.. _codingstyle: 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: 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. 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.: .. code-block:: c 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; } Don't put multiple statements on a single line unless you have something to hide: .. code-block:: c if (condition) do_this; do_something_everytime; 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. 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 strongly preferred limit. Statements longer than 80 columns will be broken into sensible chunks, unless exceeding 80 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. 3) 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: .. code-block:: c if (x is true) { we do y } This applies to all non-function statement blocks (if, switch, for, while, do). E.g.: .. code-block:: c switch (action) { case KOBJ_ADD: return "add"; case KOBJ_REMOVE: return "remove"; case KOBJ_CHANGE: return "change"; default: return NULL; } However, there is one special case, namely functions: they have the opening brace at the beginning of the next line, thus: .. code-block:: c 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: .. code-block:: c do { body of do-loop } while (condition); and .. code-block:: c 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. .. code-block:: c if (condition) action(); and .. code-block:: none if (condition) do_this(); else do_that(); 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: .. code-block:: c if (condition) { do_this(); do_that(); } else { otherwise(); } 3.1) Spaces *********** 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., .. code-block:: c s = sizeof(struct file); Do not add spaces around (inside) parenthesized expressions. This example is **bad**: .. code-block:: c 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: .. code-block:: c 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. 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 chapter 6 (Functions). 5) Typedefs ----------- Please don't use things like ``vps_t``. It's a **mistake** to use typedef for structures and pointers. When you see a .. code-block:: c vps_t a; in the source, what does it mean? In contrast, if it says .. code-block:: c 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. 6) 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. 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.: .. code-block:: c int system_is_up(void) { return system_state == SYSTEM_RUNNING; } EXPORT_SYMBOL(system_is_up); 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. 7) 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. Choose label names which say what the goto does or why the goto exists. An example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``. Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to renumber them if you ever add or remove exit paths, and they make correctness difficult to verify anyway. The rationale for using gotos 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 ;) .. code-block:: c int fun(int a) { int result = 0; char *buffer; buffer = kmalloc(SIZE, GFP_KERNEL); if (!buffer) return -ENOMEM; if (condition1) { while (loop1) { ... } result = 1; goto out_buffer; } ... out_free_buffer: kfree(buffer); return result; } A common type of bug to be aware of is ``one err bugs`` which look like this: .. code-block:: c err: kfree(foo->bar); kfree(foo); return ret; The bug in this code is that on some exit paths ``foo`` is NULL. Normally the fix for this is to split it up into two error labels ``err_free_bar:`` and ``err_free_foo:``: .. code-block:: c err_free_bar: kfree(foo->bar); err_free_foo: kfree(foo); return ret; Ideally you should simulate errors to test all exit paths. 8) 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 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. When commenting the kernel API functions, please use the kernel-doc format. See the files Documentation/kernel-documentation.rst and scripts/kernel-doc for details. The preferred style for long (multi-line) comments is: .. code-block:: c /* * This is the preferred style for multi-line * comments in the Linux kernel source code. * Please use it consistently. * * Description: A column of asterisks on the left side, * with beginning and ending almost-blank lines. */ For files in net/ and drivers/net/ the preferred style for long (multi-line) comments is a little different. .. code-block:: c /* The preferred comment style for files in net/ and drivers/net 复制代码
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