diff --git a/doc/dev_guide/c_style_guide.rst b/doc/dev_guide/c_style_guide.rst index e8aebf7141..2efd655bef 100644 --- a/doc/dev_guide/c_style_guide.rst +++ b/doc/dev_guide/c_style_guide.rst @@ -1,151 +1,48 @@ -------------------------------------------------------------------------------- - C Style Guide -------------------------------------------------------------------------------- -The project's coding style is based on a version of the Linux kernel coding style. - -The latest version of the Linux style can be found at: -http://www.kernel.org/doc/Documentation/CodingStyle - -Since it is open for changes, the version of style that we follow, -one from 2007-July-13, will be also copied later in this document. - -There are a few additional guidelines, either unique -to Tarantool or deviating from the Kernel guidelines. - -A. Chapters 10 "Kconfig configuration files", 11 "Data structures", - 13 "Printing kernel messages", 14 "Allocating memory" and 17 - "Don't re-invent the kernel macros" do not apply, since they are - specific to Linux kernel programming environment. - -B. The rest of Linux Kernel Coding Style is amended as follows: - -=========================================================== - General guidelines -=========================================================== +C Style Guide +============= We use Git for revision control. The latest development is happening in the -default branch (currently ``master``). -Our git repository is hosted on github, and can be checked out with -``git clone git://github.com/tarantool/tarantool.git`` (anonymous read-only access). +default branch (currently ``master``). Our git repository is hosted on GitHub, +and can be checked out with ``git clone git://github.com/tarantool/tarantool.git`` +(anonymous read-only access). -If you have any questions about Tarantool internals, please post them on the -developer discussion list, https://groups.google.com/forum/#!forum/tarantool. However, -please be warned: Launchpad silently deletes posts from non-subscribed members, -thus please be sure to have subscribed to the list prior to posting. Additionally, -some engineers are always present on #tarantool channel on irc.freenode.net. +If you have any questions about Tarantool internals, please post them on +`StackOverflow `_ or +ask Tarantool developers directly in `telegram `_. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Commenting style -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +**General guidelines** -Use Doxygen comment format, Javadoc flavor, i.e. `@tag` rather than `\\tag`. -The main tags in use are @param, @retval, @return, @see, @note and @todo. +The project's coding style is inspired by the `Linux kernel coding style +`_. -Every function, except perhaps a very short and obvious one, should have a -comment. A sample function comment may look like below: +However, we have some additional guidelines, either unique to Tarantool or +deviating from the Kernel guidelines. Below we rewrite the Linux kernel +coding style according to the Tarantool's style features. -.. code-block:: c - - /** Write all data to a descriptor. - * - * This function is equivalent to 'write', except it would ensure - * that all data is written to the file unless a non-ignorable - * error occurs. - * - * @retval 0 Success - * - * @retval 1 An error occurred (not EINTR) - * / - static int - write_all(int fd, void \*data, size_t len); - -Public structures and important structure members should be commented as well. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Header files -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Use header guards. Put the header guard in the first line in the header, -before the copyright or declarations. Use all-uppercase name for the header -guard. Derive the header guard name from the file name, and append _INCLUDED -to get a macro name. For example, core/log_io.h -> CORE_LOG_IO_H_INCLUDED. In -``.c`` (implementation) file, include the respective declaration header before all -other headers, to ensure that the header is self- sufficient. Header "header.h" -is self-sufficient if the following compiles without errors: - -.. code-block:: c - - #include "header.h" - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Allocating memory -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Prefer the supplied slab (salloc) and pool (palloc) allocators to malloc()/free() -for any performance-intensive or large memory allocations. Repetitive use of -malloc()/free() can lead to memory fragmentation and should therefore be avoided. - -Always free all allocated memory, even allocated at start-up. We aim at being -valgrind leak-check clean, and in most cases it's just as easy to free() the -allocated memory as it is to write a valgrind suppression. Freeing all allocated -memory is also dynamic-load friendly: assuming a plug-in can be dynamically loaded -and unloaded multiple times, reload should not lead to a memory leak. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Function naming -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Our convention is to use: - -* ``new``/``delete`` for functions which - allocate + initialize and destroy + deallocate an object, -* ``create``/``destroy`` for functions which initialize/destroy an object - but do not handle memory management, -* ``init``/``free`` for functions which initialize/destroy libraries and subsystems. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Other -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Select GNU C99 extensions are acceptable. It's OK to mix declarations and statements, -use true and false. - -The not-so-current list of all GCC C extensions can be found at: -http://gcc.gnu.org/onlinedocs/gcc-4.3.5/gcc/C-Extensions.html - -=========================================================== - Linux kernel coding style -=========================================================== +Tarantool 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: +Tarantool developers and contributors. We **insist** on following these rules +in order to make our code consistent and understandable to any developer. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 1: Indentation -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +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. +Tabs are 8 characters (8-width tabs, not 8 whitespaces), 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 +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 +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. @@ -154,231 +51,230 @@ 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.: +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 +.. code-block:: c switch (suffix) { case 'G': case 'g': - mem <<= 30; - break; + mem <<= 30; + break; case 'M': case 'm': - mem <<= 20; - break; + mem <<= 20; + break; case 'K': case 'k': - mem <<= 10; - /* fall through */ + mem <<= 10; + /* fall through */ default: - break; + break; } - Don't put multiple statements on a single line unless you have something to hide: -.. code-block:: none +.. 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. +Don't put multiple assignments on a single line either. Avoid tricky expressions. -Outside of comments, documentation and except in Kconfig, spaces are never +Outside of comments and documentation, 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 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +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 strongly +The limit on the length of lines is 80 columns and this is a strongly preferred limit. As for comments, the same limit of 80 columns is applied. -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. The -only exception to this is where exceeding 80 columns significantly increases -readability and does not hide information. - -.. code-block:: c +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. - 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 and Spaces -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 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 +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: +brace last on the line, and put the closing brace first, thus: -.. code-block:: none +.. code-block:: c if (x is true) { - we do y + we do y } This applies to all non-function statement blocks (if, switch, for, -while, do). e.g.: +while, do). E.g.: -.. code-block:: c +.. code-block:: c switch (action) { case KOBJ_ADD: - return "add"; + return "add"; case KOBJ_REMOVE: - return "remove"; + return "remove"; case KOBJ_CHANGE: - return "change"; + return "change"; default: - return NULL; + 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 +.. code-block:: c - int function(int x) + int + function(int x) { - body of function; + 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 +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 +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 +ie a ``while`` in a do-statement or an ``else`` in an if-statement, like this: -.. code-block:: c +.. code-block:: c do { - body of do-loop; + body of do-loop } while (condition); and -.. code-block:: c +.. 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 +(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 +.. code-block:: c + + if (condition) + action(); + +and + +.. code-block:: c if (condition) - action(); + do_this(); + else + do_that(); -This does not apply if one branch of a conditional statement is a single -statement. Use braces in both branches. +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 +.. code-block:: c if (condition) { - do_this(); - do_that(); + do_this(); + do_that(); } else { - otherwise(); + otherwise(); } -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 3.1: Spaces -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 3.1: Spaces +^^^^^^^^^^^^^^^^^^^ + +Tarantool 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, +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: + +.. code-block:: c -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). + if, switch, case, for, do, while -So use a space after these keywords: if, switch, case, for, do, while -but not with sizeof, typeof, alignof, or __attribute__. E.g., +but not with ``sizeof``, ``typeof``, ``alignof``, or ``__attribute__``. E.g., -.. code-block:: c +.. code-block:: c s = sizeof(struct file); Do not add spaces around (inside) parenthesized expressions. This example is **bad**: -.. code-block:: c +.. 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: +preferred use of ``*`` is adjacent to the data name or function name and not +adjacent to the type name. Examples: -.. code-block:: c +.. 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: +such as any of these:: = + - < > * / % | & ^ <= >= == != ? : -but no space after unary operators: +but no space after unary operators:: & * + - ~ ! sizeof typeof alignof __attribute__ defined -no space before the postfix increment & decrement unary operators: +no space before the postfix increment & decrement unary operators:: ++ -- -no space after the prefix increment & decrement unary operators: +no space after the prefix increment & decrement unary operators:: ++ -- -and no space around the '.' and "->" structure member 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 +Do not split a cast operator from its argument with a whitespace, +e.g. ``(ssize_t)inj->iparam``. + +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, +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 @@ -386,136 +282,125 @@ 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. +Chapter 4: Naming +~~~~~~~~~~~~~~~~~ -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 4: Naming -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -C is a Spartan language, and so should your naming be. Unlike Modula-2 +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 +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 +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 +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()". +``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 +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 +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). +For function naming we have a convention is to use: -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 5: Typedefs -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +* ``new``/``delete`` for functions which + allocate + initialize and destroy + deallocate an object, +* ``create``/``destroy`` for functions which initialize/destroy an object + but do not handle memory management, +* ``init``/``free`` for functions which initialize/destroy libraries and subsystems. -Please don't use things like "vps_t". +Chapter 5: Typedefs +~~~~~~~~~~~~~~~~~~~ -It's a _mistake_ to use typedef for structures and pointers. When you see a +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 +.. code-block:: c vps_t a; in the source, what does it mean? - In contrast, if it says -.. code-block:: c +.. code-block:: c struct virtual_container *a; -you can actually tell what "a" is. +you can actually tell what ``a`` is. -Lots of people think that typedefs "help readability". Not so. They are +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_ +#. 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 + 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. + .. note:: -(b) Clear integer types, where the abstraction _helps_ avoid confusion - whether it is "int" or "long". + 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. - u8/u16/u32 are perfectly fine typedefs, although they fit into - category (d) better than here. +#. Clear integer types, where the abstraction **helps** avoid confusion + whether it is ``int`` or ``long``. - NOTE! Again - there needs to be a _reason_ for this. If something is - "unsigned long", then there's no reason to do + u8/u16/u32 are perfectly fine typedefs, although they fit into + point 4 better than here. - .. code-block:: c + .. 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. + 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 +#. 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 +#. 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', + 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. +Chapter 6: Functions +~~~~~~~~~~~~~~~~~~~~ -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 6: Functions -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Functions should be short and sweet, and do just one thing. They should +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 +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. @@ -523,45 +408,41 @@ 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 +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 +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 confu/sed. You know you're brilliant, but maybe you'd like +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 +Although this is not required by the C language, it is preferred in Tarantool because it is a simple way to add valuable information for the reader. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 7: Centralized exiting of functions -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note that we place the function return type on the line before the name and signature. + +Chapter 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. +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 is: +The rationale for using gotos is: - unconditional statements are easier to understand and follow - nesting is reduced @@ -569,219 +450,139 @@ The rationale is: modifications are prevented - saves the compiler work to optimize redundant code away ;) -.. code-block:: c +.. code-block:: c - int fun(int a) + int + fun(int a) { - int result = 0; - char *buffer = kmalloc(SIZE); - - if (buffer == NULL) - return -ENOMEM; - - if (condition1) { - while (loop1) { - ... - } - result = 1; - goto out; + int result = 0; + char *buffer; + + buffer = kmalloc(SIZE, GFP_KERNEL); + if (!buffer) + return -ENOMEM; + + if (condition1) { + while (loop1) { + ... } - ... - out: - kfree(buffer); - return result; + result = 1; + goto out_free_buffer; + } + ... + out_free_buffer: + kfree(buffer); + return result; } -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 8: Commenting -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +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. + +Chapter 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 +write the code so that the **working** is obvious, and it's a waste of time to explain badly written code. -с + 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 +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 +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-doc-nano-HOWTO.txt and scripts/kernel-doc -for details. - -Linux style for comments is the C89 :code:`/* ... */`` style. -Don't use C99-style :code:`// ...` comments. +When commenting the Tarantool C API functions, please use Doxygen comment format, +Javadoc flavor, i.e. `@tag` rather than `\\tag`. +The main tags in use are ``@param``, ``@retval``, ``@return``, ``@see``, +``@note`` and ``@todo``. -The preferred style for long (multi-line) comments is: +Every function, except perhaps a very short and obvious one, should have a +comment. A sample function comment may look like below: -.. code-block:: c +.. code-block:: c - /* - * This is the preferred style for multi-line - * comments in the Linux kernel source code. - * Please use it consistently. + /** + * Write all data to a descriptor. + * + * This function is equivalent to 'write', except it would ensure + * that all data is written to the file unless a non-ignorable + * error occurs. * - * Description: A column of asterisks on the left side, - * with beginning and ending almost-blank lines. + * @retval 0 Success + * @retval 1 An error occurred (not EINTR) + */ + static int + write_all(int fd, void *data, size_t len); + +It's also important to comment data types, whether they are basic types or +derived ones. 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. + +Public structures and important structure members should be commented as well. + +In C comments out of functions and inside of functions should be different in +how they are started. Everything else is wrong. Below are correct examples. +``/**`` comes for documentation comments, ``/*`` for local not documented comments. +However the difference is vague already, so the rule is simple: +out of function use ``/**``, inside use ``/*``. + +.. code-block:: c + + /** + * Out of function comment, option 1. */ -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. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 9: 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: - -.. code-block:: lisp - - (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) - c-basic-offset))) - - (add-hook 'c-mode-common-hook - (lambda () - ;; Add kernel style - (c-add-style - "linux-tabs-only" - '("linux" (c-offsets-alist - (arglist-cont-nonempty - c-lineup-gcc-asm-reg - c-lineup-arglist-tabs-only)))))) - - (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") - filename)) - (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. - -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 10: 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: - -.. code-block:: kconfig - - config AUDIT - bool "Auditing support" - depends on NET - help - 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. - -Features that might still be considered unstable should be defined as -dependent on "EXPERIMENTAL": - -.. code-block:: kconfig - - config SLUB - depends on EXPERIMENTAL && !ARCH_USES_SLAB_PAGE_STRUCT - bool "SLUB (Unqueued Allocator)" - ... - -while seriously dangerous features (such as write support for certain -filesystems) should advertise this prominently in their prompt string: - -.. code-block:: kconfig - - 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. - - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 11: Data structures -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Data structures that have visibility outside the single-threaded -environment they are created and destroyed in should always have -reference counts. In the kernel, garbage collection doesn't exist (and -outside the kernel garbage collection is slow and inefficient), which -means that you absolutely _have_ to reference count all your uses. - -Reference counting means that you can avoid locking, and allows multiple -users to have access to the data structure in parallel - and not having -to worry about the structure suddenly going away from under them just -because they slept or did something else for a while. - -Note that locking is _not_ a replacement for reference counting. -Locking is used to keep data structures coherent, while reference -counting is a memory management technique. Usually both are needed, and -they are not to be confused with each other. - -Many data structures can indeed have two levels of reference counting, -when there are users of different "classes". The subclass count counts -the number of subclass users, and decrements the global count just once -when the subclass count goes to zero. - -Examples of this kind of "multi-level-reference-counting" can be found in -memory management ("struct mm_struct": mm_users and mm_count), and in -filesystem code ("struct super_block": s_count and s_active). - -Remember: if another thread can find your data structure, and you don't -have a reference count on it, you almost certainly have a bug. + /** Out of function comment, option 2. */ + + int + function() + { + /* Comment inside function, option 1. */ + + /* + * Comment inside function, option 2. + */ + } + +If a function has declaration and implementation separated, the function comment +should be for the declaration. Usually in the header file. Don't duplicate the +comment. + +A comment and the function signature should be synchronized. Double-check if the +parameter names are the same as used in the comment, and mean the same. +Especially when you change one of them - ensure you changed the other. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 12: Macros, Enums and RTL -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 9: Macros, Enums and RTL +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Names of macros defining constants and labels in enums are capitalized. -.. code-block:: c +.. code-block:: c #define CONSTANT 0x12345 @@ -794,112 +595,87 @@ Generally, inline functions are preferable to macros resembling functions. Macros with multiple statements should be enclosed in a do - while block: -.. code-block:: c +.. code-block:: c - #define macrofun(a, b, c) \ - do { \ - if (a == 5) \ - do_this(b, c); \ - } while (0) + #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: +1) macros that affect control flow: - .. code-block:: c + .. code-block:: c - #define FOO(x) \ - do { \ - if (blah(x) < 0) \ - return -EBUGGERED; \ - } while(0) + #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. + 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. -2. macros that depend on having a local variable with a magic name: +2) macros that depend on having a local variable with a magic name: - .. code-block:: c + .. code-block:: c - #define FOO(val) bar(index, val) + #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. + 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. -3. 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. +3) 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. -4. forgetting about precedence: macros defining constants using expressions - must enclose the expression in parentheses. Beware of similar issues with - macros using parameters. +4) forgetting about precedence: macros defining constants using expressions + must enclose the expression in parentheses. Beware of similar issues with + macros using parameters. - .. code-block:: c + .. code-block:: c - #define CONSTANT 0x4000 - #define CONSTEXP (CONSTANT | 3) + #define CONSTANT 0x4000 + #define CONSTEXP (CONSTANT | 3) - The cpp manual deals with macros exhaustively. The gcc internals manual also - covers RTL which is used frequently with assembly language in the kernel. +5) namespace collisions when defining local variables in macros resembling + functions: -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 13: Printing kernel messages -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + .. code-block:: c -Kernel developers like to be seen as literate. Do mind the spelling -of kernel 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. + #define FOO(x) \ + ({ \ + typeof(x) ret; \ + ret = calc_ret(x); \ + (ret); \ + }) -Kernel messages do not have to be terminated with a period. + ret is a common name for a local variable - ``__foo_ret`` is less likely + to collide with an existing variable. -Printing numbers in parentheses (%d) adds no value and should be avoided. +Chapter 10: Allocating memory +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -There are a number of driver model diagnostic macros in -which you should use to make sure messages are matched to the right device -and driver, and are tagged with the right level: dev_err(), dev_warn(), -dev_info(), and so forth. For messages that aren't associated with a -particular device, defines pr_debug() and pr_info(). +Prefer specialized allocators like ``region``, ``mempool``, ``smalloc`` to +``malloc()/free()`` for any performance-intensive or large memory allocations. +Repetitive use of ``malloc()``/``free()`` can lead to memory fragmentation +and should therefore be avoided. -Coming up with good debugging messages can be quite a challenge; and once -you have them, they can be a huge help for remote troubleshooting. Such -messages should be compiled out when the DEBUG symbol is not defined (that -is, by default they are not included). When you use dev_dbg() or pr_debug(), -that's automatic. Many subsystems have Kconfig options to turn on -DDEBUG. -A related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to the -ones already enabled by DEBUG. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 14: Allocating memory -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -The kernel provides the following general purpose memory allocators: -kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API -documentation for further information about them. - -The preferred form for passing a size of a struct is the following: - -.. code-block:: c - - p = kmalloc(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. +Always free all allocated memory, even allocated at start-up. We aim at being +valgrind leak-check clean, and in most cases it's just as easy to ``free()`` the +allocated memory as it is to write a valgrind suppression. Freeing all allocated +memory is also dynamic-load friendly: assuming a plug-in can be dynamically +loaded and unloaded multiple times, reload should not lead to a memory leak. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 15: The inline disease -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 11: 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. Abundant use of the inline keyword leads to a much bigger -kernel, which in turn slows the system as a whole down, due to a bigger -icache footprint for the CPU and simply because there is less memory +faster" speedup option called ``inline``. While the use of inlines can be +appropriate, it very often is not. Abundant use of the inline keyword leads to +a much bigger kernel, which in turn slows the system as a whole down, due to a +bigger icache footprint for the CPU and simply because there is less memory available for the pagecache. Just think about it; a pagecache miss causes a disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles that can go into these 5 milliseconds. @@ -908,8 +684,7 @@ 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 compiletime 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. +function away at compile time. 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 @@ -918,82 +693,37 @@ 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. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 16: Function return values and names -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 12: 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 -(-Exxx = failure, 0 = success) or a "succeeded" boolean (0 = failure, -non-zero = success). +failed. -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 -EBUSY 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. - -All EXPORTed functions must respect this convention, and so should all -public functions. Private (static) functions need not, but it is -recommended that they do. +In 99.99999% of all cases in Tarantool we return 0 on success, non-zero on error +(-1 usually). Errors are saved into a diagnostics area which is global per fiber. +We never return error codes as a result of a function. 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 or the ERR_PTR mechanism to report failure. - - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 17: Don't re-invent the kernel macros -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -The header file include/linux/kernel.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 - -.. code-block:: c - - #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) +this rule. Generally they indicate failure by returning some out-of-range +result. Typical examples would be functions that return pointers; they use +NULL or the mechanism to report failure. -Similarly, if you need to calculate the size of some structure member, use - -.. code-block:: c - - #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) - -There are also min() and max() macros that do strict type checking if you -need them. Feel free to peruse that header file to see what else is already -defined that you shouldn't reproduce in your code. - -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Chapter 18: Editor modelines and other cruft -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 13: 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 +indicated with special markers. For example, emacs interprets lines marked like this: -.. code-block:: none +.. code-block:: c -*- mode: c -*- Or like this: -.. code-block:: none +.. code-block:: c /* Local Variables: @@ -1003,35 +733,109 @@ Or like this: Vim interprets markers that look like this: -.. code-block:: none +.. code-block:: c /* 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 +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. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - Appendix I: References -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Chapter 14: Conditional Compilation +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Wherever possible, don't use preprocessor conditionals (``#if``, ``#ifdef``) in +.c files; doing so makes code harder to read and logic harder to follow. Instead, +use such conditionals in a header file defining functions for use in those .c +files, providing no-op stub versions in the #else case, and then call those +functions unconditionally from .c files. The compiler will avoid generating +any code for the stub calls, producing identical results, but the logic will +remain easy to follow. + +Prefer to compile out entire functions, rather than portions of functions or +portions of expressions. Rather than putting an ``#ifdef`` in an expression, +factor out part or all of the expression into a separate helper function and +apply the condition to that function. + +If you have a function or variable which may potentially go unused in a +particular configuration, and the compiler would warn about its definition +going unused, do not compile it and use #if for this. + +At the end of any non-trivial ``#if`` or ``#ifdef`` block (more than a few lines), +place a comment after the #endif on the same line, noting the conditional +expression used. For instance: + +.. code-block:: c + + #ifdef CONFIG_SOMETHING + ... + #endif /* CONFIG_SOMETHING */ + +Chapter 15: Header files +~~~~~~~~~~~~~~~~~~~~~~~~ + +Use ``#pragma once`` in the headers. As the header guards we refer to this +construction: + +.. code-block:: c + + #ifndef THE_HEADER_IS_INCLUDED + #define THE_HEADER_IS_INCLUDED + + // ... the header code ... + + #endif // THE_HEADER_IS_INCLUDED + +It works fine, but the guard name ``THE_HEADER_IS_INCLUDED`` tends to +become outdated when the file is moved or renamed. This is especially +painful with multiple files having the same name in the project, but +different path. For instance, we have 3 ``error.h`` files, which means for +each of them we need to invent a new header guard name, and not forget to +update them if the files are moved or renamed. + +For that reason we use ``#pragma once`` in all the new code, which shortens +the header file down to this: + +.. code-block:: c + + #pragma once + + // ... header code ... + +Chapter 16: Other +~~~~~~~~~~~~~~~~~ + +* We don't apply ``!`` operator to non-boolean values. It means, to check + if an integer is not 0, you use ``!= 0``. To check if a pointer is not NULL, + you use ``!= NULL``. The same for ``==``. + +* Select GNU C99 extensions are acceptable. It's OK to mix declarations and + statements, use true and false. + +* The not-so-current list of all GCC C extensions can be found at: + http://gcc.gnu.org/onlinedocs/gcc-4.3.5/gcc/C-Extensions.html + +Appendix I: References +~~~~~~~~~~~~~~~~~~~~~~ + +* `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). -* `The C Programming Language, Second Edition `_ - by Brian W. Kernighan and Dennis M. Ritchie. |br| - Prentice Hall, Inc., 1988. |br| - ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback). +* `The Practice of Programming `_ + by Brian W. Kernighan and Rob Pike. + Addison-Wesley, Inc., 1999. + ISBN 0-201-61586-X. -* `The Practice of Programming `_ - by Brian W. Kernighan and Rob Pike. |br| - Addison-Wesley, Inc., 1999. |br| - ISBN 0-201-61586-X. +* `GNU manuals `_ - where in compliance with K&R + and this text - for **cpp**, **gcc**, **gcc internals** and **indent** -* `GNU manuals `_ - where in compliance with K&R and this text - for **cpp**, **gcc**, - **gcc internals** and **indent** +* `WG14 International standardization workgroup for the programming + language C `_ -* `WG14 International standardization workgroup for the programming - language C `_ +* `Kernel CodingStyle, by greg@kroah.com at OLS 2002 + `_ -* `Kernel CodingStyle, by greg@kroah.com at OLS 2002 - `_