Linux kernel coding style This is a short document describing the preferred coding style for the linux kernel. Coding style is very personal, and I won't _force_ my views on anybody, but this is what goes for anything that I have to be able to maintain, and I'd prefer it for most other things too. Please at least consider the points made here. First off, I'd suggest printing out a copy of the GNU coding standards, and NOT read it. Burn them, it's a great symbolic gesture. Anyway, here goes: Chapter 1: Indentation 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. 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. Don't put multiple statements on a single line unless you have something to hide: if (condition) do_this; do_something_everytime; 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. Chapter 2: 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 80 columns and this is a hard limit. Statements longer than 80 columns will be broken into sensible chunks. 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. Long strings are as well broken into shorter strings. void fun(int a, int b, int c) { if (condition) printk(KERN_WARNING "Warning this is a long printk with " "3 parameters a: %u b: %u " "c: %u \n", a, b, c); else next_statement; } Chapter 3: Placing Braces 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 } 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); and 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. Chapter 4: Naming 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 next chapter. Chapter 5: Functions 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. Chapter 6: 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. 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; } ... out: kfree(buffer); return result; } Chapter 7: Commenting 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 5 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. When commenting the kernel API functions, please use the kerneldoc format. See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc for details. Chapter 8: 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 linux-c-mode () "C mode with adjusted defaults for use with the Linux kernel." (interactive) (c-mode) (c-set-style "K&R") (setq tab-width 8) (setq indent-tabs-mode t) (setq c-basic-offset 8)) This will define the M-x linux-c-mode command. When hacking on a module, if you put the string -*- linux-c -*- somewhere on the first two lines, this mode will be automatically invoked. Also, you may want to add (setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode) auto-mode-alist)) to your .emacs file if you want to have linux-c-mode switched on automagically when you edit source files under /usr/src/linux. 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. Chapter 9: Configuration-files For configuration options (arch/xxx/Kconfig, and all the Kconfig files), somewhat different indentation is used. Help text is indented with 2 spaces. if CONFIG_EXPERIMENTAL tristate C
/*
 * linux/ipc/util.h
 * Copyright (C) 1999 Christoph Rohland
 *
 * ipc helper functions (c) 1999 Manfred Spraul <manfred@colorfullife.com>
 * namespaces support.      2006 OpenVZ, SWsoft Inc.
 *                               Pavel Emelianov <xemul@openvz.org>
 */

#ifndef _IPC_UTIL_H
#define _IPC_UTIL_H

#include <linux/idr.h>
#include <linux/err.h>

#define USHRT_MAX 0xffff
#define SEQ_MULTIPLIER	(IPCMNI)

void sem_init (void);
void msg_init (void);
void shm_init (void);

struct ipc_namespace;

int sem_init_ns(struct ipc_namespace *ns);
int msg_init_ns(struct ipc_namespace *ns);
int shm_init_ns(struct ipc_namespace *ns);

void sem_exit_ns(struct ipc_namespace *ns);
void msg_exit_ns(struct ipc_namespace *ns);
void shm_exit_ns(struct ipc_namespace *ns);

struct ipc_ids {
	int in_use;
	unsigned short seq;
	unsigned short seq_max;
	struct rw_semaphore rw_mutex;
	struct idr ipcs_idr;
};

/*
 * Structure that holds the parameters needed by the ipc operations
 * (see after)
 */
struct ipc_params {
	key_t key;
	int flg;
	union {
		size_t size;	/* for shared memories */
		int nsems;	/* for semaphores */
	} u;			/* holds the getnew() specific param */
};

/*
 * Structure that holds some ipc operations. This structure is used to unify
 * the calls to sys_msgget(), sys_semget(), sys_shmget()
 *      . routine to call to create a new ipc object. Can be one of newque,
 *        newary, newseg
 *      . routine to call to check permissions for a new ipc object.
 *        Can be one of security_msg_associate, security_sem_associate,
 *        security_shm_associate
 *      . routine to call for an extra check if needed
 */
struct ipc_ops {
	int (*getnew) (struct ipc_namespace *, struct ipc_params *);
	int (*associate) (struct kern_ipc_perm *, int);
	int (*more_checks) (struct kern_ipc_perm *, struct ipc_params *);
};

struct seq_file;

void ipc_init_ids(struct ipc_ids *);
#ifdef CONFIG_PROC_FS
void __init ipc_init_proc_interface(const char *path, const char *header,
		int ids, int (*show)(struct seq_file *, void *));
#else
#define ipc_init_proc_interface(path, header, ids, show) do {} while (0)
#endif

#define IPC_SEM_IDS	0
#define IPC_MSG_IDS	1
#define IPC_SHM_IDS	2

#define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER)

/* must be called with ids->rw_mutex acquired for writing */
int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);

/* must be called with ids->rw_mutex acquired for reading */
int ipc_get_maxid(struct ipc_ids *);

/* must be called with both locks acquired. */
void ipc_rmid(struct ipc_ids *, struct kern_ipc_perm *);

/* must be called with ipcp locked */
int ipcperms(struct kern_ipc_perm *ipcp, short flg);

/* for rare, potentially huge allocations.
 * both function can sleep
 */
void* ipc_alloc(int size);
void ipc_free(void* ptr, int size);

/*
 * For allocation that need to be freed by RCU.
 * Objects are reference counted, they start with reference count 1.
 * getref increases the refcount, the putref call that reduces the recount
 * to 0 schedules the rcu destruction. Caller must guarantee locking.
 */
void* ipc_rcu_alloc(int size);
void ipc_rcu_getref(void *ptr);
void ipc_rcu_putref(void *ptr);

/*
 * ipc_lock_down: called with rw_mutex held
 * ipc_lock: called without that lock held
 */
struct kern_ipc_perm *ipc_lock_down(struct ipc_ids *, int);
struct kern_ipc_perm *ipc_lock(struct ipc_ids *, int);

void kernel_to_ipc64_perm(struct kern_ipc_perm *in, struct ipc64_perm *out);
void ipc64_perm_to_ipc_perm(struct ipc64_perm *in, struct ipc_perm *out);

#if defined(__ia64__) || defined(__x86_64__) || defined(__hppa__) || defined(__XTENSA__)
  /* On IA-64, we always use the "64-bit version" of the IPC structures.  */ 
# define ipc_parse_version(cmd)	IPC_64
#else
int ipc_parse_version (int *cmd);
#endif

extern void free_msg(struct msg_msg *msg);
extern struct msg_msg *load_msg(const void __user *src, int len);
extern int store_msg(void __user *dest, struct msg_msg *msg, int len);
extern int ipcget_new(struct ipc_namespace *, struct ipc_ids *,
			struct ipc_ops *, struct ipc_params *);
extern int ipcget_public(struct ipc_namespace *, struct ipc_ids *,
			struct ipc_ops *, struct ipc_params *);

static inline int ipc_buildid(int id, int seq)
{
	return SEQ_MULTIPLIER * seq + id;
}

/*
 * Must be called with ipcp locked
 */
static inline int ipc_checkid(struct kern_ipc_perm *ipcp, int uid)
{
	if (uid / SEQ_MULTIPLIER != ipcp->seq)
		return 1;
	return 0;
}

static inline void ipc_lock_by_ptr(struct kern_ipc_perm *perm)
{
	rcu_read_lock();
	spin_lock(&perm->lock);
}

static inline void ipc_unlock(struct kern_ipc_perm *perm)
{
	spin_unlock(&perm->lock);
	rcu_read_unlock();
}

static inline struct kern_ipc_perm *ipc_lock_check_down(struct ipc_ids *ids,
						int id)
{
	struct kern_ipc_perm *out;

	out = ipc_lock_down(ids, id);
	if (IS_ERR(out))
		return out;

	if (ipc_checkid(out, id)) {
		ipc_unlock(out);
		return ERR_PTR(-EIDRM);
	}

	return out;
}

static inline struct kern_ipc_perm *ipc_lock_check(struct ipc_ids *ids,
						int id)
{
	struct kern_ipc_perm *out;

	out = ipc_lock(ids, id);
	if (IS_ERR(out))
		return out;

	if (ipc_checkid(out, id)) {
		ipc_unlock(out);
		return ERR_PTR(-EIDRM);
	}

	return out;
}

/**
 * ipcget - Common sys_*get() code
 * @ns : namsepace
 * @ids : IPC identifier set
 * @ops : operations to be called on ipc object creation, permission checks
 *        and further checks
 * @params : the parameters needed by the previous operations.
 *
 * Common routine called by sys_msgget(), sys_semget() and sys_shmget().
 */
static inline int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
			struct ipc_ops *ops, struct ipc_params *params)
{
	if (params->key == IPC_PRIVATE)
		return ipcget_new(ns, ids, ops, params);
	else
		return ipcget_public(ns, ids, ops, params);
}

#endif

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