Coding Style

This document describes the preferred C coding style for the coreboot project. It is in many ways exactly the same as the Linux kernel coding style. In fact, most of this document has been copied from the Linux kernel coding style

The guidelines in this file should be seen as a strong suggestion, and should overrule personal preference. They may be ignored in individual instances when there are good practical reasons to do so, and reviewers are in agreement.

Any style questions that are not mentioned in here should be decided between the author and reviewers on a case-by-case basis. When modifying existing files, authors should try to match the prevalent style in that file – otherwise, they should generally match similar existing files in coreboot.

Bulk style changes to existing code (“cleanup patches”) should avoid changing existing style choices unless they actually violate this style guide, or there is broad consensus that the new version is an improvement. By default the style choices of the original author should be honored. (Note that is not part of this style guide, and neither is clang-format. These tools can be useful to find potential issues or simplify formatting in new submissions, but they were not designed to directly match this guide and may have false positives. They should not be bulk-applied to change existing code except in cases where they directly match the style guide.)


Tabs are 8 characters, and thus indentations are also 8 characters. There are heretic movements that try to make indentations 4 (or even 2!) characters deep, and that is akin to trying to define the value of PI to be 3.

Rationale: The whole idea behind indentation is to clearly define where a block of control starts and ends. Especially when you’ve been looking at your screen for 20 straight hours, you’ll find it a lot easier to see how the indentation works if you have large indentations.

Now, some people will claim that having 8-character indentations makes the code move too far to the right, and makes it hard to read on a 80-character terminal screen. The answer to that is that if you need more than 3 levels of indentation, you’re screwed anyway, and should fix your program. Note that coreboot has expanded the 80 character limit to 96 characters to allow for modern wider screens.

In short, 8-char indents make things easier to read, and have the added benefit of warning you when you’re nesting your functions too deep. Heed that warning.

The preferred way to ease multiple indentation levels in a switch statement is to align the “switch” and its subordinate “case” labels in the same column instead of “double-indenting” the “case” labels. E.g.:

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

Don’t put multiple statements on a single line unless you have something to hide:

if (condition) do_this;

Don’t put multiple assignments on a single line either. Kernel coding style is super simple. Avoid tricky expressions.

Outside of comments, documentation and except in Kconfig, spaces are never used for indentation, and the above example is deliberately broken.

Get a decent editor and don’t leave whitespace at the end of lines. This will actually keep the patch from being tested in the CI, so patches with ending whitespace cannot be merged.

Breaking long lines and strings

Coding style is all about readability and maintainability using commonly available tools.

The limit on the length of lines is 96 columns and this is a strongly preferred limit.

Statements longer than 96 columns will be broken into sensible chunks, unless exceeding 96 columns significantly increases readability and does not hide information. Descendants are always substantially shorter than the parent and are placed substantially to the right. The same applies to function headers with a long argument list. However, never break user-visible strings such as printk messages, because that breaks the ability to grep for them.

Placing Braces and Spaces

The other issue that always comes up in C styling is the placement of braces. Unlike the indent size, there are few technical reasons to choose one placement strategy over the other, but the preferred way, as shown to us by the prophets Kernighan and Ritchie, is to put the opening brace last on the line, and put the closing brace first, thusly:

if (x is true) {
	we do y

This applies to all non-function statement blocks (if, switch, for, while, do). E.g.:

switch (action) {
case KOBJ_ADD:
	return "add";
	return "remove";
	return "change";
	return NULL;

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

Heretic people all over the world have claimed that this inconsistency is … well … inconsistent, but all right-thinking people know that (a) K&R are right and (b) K&R are right. Besides, functions are special anyway (you can’t nest them in C).

Note that the closing brace is empty on a line of its own, except in the cases where it is followed by a continuation of the same statement, ie a “while” in a do-statement or an “else” in an if-statement, like this:

do {
	body of do-loop
} while (condition);


if (x == y) {
} else if (x > y) {
} else {

Rationale: K&R.

Also, note that this brace-placement also minimizes the number of empty (or almost empty) lines, without any loss of readability. Thus, as the supply of new-lines on your screen is not a renewable resource (think 25-line terminal screens here), you have more empty lines to put comments on.

Do not unnecessarily use braces where a single statement will do.

if (condition)


if (condition)

This does not apply if only one branch of a conditional statement is a single statement; in the latter case use braces in both branches:

if (condition) {
} else {


Linux kernel style for use of spaces depends (mostly) on function-versus-keyword usage. Use a space after (most) keywords. The notable exceptions are sizeof, typeof, alignof, and attribute, which look somewhat like functions (and are usually used with parentheses in Linux, although they are not required in the language, as in: “sizeof info” after “struct fileinfo info;” is declared).

So use a space after these keywords:

if, switch, case, for, do, while

but not with sizeof, typeof, alignof, or attribute. E.g.,

s = sizeof(struct file);

Do not add spaces around (inside) parenthesized expressions. This example is

  • bad*:
s = sizeof( struct file );

When declaring 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);

Use one space around (on each side of) most binary and ternary operators, such as any of these:

=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :

but no space after unary operators:

&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined

no space before the postfix increment & decrement unary operators:

++  --

no space after the prefix increment & decrement unary operators:

++  --

and no space around the ‘.’ and “->” structure member operators.

Do not leave trailing whitespace at the ends of lines. Some editors with “smart” indentation will insert whitespace at the beginning of new lines as appropriate, so you can start typing the next line of code right away. However, some such editors do not remove the whitespace if you end up not putting a line of code there, such as if you leave a blank line. As a result, you end up with lines containing trailing whitespace.

Git will warn you about patches that introduce trailing whitespace, and can optionally strip the trailing whitespace for you; however, if applying a series of patches, this may make later patches in the series fail by changing their context lines.


C is a Spartan language, and so should your naming be. Unlike Modula-2 and Pascal programmers, C programmers do not use cute names like ThisVariableIsATemporaryCounter. A C programmer would call that variable “tmp”, which is much easier to write, and not the least more difficult to understand.

HOWEVER, while mixed-case names are frowned upon, descriptive names for global variables are a must. To call a global function “foo” is a shooting offense.

GLOBAL variables (to be used only if you really need them) need to have descriptive names, as do global functions. If you have a function that counts the number of active users, you should call that “count_active_users()” or similar, you should not call it “cntusr()”.

Encoding the type of a function into the name (so-called Hungarian notation) is brain damaged - the compiler knows the types anyway and can check those, and it only confuses the programmer. No wonder MicroSoft makes buggy programs.

LOCAL variable names should be short, and to the point. If you have some random integer loop counter, it should probably be called “i”. Calling it “loop_counter” is non-productive, if there is no chance of it being mis-understood. Similarly, “tmp” can be just about any type of variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another problem, which is called the function-growth-hormone-imbalance syndrome. See chapter 6 (Functions).


Please don’t use things like “vps_t”.

It’s a mistake to use typedef for structures and pointers. When you see a

vps_t a;

in the source, what does it mean?

In contrast, if it says

struct virtual_container *a;

you can actually tell what “a” is.

Lots of people think that typedefs “help readability”. Not so. They are useful only for:

(a) totally opaque objects (where the typedef is actively used to hide what the object is).

Example: “pte_t” etc. opaque objects that you can only access using the proper accessor functions.

NOTE! Opaqueness and “accessor functions” are not good in themselves. The reason we have them for things like pte_t etc. is that there really is absolutely zero portably accessible information there.

(b) Clear integer types, where the abstraction helps avoid confusion whether it is “int” or “long”.

u8/u16/u32 are perfectly fine typedefs, although they fit into category (d) better than here.

NOTE! Again - there needs to be a reason for this. If something is “unsigned long”, then there’s no reason to do

typedef unsigned long myflags_t;

but if there is a clear reason for why it under certain circumstances might be an “unsigned int” and under other configurations might be “unsigned long”, then by all means go ahead and use a typedef.

(c) when you use sparse to literally create a new type for type-checking.

(d) New types which are identical to standard C99 types, in certain exceptional circumstances.

Although it would only take a short amount of time for the eyes and brain to become accustomed to the standard types like ‘uint32_t’, some people object to their use anyway.

Therefore, the Linux-specific ‘u8/u16/u32/u64’ types and their signed equivalents which are identical to standard types are permitted – although they are not mandatory in new code of your own.

When editing existing code which already uses one or the other set of types, you should conform to the existing choices in that code.

(e) Types safe for use in userspace.

In certain structures which are visible to userspace, we cannot require C99 types and cannot use the ‘u32’ form above. Thus, we use __u32 and similar types in all structures which are shared with userspace.

Maybe there are other cases too, but the rule should basically be to NEVER EVER use a typedef unless you can clearly match one of those rules.

In general, a pointer, or a struct that has elements that can reasonably be directly accessed should never be a typedef.


Functions should be short and sweet, and do just one thing. They should fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the complexity and indentation level of that function. So, if you have a conceptually simple function that is just one long (but simple) case-statement, where you have to do lots of small things for a lot of different cases, it’s OK to have a longer function.

However, if you have a complex function, and you suspect that a less-than-gifted first-year high-school student might not even understand what the function is all about, you should adhere to the maximum limits all the more closely. Use helper functions with descriptive names (you can ask the compiler to in-line them if you think it’s performance-critical, and it will probably do a better job of it than you would have done).

Another measure of the function is the number of local variables. They shouldn’t exceed 5-10, or you’re doing something wrong. Re-think the function, and split it into smaller pieces. A human brain can generally easily keep track of about 7 different things, anything more and it gets confused. You know you’re brilliant, but maybe you’d like to understand what you did 2 weeks from now.

In source files, separate functions with one blank line. If the function is exported, the EXPORT* macro for it should follow immediately after the closing function brace line. E.g.:

int system_is_up(void)
	return system_state == SYSTEM_RUNNING;

In function prototypes, include parameter names with their data types. Although this is not required by the C language, it is preferred in Linux because it is a simple way to add valuable information for the reader.

Centralized exiting of functions

Albeit deprecated by some people, the equivalent of the goto statement is used frequently by compilers in form of the unconditional jump instruction.

The goto statement comes in handy when a function exits from multiple locations and some common work such as cleanup has to be done. If there is no cleanup needed then just return directly.

The rationale is:

  • unconditional statements are easier to understand and follow
  • nesting is reduced
  • errors by not updating individual exit points when making modifications are prevented
  • saves the compiler work to optimize redundant code away ;)
int fun(int a)
	int result = 0;
	char *buffer = kmalloc(SIZE);

	if (buffer == NULL)
		return -ENOMEM;

	if (condition1) {
		while (loop1) {
		result = 1;
		goto out;
	return result;


Comments are good, but there is also a danger of over-commenting. NEVER try to explain HOW your code works in a comment: it’s much better to write the code so that the working is obvious, and it’s a waste of time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably go back to chapter 6 for a while. You can make small comments to note or warn about something particularly clever (or ugly), but try to avoid excess. Instead, put the comments at the head of the function, telling people what it does, and possibly WHY it does it.

coreboot style for comments is the C89 “/* … */” style. You may also use C99-style “// …” comments for single-line comments.

The preferred style for short (multi-line) comments is:

/* This is the preferred style for short multi-line
   comments in the coreboot source code.
   Please use it consistently. */

The preferred style for long (multi-line) comments is:

 * This is the preferred style for multi-line
 * comments in the coreboot source code.
 * Please use it consistently.
 * Description:  A column of asterisks on the left side,
 * with beginning and ending almost-blank lines.

It’s also important to comment data, whether they are basic types or derived types. To this end, use just one data declaration per line (no commas for multiple data declarations). This leaves you room for a small comment on each item, explaining its use.

You’ve made a mess of it

That’s OK, we all do. You’ve probably been told by your long-time Unix user helper that “GNU emacs” automatically formats the C sources for you, and you’ve noticed that yes, it does do that, but the defaults it uses are less than desirable (in fact, they are worse than random typing - an infinite number of monkeys typing into GNU emacs would never make a good program).

So, you can either get rid of GNU emacs, or change it to use saner values. To do the latter, you can stick the following in your .emacs file:

(defun c-lineup-arglist-tabs-only (ignored)
  "Line up argument lists by tabs, not spaces"
  (let* ((anchor (c-langelem-pos c-syntactic-element))
	 (column (c-langelem-2nd-pos c-syntactic-element))
	 (offset (- (1+ column) anchor))
	 (steps (floor offset c-basic-offset)))
    (* (max steps 1)

(add-hook 'c-mode-common-hook
          (lambda ()
            ;; Add kernel style
             '("linux" (c-offsets-alist

(add-hook 'c-mode-hook
          (lambda ()
            (let ((filename (buffer-file-name)))
              ;; Enable kernel mode for the appropriate files
              (when (and filename
                         (string-match (expand-file-name "~/src/linux-trees")
                (setq indent-tabs-mode t)
                (c-set-style "linux-tabs-only")))))

This will make emacs go better with the kernel coding style for C files below ~/src/linux-trees. Obviously, this should be updated to match your own paths for coreboot.

But even if you fail in getting emacs to do sane formatting, not everything is lost: use “indent”.

Now, again, GNU indent has the same brain-dead settings that GNU emacs has, which is why you need to give it a few command line options. However, that’s not too bad, because even the makers of GNU indent recognize the authority of K&R (the GNU people aren’t evil, they are just severely misguided in this matter), so you just give indent the options “-kr -i8” (stands for “K&R, 8 character indents”), or use “scripts/Lindent”, which indents in the latest style.

“indent” has a lot of options, and especially when it comes to comment re-formatting you may want to take a look at the man page. But remember: “indent” is not a fix for bad programming.

Kconfig configuration files

For all of the Kconfig* configuration files throughout the source tree, the indentation is somewhat different. Lines under a “config” definition are indented with one tab, while help text is indented an additional two spaces. Example:

config AUDIT
	bool "Auditing support"
	depends on NET
	  Enable auditing infrastructure that can be used with another
	  kernel subsystem, such as SELinux (which requires this for
	  logging of avc messages output).  Does not do system-call
	  auditing without CONFIG_AUDITSYSCALL.

Seriously dangerous features (such as write support for certain filesystems) should advertise this prominently in their prompt string:

config ADFS_FS_RW
	bool "ADFS write support (DANGEROUS)"
	depends on ADFS_FS

For full documentation on the configuration files, see the file Documentation/kbuild/kconfig-language.txt.

Macros, Enums and RTL

Names of macros defining constants and labels in enums are capitalized.

#define CONSTANT 0x12345

Enums are preferred when defining several related constants.

CAPITALIZED macro names are appreciated but macros resembling functions may be named in lower case.

Generally, inline functions are preferable to macros resembling functions.

Macros with multiple statements should be enclosed in a do - while block:

#define macrofun(a, b, c)   \
    do {                    \
        if (a == 5)         \
            do_this(b, c);  \
    } while (0)

Things to avoid when using macros:

  1. macros that affect control flow:
#define FOO(x)              \
    do {                        \
	    if (blah(x) < 0)        \
		    return -EBUGGERED;  \
    } while(0)

is a very bad idea. It looks like a function call but exits the “calling” function; don’t break the internal parsers of those who will read the code.

  1. macros that depend on having a local variable with a magic name:
#define FOO(val) bar(index, val)

might look like a good thing, but it’s confusing as hell when one reads the code and it’s prone to breakage from seemingly innocent changes.

  1. macros with arguments that are used as l-values: FOO(x) = y; will bite you if somebody e.g. turns FOO into an inline function.
  2. forgetting about precedence: macros defining constants using expressions must enclose the expression in parentheses. Beware of similar issues with macros using parameters.
#define CONSTANT 0x4000

The cpp manual deals with macros exhaustively. The gcc internals manual also covers RTL which is used frequently with assembly language in the kernel.

Printing coreboot messages

coreboot developers like to be seen as literate. Do mind the spelling of coreboot messages to make a good impression. Do not use crippled words like “dont”; use “do not” or “don’t” instead. Make the messages concise, clear, and unambiguous.

coreboot messages do not have to be terminated with a period.

Printing numbers in parentheses (%d) adds no value and should be avoided.

Allocating memory

coreboot provides a single general purpose memory allocator: malloc()

The preferred form for passing a size of a struct is the following:

p = malloc(sizeof(*p));

The alternative form where struct name is spelled out hurts readability and introduces an opportunity for a bug when the pointer variable type is changed but the corresponding sizeof that is passed to a memory allocator is not.

Casting the return value which is a void pointer is redundant. The conversion from void pointer to any other pointer type is guaranteed by the C programming language.

You should contain your memory usage to stack variables whenever possible. Only use malloc() as a last resort. In ramstage, you may also be able to get away with using static variables. Never use malloc() outside of ramstage.

Since coreboot only runs for a very short time, there is no memory deallocator, although a corresponding free() is offered. It is a no-op. Use of free() is not required though it is accepted. It is useful when sharing code with other codebases that make use of free().

The inline disease

There appears to be a common misperception that gcc has a magic “make me faster” speedup option called “inline”. While the use of inlines can be appropriate (for example as a means of replacing macros, see Chapter 12), it very often is not.

A reasonable rule of thumb is to not put inline at functions that have more than 3 lines of code in them. An exception to this rule are the cases where a parameter is known to be a compile time constant, and as a result of this constantness you know the compiler will be able to optimize most of your function away at compile time. For a good example of this later case, see the kmalloc() inline function.

Often people argue that adding inline to functions that are static and used only once is always a win since there is no space tradeoff. While this is technically correct, gcc is capable of inlining these automatically without help, and the maintenance issue of removing the inline when a second user appears outweighs the potential value of the hint that tells gcc to do something it would have done anyway.

Function return values and names

Functions can return values of many different kinds, and one of the most common is a value indicating whether the function succeeded or failed. Such a value can be represented as an error-code integer (CB_ERR_xxx (negative number) = failure, CB_SUCCESS (0) = success) or a “succeeded” boolean (0 = failure, non-zero = success).

Mixing up these two sorts of representations is a fertile source of difficult-to-find bugs. If the C language included a strong distinction between integers and booleans then the compiler would find these mistakes for us… but it doesn’t. To help prevent such bugs, always follow this convention:

If the name of a function is an action or an imperative command, the function should return an error-code integer.  If the name is a predicate, the function should return a ”succeeded” boolean.

For example, “add work” is a command, and the add_work() function returns 0 for success or CB_ERR for failure. In the same way, “PCI device present” is a predicate, and the pci_dev_present() function returns 1 if it succeeds in finding a matching device or 0 if it doesn’t.

Functions whose return value is the actual result of a computation, rather than an indication of whether the computation succeeded, are not subject to this rule. Generally they indicate failure by returning some out-of-range result. Typical examples would be functions that return pointers; they use NULL to report failure.

Error handling, assertions and die()

As firmware, coreboot has no means to let the user interactively fix things when something goes wrong. We either succeed to boot or the device becomes a brick that must be recovered through complicated external means (e.g. a flash programmer). Therefore, coreboot code should strive to continue booting wherever possible.

In most cases, errors should be handled by logging a message of at least BIOS_ERR level, returning out of the function stack for the failed feature, and then continuing execution. For example, if a function reading the EDID of an eDP display panel encounters an I2C error, it should print a “cannot read EDID” message and return an error code. The calling display initialization function knows that without the EDID there is no way to initialize the display correctly, so it will also immediately return with an error code without running its remaining code that would initialize the SoC’s display controller. Execution returns further up the function stack to the mainboard initialization code which continues booting despite the failed display initialization, since display functionality is non-essential to the system. (Code is encouraged but not required to use enum cb_err error codes to return these errors.)

coreboot also has the die() function that completely halts execution. die() should only be used as a last resort, since it results in the worst user experience (bricked system). It is generally preferrable to continue executing even after a problem was encountered that might be fatal (e.g. SPI clock couldn’t be configured correctly), because a slight chance of successfully booting is still better than not booting at all. The only cases where die() should be used are:

  1. There is no (simple) way to continue executing. For example, when loading the next stage from SPI flash fails, we don’t have any more code to execute. When memory initialization fails, we have no space to load the ramstage into.
  2. Continuing execution would pose a security risk. All security features in coreboot are optional, but when they are configured in the user must be able to rely on them. For example, if CBFS verification is enabled and the file hash when loading the romstage doesn’t match what it should be, it is better to stop execution than to jump to potentially malicious code.

In addition to normal error logging with printk(), coreboot also offers the assert() macro. assert() should be used judiciously to confirm that conditions are true which the programmer knows to be true, in order to catch programming errors and incorrect assumptions. It is therefore different from a normal if ()-check that is used to actually test for things which may turn out to be true or false based on external conditions. For example, anything that involves communicating with hardware, such as whether an attempt to read from SPI flash succeeded, should not use assert() and should instead just be checked with a normal if () and subsequent manual error handling. Hardware can always fail for various reasons and the programmer can never 100% assume in advance that it will work as expected. On the other hand, if a function takes a pointer parameter ctx and the contract for that function (as documented in a comment above its declaration) specifies that this parameter should point to a valid context structure, then adding an assert(ctx) line to that function may be a good idea. The programmer knows that this function should never be called with a NULL pointer (because that’s how it is specified), and if it was actually called with a NULL pointer that would indicate a programming error on account of the caller.

assert() can be configured to either just print an error message and continue execution (default), or call die() (when CONFIG_FATAL_ASSERTS is set). Developers are encouraged to always test their code with this option enabled to make assertion errors (and therefore bugs) more easy to notice. Since assertions thus do not always stop execution, they should never be relied upon to be the sole guard against conditions that really need to stop execution (e.g. security guarantees should never be enforced only by assert()).

Headers and includes

Headers should always be included at the top of the file. Includes should always use the #include <file.h> notation, except for rare cases where a file in the same directory that is not part of a normal include path gets included (e.g. local headers in mainboard directories), which should use #include "file.h". Local “file.h” includes should always come separately after all <file.h> includes. Headers that can be included from both assembly files and .c files should keep all C code wrapped in #ifndef __ASSEMBLER__ blocks, including includes to other headers that don’t follow that provision. Where a specific include order is required for technical reasons, it should be clearly documented with comments. This should not be the norm.

Files should generally include every header they need a definition from directly (and not include any unnecessary extra headers). Excepted from this are certain headers that intentionally chain-include other headers which logically belong to them and are just factored out into a separate location for implementation or organizatory reasons. This could be because part of the definitions is generic and part SoC-specific (e.g. <gpio.h> chain-including <soc/gpio.h>), architecture-specific (e.g. <device/mmio.h> chain-including <arch/mmio.h>), separated out into commonlib[/bsd] for sharing/license reasons (e.g. <cbfs.h> chain-including <commonlib/bsd/cbfs_serialized.h>) or just split out to make organizing subunits of a larger header easier. This can also happen when certain definitions need to be in a specific header for legacy POSIX reasons but we would like to logically group them together (e.g. uintptr_t is in <stdint.h> and size_t in <stddef.h>, but it’s nicer to be able to just include <types.h> and get all the common type and helper function stuff we need everywhere).

The headers <kconfig.h>, <rules.h> and <commonlib/bsd/compiler.h> are always automatically included in all compilation units by the build system and should not be included manually.

Don’t re-invent common macros

The header file src/commonlib/bsd/include/commonlib/bsd/helpers.h contains a number of macros that you should use, rather than explicitly coding some variant of them yourself. For example, if you need to calculate the length of an array, take advantage of the macro

#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

Editor modelines and other cruft

Some editors can interpret configuration information embedded in source files, indicated with special markers. For example, emacs interprets lines marked like this:

-*- mode: c -*-

Or like this:

Local Variables:
compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"

Vim interprets markers that look like this:

/* vim:set sw=8 noet */

Do not include any of these in source files. People have their own personal editor configurations, and your source files should not override them. This includes markers for indentation and mode configuration. People may use their own custom mode, or may have some other magic method for making indentation work correctly.

Inline assembly

In architecture-specific code, you may need to use inline assembly to interface with CPU or platform functionality. Don’t hesitate to do so when necessary. However, don’t use inline assembly gratuitously when C can do the job. You can and should poke hardware from C when possible.

Consider writing simple helper functions that wrap common bits of inline assembly, rather than repeatedly writing them with slight variations. Remember that inline assembly can use C parameters.

Large, non-trivial assembly functions should go in .S files, with corresponding C prototypes defined in C header files. The C prototypes for assembly functions should use “asmlinkage”.

You may need to mark your asm statement as volatile, to prevent GCC from removing it if GCC doesn’t notice any side effects. You don’t always need to do so, though, and doing so unnecessarily can limit optimization.

When writing a single inline assembly statement containing multiple instructions, put each instruction on a separate line in a separate quoted string, and end each string except the last with nt to properly indent the next instruction in the assembly output:

asm ("magic %reg1, #42nt"
	"more_magic %reg2, %reg3"
	: /* outputs */ : /* inputs */ : /* clobbers */);

GCC extensions

GCC is the only officially-supported compiler for coreboot, and a variety of its C language extensions are heavily used throughout the code base. There have been occasional attempts to add clang as a second compiler option, which is generally compatible to the same language extensions that have been long-established by GCC.

Some GCC extensions (e.g. inline assembly) are basically required for proper firmware development. Others enable more safe or flexible coding patterns than can be expressed with standard C (e.g. statement expressions and typeof() to avoid double evaluation in macros like MAX()). Yet others just add some simple convenience and reduce boilerplate (e.g. void * arithmetic).

Since some GCC extensions are necessary either way, there is no gain from avoiding other GCC extensions elsewhere. The use of all official GCC extensions is expressly allowed within coreboot. In cases where an extension can be replaced by a 100% equivalent C standard feature with no extra boilerplate or loss of readability, the C standard feature should be preferred (this usually only happens when GCC retains an older pre-standardization extension for backwards compatibility, e.g. the old pre-C99 syntax for designated initializers). But if there is any advantage offered by the GCC extension (e.g. using GCC zero-length arrays instead of C99 variable-length arrays because they don’t inhibit sizeof()), there is no reason to deprive ourselves of that, and “this is not C standard compliant” should not be a reason to argue against its use in reviews.

This rule only applies to explicit GCC extensions listed in the “Extensions to the C Language Family” section of the GCC manual. Code should never rely on incidental GCC translation behavior that is not explicitly documented as a feature and could change at any moment.


Because refactoring existing code can add bugs to tested code, any refactors should be done only with serious consideration. Refactoring for style differences should only be done if the existing style conflicts with a documented coreboot guideline. If you believe that the style should be modified, the pros and cons can be discussed on the mailing list and in the coreboot leadership meeting.

Similarly, the original author should be respected. Changing working code simply because of a stylistic disagreement is prohibited. This is not saying that refactors that are objectively better (simpler, faster, easier to understand) are not allowed, but there has to be a definite improvement, not simply stylistic changes.

Basically, when refactoring code, there should be a clear benefit to the project and codebase. The reviewers and submitters get to make the call on how to interpret this.

When refactoring, adding unit tests to verify that the post-change functionality matches or improves upon pre-change functionality is encouraged.


The C Programming Language, Second Edition by Brian W. Kernighan and Dennis M. Ritchie. Prentice Hall, Inc., 1988. ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). URL: or

The Practice of Programming by Brian W. Kernighan and Rob Pike. Addison-Wesley, Inc., 1999. ISBN 0-201-61586-X. URL: or

GNU manuals - where in compliance with K&R and this text - for cpp, gcc, gcc internals and indent, all available from

WG14 is the international standardization working group for the programming language C, URL:

Kernel CodingStyle, by at OLS 2002: