC3
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included, formatted as a draft, can be downloaded from:
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`<http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf>`_
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The C language specification defines regions of undefined behavior and
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implementation defined behavior (to give compiler authors enough leeway to
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produce better code). In general, code in QEMU should follow the language
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specification and avoid both undefined and implementation defined
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constructs. ("It works fine on the gcc I tested it with" is not a valid
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argument...) However there are a few areas where we allow ourselves to
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assume certain behaviors because in practice all the platforms we care about
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behave in the same way and writing strictly conformant code would be
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painful. These are:
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* you may assume that integers are 2s complement representation
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* you may assume that right shift of a signed integer duplicates
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the sign bit (ie it is an arithmetic shift, not a logical shift)
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In addition, QEMU assumes that the compiler does not use the latitude
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given in C99 and C11 to treat aspects of signed '<<' as undefined, as
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documented in the GNU Compiler Collection manual starting at version 4.0.
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Automatic memory deallocation
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=============================
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QEMU has a mandatory dependency either the GCC or CLang compiler. As
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such it has the freedom to make use of a C language extension for
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automatically running a cleanup function when a stack variable goes
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out of scope. This can be used to simplify function cleanup paths,
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often allowing many goto jumps to be eliminated, through automatic
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free'ing of memory.
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The GLib2 library provides a number of functions/macros for enabling
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automatic cleanup:
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`<https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html>`_
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Most notably:
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* g_autofree - will invoke g_free() on the variable going out of scope
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* g_autoptr - for structs / objects, will invoke the cleanup func created
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by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is
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supported for most GLib data types and GObjects
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For example, instead of
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.. code-block:: c
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int somefunc(void) {
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int ret = -1;
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char *foo = g_strdup_printf("foo%", "wibble");
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GList *bar = .....
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if (eek) {
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goto cleanup;
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}
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ret = 0;
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cleanup:
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g_free(foo);
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g_list_free(bar);
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return ret;
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}
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Using g_autofree/g_autoptr enables the code to be written as:
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.. code-block:: c
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int somefunc(void) {
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g_autofree char *foo = g_strdup_printf("foo%", "wibble");
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g_autoptr (GList) bar = .....
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if (eek) {
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return -1;
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}
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return 0;
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}
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While this generally results in simpler, less leak-prone code, there
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are still some caveats to beware of
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* Variables declared with g_auto* MUST always be initialized,
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otherwise the cleanup function will use uninitialized stack memory
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* If a variable declared with g_auto* holds a value which must
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live beyond the life of the function, that value must be saved
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and the original variable NULL'd out. This can be simpler using
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g_steal_pointer
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|
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.. code-block:: c
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char *somefunc(void) {
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g_autofree char *foo = g_strdup_printf("foo%", "wibble");
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g_autoptr (GList) bar = .....
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if (eek) {
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return NULL;
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}
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return g_steal_pointer(&foo);
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}
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QEMU Specific Idioms
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********************
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Error handling and reporting
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============================
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Reporting errors to the human user
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----------------------------------
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Do not use printf(), fprintf() or monitor_printf(). Instead, use
|
|
error_report() or error_vreport() from error-report.h. This ensures the
|
|
error is reported in the right place (current monitor or stderr), and in
|
|
a uniform format.
|
|
|
|
Use error_printf() & friends to print additional information.
|
|
|
|
error_report() prints the current location. In certain common cases
|
|
like command line parsing, the current location is tracked
|
|
automatically. To manipulate it manually, use the loc_``*``() from
|
|
error-report.h.
|
|
|
|
Propagating errors
|
|
------------------
|
|
|
|
An error can't always be reported to the user right where it's detected,
|
|
but often needs to be propagated up the call chain to a place that can
|
|
handle it. This can be done in various ways.
|
|
|
|
The most flexible one is Error objects. See error.h for usage
|
|
information.
|
|
|
|
Use the simplest suitable method to communicate success / failure to
|
|
callers. Stick to common methods: non-negative on success / -1 on
|
|
error, non-negative / -errno, non-null / null, or Error objects.
|
|
|
|
Example: when a function returns a non-null pointer on success, and it
|
|
can fail only in one way (as far as the caller is concerned), returning
|
|
null on failure is just fine, and certainly simpler and a lot easier on
|
|
the eyes than propagating an Error object through an Error ``*````*`` parameter.
|
|
|
|
Example: when a function's callers need to report details on failure
|
|
only the function really knows, use Error ``*````*``, and set suitable errors.
|
|
|
|
Do not report an error to the user when you're also returning an error
|
|
for somebody else to handle. Leave the reporting to the place that
|
|
consumes the error returned.
|
|
|
|
Handling errors
|
|
---------------
|
|
|
|
Calling exit() is fine when handling configuration errors during
|
|
startup. It's problematic during normal operation. In particular,
|
|
monitor commands should never exit().
|
|
|
|
Do not call exit() or abort() to handle an error that can be triggered
|
|
by the guest (e.g., some unimplemented corner case in guest code
|
|
translation or device emulation). Guests should not be able to
|
|
terminate QEMU.
|
|
|
|
Note that &error_fatal is just another way to exit(1), and &error_abort
|
|
is just another way to abort().
|
|
|
|
|
|
trace-events style
|
|
==================
|
|
|
|
0x prefix
|
|
---------
|
|
|
|
In trace-events files, use a '0x' prefix to specify hex numbers, as in:
|
|
|
|
.. code-block::
|
|
|
|
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):
|
|
|
|
.. code-block::
|
|
|
|
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.:
|
|
|
|
.. code-block::
|
|
|
|
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.
|
|
|
|
'#' 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
|