mirror of
https://github.com/qemu/qemu.git
synced 2024-12-05 17:53:36 +08:00
9f8efa74d3
Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
642 lines
21 KiB
ReStructuredText
642 lines
21 KiB
ReStructuredText
=================
|
|
QEMU Coding Style
|
|
=================
|
|
|
|
.. contents:: Table of Contents
|
|
|
|
Please use the script checkpatch.pl in the scripts directory to check
|
|
patches before submitting.
|
|
|
|
Formatting and style
|
|
********************
|
|
|
|
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 has 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.
|
|
|
|
Multiline Indent
|
|
----------------
|
|
|
|
There are several places where indent is necessary:
|
|
|
|
* if/else
|
|
* while/for
|
|
* function definition & call
|
|
|
|
When breaking up a long line to fit within line width, we need a proper indent
|
|
for the following lines.
|
|
|
|
In case of if/else, while/for, align the secondary lines just after the
|
|
opening parenthesis of the first.
|
|
|
|
For example:
|
|
|
|
.. code-block:: c
|
|
|
|
if (a == 1 &&
|
|
b == 2) {
|
|
|
|
while (a == 1 &&
|
|
b == 2) {
|
|
|
|
In case of function, there are several variants:
|
|
|
|
* 4 spaces indent from the beginning
|
|
* align the secondary lines just after the opening parenthesis of the first
|
|
|
|
For example:
|
|
|
|
.. code-block:: c
|
|
|
|
do_something(x, y,
|
|
z);
|
|
|
|
do_something(x, y,
|
|
z);
|
|
|
|
do_something(x, do_another(y,
|
|
z));
|
|
|
|
Line width
|
|
==========
|
|
|
|
Lines should be 80 characters; try not to make them longer.
|
|
|
|
Sometimes it is hard to do, especially when dealing with QEMU subsystems
|
|
that use long function or symbol names. Even in that case, do not make
|
|
lines much longer than 80 characters.
|
|
|
|
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.
|
|
* The four-space indentation makes the most common excuse ("But look
|
|
at all that white space on the left!") moot.
|
|
* It is the QEMU coding style.
|
|
|
|
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.
|
|
|
|
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:
|
|
|
|
.. code-block:: c
|
|
|
|
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:
|
|
|
|
.. code-block:: c
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
Conditional statements
|
|
======================
|
|
|
|
When comparing a variable for (in)equality with a constant, list the
|
|
constant on the right, as in:
|
|
|
|
.. code-block:: c
|
|
|
|
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.
|
|
|
|
Comment style
|
|
=============
|
|
|
|
We use traditional C-style /``*`` ``*``/ comments and avoid // comments.
|
|
|
|
Rationale: The // form is valid in C99, so this is purely a matter of
|
|
consistency of style. The checkpatch script will warn you about this.
|
|
|
|
Multiline comment blocks should have a row of stars on the left,
|
|
and the initial /``*`` and terminating ``*``/ both on their own lines:
|
|
|
|
.. code-block:: c
|
|
|
|
/*
|
|
* like
|
|
* this
|
|
*/
|
|
|
|
This is the same format required by the Linux kernel coding style.
|
|
|
|
(Some of the existing comments in the codebase use the GNU Coding
|
|
Standards form which does not have stars on the left, or other
|
|
variations; avoid these when writing new comments, but don't worry
|
|
about converting to the preferred form unless you're editing that
|
|
comment anyway.)
|
|
|
|
Rationale: Consistency, and ease of visually picking out a multiline
|
|
comment from the surrounding code.
|
|
|
|
Language usage
|
|
**************
|
|
|
|
Preprocessor
|
|
============
|
|
|
|
Variadic macros
|
|
---------------
|
|
|
|
For variadic macros, stick with this C99-like syntax:
|
|
|
|
.. code-block:: c
|
|
|
|
#define DPRINTF(fmt, ...) \
|
|
do { printf("IRQ: " fmt, ## __VA_ARGS__); } while (0)
|
|
|
|
Include directives
|
|
------------------
|
|
|
|
Order include directives as follows:
|
|
|
|
.. code-block:: c
|
|
|
|
#include "qemu/osdep.h" /* Always first... */
|
|
#include <...> /* then system headers... */
|
|
#include "..." /* and finally QEMU headers. */
|
|
|
|
The "qemu/osdep.h" header contains preprocessor macros that affect the behavior
|
|
of core system headers like <stdint.h>. It must be the first include so that
|
|
core system headers included by external libraries get the preprocessor macros
|
|
that QEMU depends on.
|
|
|
|
Do not include "qemu/osdep.h" from header files since the .c file will have
|
|
already included it.
|
|
|
|
C types
|
|
=======
|
|
|
|
It should be common sense to use the right type, but we have collected
|
|
a few useful guidelines here.
|
|
|
|
Scalars
|
|
-------
|
|
|
|
If you're using "int" or "long", odds are good that there's a better type.
|
|
If a variable is counting something, it should be declared with an
|
|
unsigned type.
|
|
|
|
If it's host memory-size related, size_t should be a good choice (use
|
|
ssize_t only if required). Guest RAM memory offsets must use ram_addr_t,
|
|
but only for RAM, it may not cover whole guest address space.
|
|
|
|
If it's file-size related, use off_t.
|
|
If it's file-offset related (i.e., signed), use off_t.
|
|
If it's just counting small numbers use "unsigned int";
|
|
(on all but oddball embedded systems, you can assume that that
|
|
type is at least four bytes wide).
|
|
|
|
In the event that you require a specific width, use a standard type
|
|
like int32_t, uint32_t, uint64_t, etc. The specific types are
|
|
mandatory for VMState fields.
|
|
|
|
Don't use Linux kernel internal types like u32, __u32 or __le32.
|
|
|
|
Use hwaddr for guest physical addresses except pcibus_t
|
|
for PCI addresses. In addition, ram_addr_t is a QEMU internal address
|
|
space that maps guest RAM physical addresses into an intermediate
|
|
address space that can map to host virtual address spaces. Generally
|
|
speaking, the size of guest memory can always fit into ram_addr_t but
|
|
it would not be correct to store an actual guest physical address in a
|
|
ram_addr_t.
|
|
|
|
For CPU virtual addresses there are several possible types.
|
|
vaddr is the best type to use to hold a CPU virtual address in
|
|
target-independent code. It is guaranteed to be large enough to hold a
|
|
virtual address for any target, and it does not change size from target
|
|
to target. It is always unsigned.
|
|
target_ulong is a type the size of a virtual address on the CPU; this means
|
|
it may be 32 or 64 bits depending on which target is being built. It should
|
|
therefore be used only in target-specific code, and in some
|
|
performance-critical built-per-target core code such as the TLB code.
|
|
There is also a signed version, target_long.
|
|
abi_ulong is for the ``*``-user targets, and represents a type the size of
|
|
'void ``*``' in that target's ABI. (This may not be the same as the size of a
|
|
full CPU virtual address in the case of target ABIs which use 32 bit pointers
|
|
on 64 bit CPUs, like sparc32plus.) Definitions of structures that must match
|
|
the target's ABI must use this type for anything that on the target is defined
|
|
to be an 'unsigned long' or a pointer type.
|
|
There is also a signed version, abi_long.
|
|
|
|
Of course, take all of the above with a grain of salt. If you're about
|
|
to use some system interface that requires a type like size_t, pid_t or
|
|
off_t, use matching types for any corresponding variables.
|
|
|
|
Also, if you try to use e.g., "unsigned int" as a type, and that
|
|
conflicts with the signedness of a related variable, sometimes
|
|
it's best just to use the *wrong* type, if "pulling the thread"
|
|
and fixing all related variables would be too invasive.
|
|
|
|
Finally, while using descriptive types is important, be careful not to
|
|
go overboard. If whatever you're doing causes warnings, or requires
|
|
casts, then reconsider or ask for help.
|
|
|
|
Pointers
|
|
--------
|
|
|
|
Ensure that all of your pointers are "const-correct".
|
|
Unless a pointer is used to modify the pointed-to storage,
|
|
give it the "const" attribute. That way, the reader knows
|
|
up-front that this is a read-only pointer. Perhaps more
|
|
importantly, if we're diligent about this, when you see a non-const
|
|
pointer, you're guaranteed that it is used to modify the storage
|
|
it points to, or it is aliased to another pointer that is.
|
|
|
|
Typedefs
|
|
--------
|
|
|
|
Typedefs are used to eliminate the redundant 'struct' keyword, since type
|
|
names have a different style than other identifiers ("CamelCase" versus
|
|
"snake_case"). Each named struct type should have a CamelCase name and a
|
|
corresponding typedef.
|
|
|
|
Since certain C compilers choke on duplicated typedefs, you should avoid
|
|
them and declare a typedef only in one header file. For common types,
|
|
you can use "include/qemu/typedefs.h" for example. However, as a matter
|
|
of convenience it is also perfectly fine to use forward struct
|
|
definitions instead of typedefs in headers and function prototypes; this
|
|
avoids problems with duplicated typedefs and reduces the need to include
|
|
headers from other headers.
|
|
|
|
Reserved namespaces in C and POSIX
|
|
----------------------------------
|
|
|
|
Underscore capital, double underscore, and underscore 't' suffixes should be
|
|
avoided.
|
|
|
|
Low level memory management
|
|
===========================
|
|
|
|
Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign
|
|
APIs is not allowed in the QEMU codebase. Instead of these routines,
|
|
use the GLib memory allocation routines g_malloc/g_malloc0/g_new/
|
|
g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree
|
|
APIs.
|
|
|
|
Please note that g_malloc will exit on allocation failure, so there
|
|
is no need to test for failure (as you would have to with malloc).
|
|
Calling g_malloc with a zero size is valid and will return NULL.
|
|
|
|
Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following
|
|
reasons:
|
|
|
|
* It catches multiplication overflowing size_t;
|
|
* It returns T ``*`` instead of void ``*``, letting compiler catch more type errors.
|
|
|
|
Declarations like
|
|
|
|
.. code-block:: c
|
|
|
|
T *v = g_malloc(sizeof(*v))
|
|
|
|
are acceptable, though.
|
|
|
|
Memory allocated by qemu_memalign or qemu_blockalign must be freed with
|
|
qemu_vfree, since breaking this will cause problems on Win32.
|
|
|
|
String manipulation
|
|
===================
|
|
|
|
Do not use the strncpy function. As mentioned in the man page, it does *not*
|
|
guarantee a NULL-terminated buffer, which makes it extremely dangerous to use.
|
|
It also zeros trailing destination bytes out to the specified length. Instead,
|
|
use this similar function when possible, but note its different signature:
|
|
|
|
.. code-block:: c
|
|
|
|
void pstrcpy(char *dest, int dest_buf_size, const char *src)
|
|
|
|
Don't use strcat because it can't check for buffer overflows, but:
|
|
|
|
.. code-block:: c
|
|
|
|
char *pstrcat(char *buf, int buf_size, const char *s)
|
|
|
|
The same limitation exists with sprintf and vsprintf, so use snprintf and
|
|
vsnprintf.
|
|
|
|
QEMU provides other useful string functions:
|
|
|
|
.. code-block:: c
|
|
|
|
int strstart(const char *str, const char *val, const char **ptr)
|
|
int stristart(const char *str, const char *val, const char **ptr)
|
|
int qemu_strnlen(const char *s, int max_len)
|
|
|
|
There are also replacement character processing macros for isxyz and toxyz,
|
|
so instead of e.g. isalnum you should use qemu_isalnum.
|
|
|
|
Because of the memory management rules, you must use g_strdup/g_strndup
|
|
instead of plain strdup/strndup.
|
|
|
|
Printf-style functions
|
|
======================
|
|
|
|
Whenever you add a new printf-style function, i.e., one with a format
|
|
string argument and following "..." in its prototype, be sure to use
|
|
gcc's printf attribute directive in the prototype.
|
|
|
|
This makes it so gcc's -Wformat and -Wformat-security options can do
|
|
their jobs and cross-check format strings with the number and types
|
|
of arguments.
|
|
|
|
C standard, implementation defined and undefined behaviors
|
|
==========================================================
|
|
|
|
C code in QEMU should be written to the C99 language specification. A copy
|
|
of the final version of the C99 standard with corrigenda TC1, TC2, and TC3
|
|
included, formatted as a draft, can be downloaded from:
|
|
|
|
`<http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf>`_
|
|
|
|
The C language specification defines regions of undefined behavior and
|
|
implementation defined behavior (to give compiler authors enough leeway to
|
|
produce better code). In general, code in QEMU should follow the language
|
|
specification and avoid both undefined and implementation defined
|
|
constructs. ("It works fine on the gcc I tested it with" is not a valid
|
|
argument...) However there are a few areas where we allow ourselves to
|
|
assume certain behaviors because in practice all the platforms we care about
|
|
behave in the same way and writing strictly conformant code would be
|
|
painful. These are:
|
|
|
|
* you may assume that integers are 2s complement representation
|
|
* you may assume that right shift of a signed integer duplicates
|
|
the sign bit (ie it is an arithmetic shift, not a logical shift)
|
|
|
|
In addition, QEMU assumes that the compiler does not use the latitude
|
|
given in C99 and C11 to treat aspects of signed '<<' as undefined, as
|
|
documented in the GNU Compiler Collection manual starting at version 4.0.
|
|
|
|
Automatic memory deallocation
|
|
=============================
|
|
|
|
QEMU has a mandatory dependency either the GCC or CLang compiler. As
|
|
such it has the freedom to make use of a C language extension for
|
|
automatically running a cleanup function when a stack variable goes
|
|
out of scope. This can be used to simplify function cleanup paths,
|
|
often allowing many goto jumps to be eliminated, through automatic
|
|
free'ing of memory.
|
|
|
|
The GLib2 library provides a number of functions/macros for enabling
|
|
automatic cleanup:
|
|
|
|
`<https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html>`_
|
|
|
|
Most notably:
|
|
|
|
* g_autofree - will invoke g_free() on the variable going out of scope
|
|
|
|
* g_autoptr - for structs / objects, will invoke the cleanup func created
|
|
by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is
|
|
supported for most GLib data types and GObjects
|
|
|
|
For example, instead of
|
|
|
|
.. code-block:: c
|
|
|
|
int somefunc(void) {
|
|
int ret = -1;
|
|
char *foo = g_strdup_printf("foo%", "wibble");
|
|
GList *bar = .....
|
|
|
|
if (eek) {
|
|
goto cleanup;
|
|
}
|
|
|
|
ret = 0;
|
|
|
|
cleanup:
|
|
g_free(foo);
|
|
g_list_free(bar);
|
|
return ret;
|
|
}
|
|
|
|
Using g_autofree/g_autoptr enables the code to be written as:
|
|
|
|
.. code-block:: c
|
|
|
|
int somefunc(void) {
|
|
g_autofree char *foo = g_strdup_printf("foo%", "wibble");
|
|
g_autoptr (GList) bar = .....
|
|
|
|
if (eek) {
|
|
return -1;
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
While this generally results in simpler, less leak-prone code, there
|
|
are still some caveats to beware of
|
|
|
|
* Variables declared with g_auto* MUST always be initialized,
|
|
otherwise the cleanup function will use uninitialized stack memory
|
|
|
|
* If a variable declared with g_auto* holds a value which must
|
|
live beyond the life of the function, that value must be saved
|
|
and the original variable NULL'd out. This can be simpler using
|
|
g_steal_pointer
|
|
|
|
|
|
.. code-block:: c
|
|
|
|
char *somefunc(void) {
|
|
g_autofree char *foo = g_strdup_printf("foo%", "wibble");
|
|
g_autoptr (GList) bar = .....
|
|
|
|
if (eek) {
|
|
return NULL;
|
|
}
|
|
|
|
return g_steal_pointer(&foo);
|
|
}
|
|
|
|
|
|
QEMU Specific Idioms
|
|
********************
|
|
|
|
Error handling and reporting
|
|
============================
|
|
|
|
Reporting errors to the human user
|
|
----------------------------------
|
|
|
|
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
|