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Coding Style for C style languages
This document serves as a reference for the coding style used for this project for C++ and C-family languages. This style will be preferred and any submissions or pull requests that do not respect the style may be removed or turned away from the master repository. Credit should go where credit is due: this page is largely copied over from the Project Tox wiki.
- Indentation
- Breaking long lines and strings
- Placing braces and spaces
- Naming
- Typedefs
- Functions
- Commenting
- Data structures
- Macros, Enums, and RTL
- Allocating Memory
- Inlining
- Function return values and names
Indentation will be tab-centric, and source code should be expected to have tabs for whitespace, not spaces. The recommended spacing is 1 tab = 4 spaces. There are a plethora of text editors and IDEs out there that can do the conversion for you, so this should not bring great difficulty. That said, whitespace at the end of lines is also the devil. Please use a proper editor and remove trailing whitespace at the end of lines where possible.
When dealing with multiple indentation levels like switch statements, the preferred way is to align the switch statement with the subordinate case labels, like so:
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;
}
It is also important that you place control statements on multiple lines, and use the corresponding indentation appropriately. For example, it is NOT okay to place multiple statements on a single line, and then try to hide the second function as follows:
if (condition) do_this();
do_something_everytime();
Multiple assignments on a single line are not okay either. Make sure your code is readable first, we can optimize later.
Coding style is all about readability and maintainability using commonly available tools.
The limit on the length of lines is 120 columns and this is a strongly preferred limit.
Statements longer than 120 columns will be broken into sensible chunks, unless exceeding 120 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 use regular expressions to search for/through them.
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 is to put the opening brace last on the line, and put the closing brace first. This notation prevents our code from having a series of lines wasted by a single opening brace, as follows:
if (x is true) {
we do y
}
The above is correct. The below is NOT CORRECT:
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";
case KOBJ_REMOVE:
return "remove";
case KOBJ_CHANGE:
return "change";
default:
return NULL;
}
Notice above that it was mentioned that this applies for all non-function statement blocks. The exception then, is when we are defining functions, where the opening curly brace will have its own line. See:
int function(int x)
{
body of function
}
The inconsistency here is, well... inconsistent. However, this is mostly for code-folding purposes within editors such as emacs or vim, where you can still see the full function prototype of a folded function without having the full implementation visible and taking up lines on the screen.
As a special note it should always be the case that a closing brace is on a line of its own, except in cases such as a do-while loop, else, else-if, or catch clause where placing the continuation of the statement makes sense:
do {
body of do-loop
} while (condition);
and
if (x == y) {
..
} else if (x > y) {
...
} else {
....
}
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. This is the case when using control statements that only execute one line of code afterwards.
if (condition)
action();
and
if (condition)
do_this();
else
do_that();
This does not apply in the case where one branch has multiple statements and the others all contain a single statement. In this case stick to using braces for all branches to avoid confusion.
if (condition) {
do_this();
do_that();
} else {
otherwise();
}
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:
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.
Unlike previous sections, variable naming will not necessarily follow the K&R standard. Since this project will likely use more C++ than C code, certain naming conventions for object-oriented programming will be used here. However, the following will be a general guideline for naming things:
Make variable names descriptive, but not excessively long. For example, one would name a temporary variable 'tmp', or 'temp', as opposed to something such as 'temporaryObject'. However, excessively short names outside of common mathematical notations are frowned upon. e.g.: calling the first design matrix of a Least-Squares solution 'A' is okay, but naming your fstream object 'o' is unacceptable. In these cases, we should strive to use carrotCase naming where applicable. This will also be the same naming convention for function names as well.
int a;
double precision;
std::fstream outFile;
void thisIsAFunction(... arg list ...);
With regards to class definitions, the best practice for naming class types is 'MixedCase'. Unlike carrot case naming, we use capitals at the beginning of the class name.
class MyClass {
...
};
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 canno t require C99 types and cannot use the 'u32' form above. Thus, we use __u32 and similar types in all structures which are s hared 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;
}
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 because it is a simple way to add valuable information for the reader. The exception to this rule is for functions that require a prototype, but no implementation / definition. Cases like this include functions that have locked access in classes by the private: keyword. e.g.:
class Uncopyable {
protected: // allow construction
Uncopyable() {} // and destruction of
~Uncopyable() {} // derived objects...
private:
Uncopyable(const Uncopyable&); // ...but prevent copying
Uncopyable& operator=(const Uncopyable&);
};
class Inherited: private Uncopyable { // compiler won't default
... // declare copy constructor or
}; // copy assign. operator
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 refer back to 6. Functions 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.
The style for comments is the C89 "/* ... */" style. Don't use C99-style "// ..." comments if possible.
The preferred style for long (multi-line) comments is:
/*
* This is the preferred style for multi-line
* comments in the Project Tox source code.
* Please use it consistently.
*
* Description: A column of asterisks on the left side,
* with beginning and ending almost-blank lines.
*/
If there is ever a scenario where one (or two very short (< 50 chars)) line(s) of comments will work, then it is acceptable to use C99-style comments. With regards to headers for the beginning of new files or libraries, it is not necessary to apply any specific convention, but please include author name, the date, the name of the file you're editing and a short description of what the file contains.
Lastly, 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) if the purpose of the data structure or object is not directly relevant. This leaves you room for a small comment on each item, explaining its use.
Data structures that have visibility outside the single-threaded environment they are created and destroyed in should always have reference counts. 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.
Note: this shouldn't affect our project too much, but bear in mind that it will be necessary for us to consider how we formulate data structures and implement our object types. The best way is to use RAII and PIMPL idioms as much as possible, without obfuscating or derailing the code.
For the most part, macros are frowned upon in C++ with the introduction of the const keyword. Attempt to use macros as little as possible, but follow these guidelines if that is an impossible task.
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:
- 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.
- 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.
-
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.
-
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
#define CONSTEXP (CONSTANT | 3)
This section is largely irrelevant for our needs, as we will mostly be adopting a RAII approach with our C++ classes, as opposed to manual allocation. However, should the need arise, the coding practices below are recommended for explicit allocation of memory:
The kernel provides the following general purpose memory allocators: kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and vzalloc().
The preferred form for passing a size of a struct is the following:
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.
The preferred form for allocating an array is the following:
p = kmalloc_array(n, sizeof(...), ...);
The preferred form for allocating a zeroed array is the following:
p = kcalloc(n, sizeof(...), ...);
Both forms check for overflow on the allocation size n * sizeof(...), and return NULL if that occurred.
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 12 Function return values and names), 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.
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.
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.
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).
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 addWork() function returns 0 for success or -EBUSY for failure. In the same way, "PCI device present" is a predicate, and the pciDevPresent() function returns 1 if it succeeds in finding a matching device or 0 if it doesn't. In C++, the 1 and 0 can be replaced by 'true' and 'false', if returning bool in that context makes more sense than explicit integers.
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.
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.