Checkpatch¶

Checkpatch (scripts/checkpatch.pl) is a perl script which checks for trivial style violations in patches and optionally corrects them. Checkpatch can also be run on file contexts and without the kernel tree.

Checkpatch is not always right. Your judgement takes precedence over checkpatch messages. If your code looks better with the violations, then its probably best left alone.

Options¶

This section will describe the options checkpatch can be run with.

Usage:

./scripts/checkpatch.pl [OPTION]... [FILE]...

Available options:

-q, –quiet

Enable quiet mode.

-v, –verbose Enable verbose mode. Additional verbose test descriptions are output so as to provide information on why that particular message is shown.

–no-tree

Run checkpatch without the kernel tree.
–no-signoff

Disable the ‘Signed-off-by’ line check. The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch.

Example:
Signed-off-by: Random J Developer <random@developer.example.org>
Setting this flag effectively stops a message for a missing signed-off-by line in a patch context.
–patch

Treat FILE as a patch. This is the default option and need not be explicitly specified.

–emacs

Set output to emacs compile window format. This allows emacs users to jump from the error in the compile window directly to the offending line in the patch.

–terse

Output only one line per report.

–showfile

Show the diffed file position instead of the input file position.

-g, –git

Treat FILE as a single commit or a git revision range.

Single commit with:

<rev>

<rev>^

<rev>~n

Multiple commits with:

<rev1>..<rev2>

<rev1>…<rev2>

<rev>-<count>

-f, –file

Treat FILE as a regular source file. This option must be used when running checkpatch on source files in the kernel.

–subjective, –strict

Enable stricter tests in checkpatch. By default the tests emitted as CHECK do not activate by default. Use this flag to activate the CHECK tests.

–list-types

Every message emitted by checkpatch has an associated TYPE. Add this flag to display all the types in checkpatch.

Note that when this flag is active, checkpatch does not read the input FILE, and no message is emitted. Only a list of types in checkpatch is output.
–types TYPE(,TYPE2…)

Only display messages with the given types.

Example:
./scripts/checkpatch.pl mypatch.patch --types EMAIL_SUBJECT,BRACES
–ignore TYPE(,TYPE2…)

Checkpatch will not emit messages for the specified types.

Example:
./scripts/checkpatch.pl mypatch.patch --ignore EMAIL_SUBJECT,BRACES
–show-types

By default checkpatch doesn’t display the type associated with the messages. Set this flag to show the message type in the output.

–max-line-length=n

Set the max line length (default 100). If a line exceeds the specified length, a LONG_LINE message is emitted.

The message level is different for patch and file contexts. For patches, a WARNING is emitted. While a milder CHECK is emitted for files. So for file contexts, the –strict flag must also be enabled.

–min-conf-desc-length=n

Set the Kconfig entry minimum description length, if shorter, warn.

–tab-size=n

Set the number of spaces for tab (default 8).

–root=PATH

PATH to the kernel tree root.

This option must be specified when invoking checkpatch from outside the kernel root.

–no-summary

Suppress the per file summary.

–mailback

Only produce a report in case of Warnings or Errors. Milder Checks are excluded from this.

–summary-file

Include the filename in summary.

–debug KEY=[0|1]

Turn on/off debugging of KEY, where KEY is one of ‘values’, ‘possible’, ‘type’, and ‘attr’ (default is all off).

–fix

This is an EXPERIMENTAL feature. If correctable errors exists, a file <inputfile>.EXPERIMENTAL-checkpatch-fixes is created which has the automatically fixable errors corrected.

–fix-inplace

EXPERIMENTAL - Similar to –fix but input file is overwritten with fixes.

DO NOT USE this flag unless you are absolutely sure and you have a backup in place.

–ignore-perl-version

Override checking of perl version. Runtime errors maybe encountered after enabling this flag if the perl version does not meet the minimum specified.

–codespell

Use the codespell dictionary for checking spelling errors.

–codespellfile

Use the specified codespell file. Default is ‘/usr/share/codespell/dictionary.txt’.

–typedefsfile

Read additional types from this file.

–color[=WHEN]

Use colors ‘always’, ‘never’, or only when output is a terminal (‘auto’). Default is ‘auto’.

–kconfig-prefix=WORD

Use WORD as a prefix for Kconfig symbols (default is CONFIG_).

-h, –help, –version

Display the help text.

Message Levels¶

Messages in checkpatch are divided into three levels. The levels of messages in checkpatch denote the severity of the error. They are:

ERROR

This is the most strict level. Messages of type ERROR must be taken seriously as they denote things that are very likely to be wrong.

WARNING

This is the next stricter level. Messages of type WARNING requires a more careful review. But it is milder than an ERROR.

CHECK

This is the mildest level. These are things which may require some thought.

Type Descriptions¶

This section contains a description of all the message types in checkpatch.

Allocation style¶

ALLOC_ARRAY_ARGS
The first argument for kcalloc or kmalloc_array should be the number of elements. sizeof() as the first argument is generally wrong. See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html

ALLOC_SIZEOF_STRUCT
The allocation style is bad. In general for family of allocation functions using sizeof() to get memory size, constructs like:
p = alloc(sizeof(struct foo), ...)
should be:
p = alloc(sizeof(*p), ...)
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#allocating-memory
ALLOC_WITH_MULTIPLY
Prefer kmalloc_array/kcalloc over kmalloc/kzalloc with a sizeof multiply. See: https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html

API usage¶

ARCH_DEFINES
Architecture specific defines should be avoided wherever possible.

ARCH_INCLUDE_LINUX
Whenever asm/file.h is included and linux/file.h exists, a conversion can be made when linux/file.h includes asm/file.h. However this is not always the case (See signal.h). This message type is emitted only for includes from arch/.

AVOID_BUG
BUG() or BUG_ON() should be avoided totally. Use WARN() and WARN_ON() instead, and handle the “impossible” error condition as gracefully as possible. See: https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

CONSIDER_KSTRTO
The simple_strtol(), simple_strtoll(), simple_strtoul(), and simple_strtoull() functions explicitly ignore overflows, which may lead to unexpected results in callers. The respective kstrtol(), kstrtoll(), kstrtoul(), and kstrtoull() functions tend to be the correct replacements. See: https://www.kernel.org/doc/html/latest/process/deprecated.html#simple-strtol-simple-strtoll-simple-strtoul-simple-strtoull

LOCKDEP
The lockdep_no_validate class was added as a temporary measure to prevent warnings on conversion of device->sem to device->mutex. It should not be used for any other purpose. See: https://lore.kernel.org/lkml/1268959062.9440.467.camel@laptop/

MALFORMED_INCLUDE
The #include statement has a malformed path. This has happened because the author has included a double slash “//” in the pathname accidentally.

USE_LOCKDEP
lockdep_assert_held() annotations should be preferred over assertions based on spin_is_locked() See: https://www.kernel.org/doc/html/latest/locking/lockdep-design.html#annotations

UAPI_INCLUDE
No #include statements in include/uapi should use a uapi/ path.

Comment style¶

BLOCK_COMMENT_STYLE
The comment style is incorrect. The preferred style for multi- line comments is:
/*
* This is the preferred style
* for multi line comments.
*/
The networking comment style is a bit different, with the first line not empty like the former:
/* This is the preferred comment style
* for files in net/ and drivers/net/
*/
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting
C99_COMMENTS
C99 style single line comments (//) should not be used. Prefer the block comment style instead. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#commenting

Commit message¶

BAD_SIGN_OFF
The signed-off-by line does not fall in line with the standards specified by the community. See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#developer-s-certificate-of-origin-1-1

BAD_STABLE_ADDRESS_STYLE
The email format for stable is incorrect. Some valid options for stable address are:
1. stable@vger.kernel.org
2. stable@kernel.org
For adding version info, the following comment style should be used:
stable@vger.kernel.org # version info
COMMIT_COMMENT_SYMBOL
Commit log lines starting with a ‘#’ are ignored by git as comments. To solve this problem addition of a single space infront of the log line is enough.

COMMIT_MESSAGE
The patch is missing a commit description. A brief description of the changes made by the patch should be added. See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

MISSING_SIGN_OFF
The patch is missing a Signed-off-by line. A signed-off-by line should be added according to Developer’s certificate of Origin. See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

NO_AUTHOR_SIGN_OFF
The author of the patch has not signed off the patch. It is required that a simple sign off line should be present at the end of explanation of the patch to denote that the author has written it or otherwise has the rights to pass it on as an open source patch. See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

DIFF_IN_COMMIT_MSG
Avoid having diff content in commit message. This causes problems when one tries to apply a file containing both the changelog and the diff because patch(1) tries to apply the diff which it found in the changelog. See: https://lore.kernel.org/lkml/20150611134006.9df79a893e3636019ad2759e@linux-foundation.org/

GERRIT_CHANGE_ID
To be picked up by gerrit, the footer of the commit message might have a Change-Id like:
Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
Signed-off-by: A. U. Thor <author@example.com>
The Change-Id line must be removed before submitting.
GIT_COMMIT_ID
The proper way to reference a commit id is: commit <12+ chars of sha1> (“<title line>”)

An example may be:
Commit e21d2170f36602ae2708 ("video: remove unnecessary
platform_set_drvdata()") removed the unnecessary
platform_set_drvdata(), but left the variable "dev" unused,
delete it.
See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes

Comparison style¶

ASSIGN_IN_IF
Do not use assignments in if condition. Example:
if ((foo = bar(...)) < BAZ) {
should be written as:
foo = bar(...);
if (foo < BAZ) {
BOOL_COMPARISON
Comparisons of A to true and false are better written as A and !A. See: https://lore.kernel.org/lkml/1365563834.27174.12.camel@joe-AO722/

COMPARISON_TO_NULL
Comparisons to NULL in the form (foo == NULL) or (foo != NULL) are better written as (!foo) and (foo).

CONSTANT_COMPARISON
Comparisons with a constant or upper case identifier on the left side of the test should be avoided.

Macros, Attributes and Symbols¶

ARRAY_SIZE
The ARRAY_SIZE(foo) macro should be preferred over sizeof(foo)/sizeof(foo[0]) for finding number of elements in an array.

The macro is defined in include/linux/kernel.h:
#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
AVOID_EXTERNS
Function prototypes don’t need to be declared extern in .h files. It’s assumed by the compiler and is unnecessary.

AVOID_L_PREFIX
Local symbol names that are prefixed with .L should be avoided, as this has special meaning for the assembler; a symbol entry will not be emitted into the symbol table. This can prevent objtool from generating correct unwind info.

Symbols with STB_LOCAL binding may still be used, and .L prefixed local symbol names are still generally usable within a function, but .L prefixed local symbol names should not be used to denote the beginning or end of code regions via SYM_CODE_START_LOCAL/SYM_CODE_END

BIT_MACRO
Defines like: 1 << <digit> could be BIT(digit). The BIT() macro is defined in include/linux/bitops.h:
#define BIT(nr)         (1UL << (nr))
CONST_READ_MOSTLY
When a variable is tagged with the __read_mostly annotation, it is a signal to the compiler that accesses to the variable will be mostly reads and rarely(but NOT never) a write.

const __read_mostly does not make any sense as const data is already read-only. The __read_mostly annotation thus should be removed.

DATE_TIME
It is generally desirable that building the same source code with the same set of tools is reproducible, i.e. the output is always exactly the same.

The kernel does not use the __DATE__ and __TIME__ macros, and enables warnings if they are used as they can lead to non-deterministic builds. See: https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html#timestamps

DEFINE_ARCH_HAS
The ARCH_HAS_xyz and ARCH_HAVE_xyz patterns are wrong.

For big conceptual features use Kconfig symbols instead. And for smaller things where we have compatibility fallback functions but want architectures able to override them with optimized ones, we should either use weak functions (appropriate for some cases), or the symbol that protects them should be the same symbol we use. See: https://lore.kernel.org/lkml/CA+55aFycQ9XJvEOsiM3txHL5bjUc8CeKWJNR_H+MiicaddB42Q@mail.gmail.com/

INIT_ATTRIBUTE
Const init definitions should use __initconst instead of __initdata.

Similarly init definitions without const require a separate use of const.

INLINE_LOCATION
The inline keyword should sit between storage class and type.

For example, the following segment:
inline static int example_function(void)
{
        ...
}
should be:
static inline int example_function(void)
{
        ...
}
MULTISTATEMENT_MACRO_USE_DO_WHILE
Macros with multiple statements should be enclosed in a do - while block. Same should also be the case for macros starting with if to avoid logic defects:
#define macrofun(a, b, c)                 \
  do {                                    \
          if (a == 5)                     \
                  do_this(b, c);          \
  } while (0)
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl
WEAK_DECLARATION
Using weak declarations like __attribute__((weak)) or __weak can have unintended link defects. Avoid using them.

Functions and Variables¶

CAMELCASE
Avoid CamelCase Identifiers. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#naming

FUNCTION_WITHOUT_ARGS
Function declarations without arguments like:
int foo()
should be:
int foo(void)
GLOBAL_INITIALISERS
Global variables should not be initialized explicitly to 0 (or NULL, false, etc.). Your compiler (or rather your loader, which is responsible for zeroing out the relevant sections) automatically does it for you.

INITIALISED_STATIC
Static variables should not be initialized explicitly to zero. Your compiler (or rather your loader) automatically does it for you.

RETURN_PARENTHESES
return is not a function and as such doesn’t need parentheses:
return (bar);
can simply be:
return bar;

Spacing and Brackets¶

ASSIGNMENT_CONTINUATIONS
Assignment operators should not be written at the start of a line but should follow the operand at the previous line.

BRACES
The placement of braces is stylistically incorrect. The preferred way is to put the opening brace last on the line, and put the closing brace first:
if (x is true) {
        we do y
}
This applies for all non-functional blocks. However, there is one special case, namely functions: they have the opening brace at the beginning of the next line, thus:
int function(int x)
{
        body of function
}
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces
BRACKET_SPACE
Whitespace before opening bracket ‘[‘ is prohibited. There are some exceptions:
With a type on the left:
;int [] a;
At the beginning of a line for slice initialisers:
[0...10] = 5,
Inside a curly brace:
= { [0...10] = 5 }
CODE_INDENT
Code indent should use tabs instead of spaces. Outside of comments, documentation and Kconfig, spaces are never used for indentation. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation

CONCATENATED_STRING
Concatenated elements should have a space in between. Example:
printk(KERN_INFO"bar");
should be:
printk(KERN_INFO "bar");
ELSE_AFTER_BRACE
else { should follow the closing block } on the same line. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

LINE_SPACING
Vertical space is wasted given the limited number of lines an editor window can display when multiple blank lines are used. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

OPEN_BRACE
The opening brace should be following the function definitions on the next line. For any non-functional block it should be on the same line as the last construct. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

POINTER_LOCATION
When using pointer data or a function that returns a pointer type, the preferred use of * is adjacent to the data name or function name and not adjacent to the type name. Examples:
char *linux_banner;
unsigned long long memparse(char *ptr, char **retptr);
char *match_strdup(substring_t *s);
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces
SPACING
Whitespace style used in the kernel sources is described in kernel docs. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

SWITCH_CASE_INDENT_LEVEL
switch should be at the same indent as case. Example:
switch (suffix) {
case 'G':
case 'g':
        mem <<= 30;
        break;
case 'M':
case 'm':
        mem <<= 20;
        break;
case 'K':
case 'k':
        mem <<= 10;
        /* fall through */
default:
        break;
}
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#indentation
TRAILING_WHITESPACE
Trailing whitespace should always be removed. Some editors highlight the trailing whitespace and cause visual distractions when editing files. See: https://www.kernel.org/doc/html/latest/process/coding-style.html#spaces

WHILE_AFTER_BRACE
while should follow the closing bracket on the same line:
do {
        ...
} while(something);
See: https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces

Others¶

CONFIG_DESCRIPTION
Kconfig symbols should have a help text which fully describes it.

CORRUPTED_PATCH
The patch seems to be corrupted or lines are wrapped. Please regenerate the patch file before sending it to the maintainer.

DOS_LINE_ENDINGS
For DOS-formatted patches, there are extra ^M symbols at the end of the line. These should be removed.

EXECUTE_PERMISSIONS
There is no reason for source files to be executable. The executable bit can be removed safely.

NON_OCTAL_PERMISSIONS
Permission bits should use 4 digit octal permissions (like 0700 or 0444). Avoid using any other base like decimal.

NOT_UNIFIED_DIFF
The patch file does not appear to be in unified-diff format. Please regenerate the patch file before sending it to the maintainer.

PRINTF_0XDECIMAL
Prefixing 0x with decimal output is defective and should be corrected.

TRAILING_STATEMENTS
Trailing statements (for example after any conditional) should be on the next line. Like:
if (x == y) break;
should be:
if (x == y)
        break;