Checkpatch (scripts/ 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.


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


./scripts/ [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.


    Signed-off-by: Random J Developer <>

    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.


    ./scripts/ mypatch.patch --types EMAIL_SUBJECT,BRACES
  • –ignore TYPE(,TYPE2…)

    Checkpatch will not emit messages for the specified types.


    ./scripts/ 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:


    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.


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


    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


The first argument for kcalloc or kmalloc_array should be the number of elements. sizeof() as the first argument is generally wrong. See:


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), ...)



Prefer kmalloc_array/kcalloc over kmalloc/kzalloc with a sizeof multiply. See:

API usage


Architecture specific defines should be avoided wherever possible.


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/.


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:


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:


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:


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


lockdep_assert_held() annotations should be preferred over assertions based on spin_is_locked() See:


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

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/



C99 style single line comments (//) should not be used. Prefer the block comment style instead. See:

Commit message


The signed-off-by line does not fall in line with the standards specified by the community. See:


The email format for stable is incorrect. Some valid options for stable address are:


For adding version info, the following comment style should be used: # version info

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.


The patch is missing a commit description. A brief description of the changes made by the patch should be added. See:


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:


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:


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:


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 <>

The Change-Id line must be removed before submitting.


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.


Comparison style


Do not use assignments in if condition. Example:

if ((foo = bar(...)) < BAZ) {

should be written as:

foo = bar(...);
if (foo < BAZ) {

Comparisons of A to true and false are better written as A and !A. See:


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


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

Macros, Attributes and Symbols


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]))

Function prototypes don’t need to be declared extern in .h files. It’s assumed by the compiler and is unnecessary.


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


Defines like: 1 << <digit> could be BIT(digit). The BIT() macro is defined in include/linux/bitops.h:

#define BIT(nr)         (1UL << (nr))

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.


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:


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:


Const init definitions should use __initconst instead of __initdata.

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


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)

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)



Using weak declarations like __attribute__((weak)) or __weak can have unintended link defects. Avoid using them.

Functions and Variables


Avoid CamelCase Identifiers. See:


Function declarations without arguments like:

int foo()

should be:

int foo(void)

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.


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


return is not a function and as such doesn’t need parentheses:

return (bar);

can simply be:

return bar;

Spacing and Brackets


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


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



Whitespace before opening bracket ‘[‘ is prohibited. There are some exceptions:

  1. With a type on the left:

    ;int [] a;
  2. At the beginning of a line for slice initialisers:

    [0...10] = 5,
  3. Inside a curly brace:

    = { [0...10] = 5 }

Code indent should use tabs instead of spaces. Outside of comments, documentation and Kconfig, spaces are never used for indentation. See:


Concatenated elements should have a space in between. Example:


should be:

printk(KERN_INFO "bar");

else { should follow the closing block } on the same line. See:


Vertical space is wasted given the limited number of lines an editor window can display when multiple blank lines are used. See:


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:


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);



Whitespace style used in the kernel sources is described in kernel docs. See:


switch should be at the same indent as case. Example:

switch (suffix) {
case 'G':
case 'g':
        mem <<= 30;
case 'M':
case 'm':
        mem <<= 20;
case 'K':
case 'k':
        mem <<= 10;
        /* fall through */



Trailing whitespace should always be removed. Some editors highlight the trailing whitespace and cause visual distractions when editing files. See:


while should follow the closing bracket on the same line:

do {
} while(something);




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


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


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


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


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


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


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


Trailing statements (for example after any conditional) should be on the next line. Like:

if (x == y) break;

should be:

if (x == y)