Note
This coding style document has been borrowed from the Linux coding style.
8. “C” coding conventions (Cconv)¶
8.1. Indentation¶
Tabs are 8 characters, and thus indentations are also 8 characters.
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.:
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:
if (condition) do_this;
do_something_everytime;
Don’t put multiple assignments on a single line either. Coding style is super simple. Avoid tricky expressions.
Outside of comments, documentation and except in some files where it is required like Kconfig Linux kernel, 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.
8.2. Breaking long lines and strings¶
Coding style is all about readability and maintainability using commonly available tools.
The limit on the length of lines should correspond to what a modern screen and editor is reasonibly able to display before the reader has to scroll horizontally (even if it is acceptable to scroll over a few characters).
Statements too long 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. However, avoid to break user-visible strings such as printk messages, because that breaks the ability to grep for them.
8.3. 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:
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;
}
However, there is one special case, namely functions: they may have the opening brace at the beginning of the next line, thus:
int function(int x)
{
body of function
}
Note that the closing brace is empty on a line of its own, except in
the cases where it is followed by a continuation of the same statement,
ie a while in a do-statement or an else in an if-statement, like
this:
do {
body of do-loop
} while (condition);
and
if (x == y) {
..
} else if (x > y) {
...
} else {
....
}
Rationale: K&R.
Also, note that this brace-placement also minimizes the number of empty (or almost empty) lines, without any loss of readability.
Do not unnecessarily use braces where a single statement will do.
if (condition)
action();
and
if (condition)
do_this();
else
do_that();
Also, prefer using braces when a loop contains more than a single simple statement:
while (condition) {
if (test)
do_something();
}
8.3.1. Spaces¶
Use a space after (most) keywords. The notable exceptions are sizeof, typeof, alignof,
and __attribute__, which look somewhat like functions (and are usually used with parentheses in Linux,
although they are not required in the language, as in: sizeof info after
struct fileinfo info; is declared).
So use a space after these keywords:
if, switch, case, for, do, while
but not with sizeof, typeof, alignof, or __attribute__. E.g.,
s = sizeof(struct file);
Do not add spaces around (inside) parenthesized expressions. This example is bad:
s = sizeof( struct file );
When declaring pointer data or a function that returns a pointer type, the
preferred use of * is adjacent to the data name or function name and not
adjacent to the type name. Examples:
char *linux_banner;
unsigned long long memparse(char *ptr, char **retptr);
char *match_strdup(substring_t *s);
Use one space around (on each side of) most binary and ternary operators, such as any of these:
= + - < > * / % | & ^ <= >= == != ? :
but no space after unary operators:
& * + - ~ ! sizeof typeof alignof __attribute__ defined
no space before the postfix increment & decrement unary operators:
++ --
no space after the prefix increment & decrement unary operators:
++ --
and no space around the . and -> structure member operators.
Do not leave trailing whitespace at the ends of lines. Some editors with
smart indentation will insert whitespace at the beginning of new lines as
appropriate, so you can start typing the next line of code right away.
However, some such editors do not remove the whitespace if you end up not
putting a line of code there, such as if you leave a blank line. As a result,
you end up with lines containing trailing whitespace.
Git will warn you about patches that introduce trailing whitespace, and can optionally strip the trailing whitespace for you; however, if applying a series of patches, this may make later patches in the series fail by changing their context lines.
8.4. 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).
8.5. Typedefs¶
The definition of a type should be either to hide the platform-dependent
definition of a type or to define a struct and having a more readable
type rather than struct sensor. Put a _t as suffix of a type
definition. For example, sensor_t.
Regarding the platform-dependent definition, it helps to define clear integer types,
where the abstraction helps avoid confusion whether it is int or long.
u8/u16/u32 are perfectly fine typedefs
NEVER use a typedef to hide a pointer except for the pointer to a function. For example:
typedef (void)(*sigint_fn_t)(int sig)
is a correct usage of typedef.
8.6. Functions¶
Functions should be short and sweet, and do just one thing.
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.
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.
8.7. 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 ;)
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_free_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:
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::
err_free_bar:
kfree(foo->bar);
err_free_foo:
kfree(foo);
return ret;
Ideally you should simulate errors to test all exit paths.
8.8. 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.
The preferred style for long (multi-line) comments is:
/*
* This is the preferred style for multi-line
* comments in the source code. Please use it consistently.
*
* Description: A column of asterisks on the left side,
* with beginning and ending almost-blank lines.
*/
It’s also important to comment data, whether they are basic types or derived types. To this end, use just one data declaration per line (no commas for multiple data declarations). This leaves you room for a small comment on each item, explaining its use.
8.9. Macros, Enums and RTL¶
Names of macros defining constants and labels in enums are capitalized.
#define CONSTANT 0x12345
Enums are preferred when defining several related constants.
CAPITALIZED macro names are appreciated but macros resembling functions may be named in lower case.
Generally, inline functions are preferable to macros resembling functions.
Macros with multiple statements should be enclosed in a do - while block:
#define macrofun(a, b, c) \
do { \
if (a == 5) \
do_this(b, c); \
} while (0)
Things to avoid when using macros:
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.
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.
#define CONSTANT 0x4000
#define CONSTEXP (CONSTANT | 3)
5) namespace collisions when defining local variables in macros resembling functions:
#define FOO(x) \
({ \
typeof(x) ret; \
ret = calc_ret(x); \
(ret); \
})
ret is a common name for a local variable - __foo_ret is less likely to collide with an existing variable.
The cpp manual deals with macros exhaustively. The gcc internals manual also covers RTL which is used frequently with assembly language in the kernel.
8.10. Printing logging messages¶
Do mind the spelling of 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.
Usually, messages do not have to be terminated with a period.
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. However debug message printing is handled differently than printing other non-debug messages.
8.10.1. Syslog-ng¶
Syslog-ng enables logging messages in various forms and configurations.
It can be used to log message on the console and/or in files typically
stored in /var/log directory.
8.11. 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).
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.
Functions whose return value is the actual result of a computation, rather than an indication of whether the computation succeeded, are not subject to this rule. Generally they indicate failure by returning some out-of-range result. Typical examples would be functions that return pointers; they use NULL to report failure.
8.12. Inline assembly¶
In architecture-specific code, you may need to use inline assembly to interface with CPU or platform functionality. Don’t hesitate to do so when necessary. However, don’t use inline assembly gratuitously when C can do the job. You can and should poke hardware from C when possible.
Consider writing simple helper functions that wrap common bits of inline assembly, rather than repeatedly writing them with slight variations. Remember that inline assembly can use C parameters.
Large, non-trivial assembly functions should go in .S files, with corresponding
C prototypes defined in C header files. The C prototypes for assembly
functions should use asmlinkage.
You may need to mark your asm statement as volatile, to prevent GCC from removing it if GCC doesn’t notice any side effects. You don’t always need to do so, though, and doing so unnecessarily can limit optimization.
When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate quoted
string, and end each string except the last with \n\t to properly indent
the next instruction in the assembly output:
asm ("magic %reg1, #42\n\t"
"more_magic %reg2, %reg3"
: /* outputs */ : /* inputs */ : /* clobbers */);
8.13. Conditional Compilation¶
Using #if or #ifdef block should always have a comment on the #else or #endif statement with the name of the condition, like this:
#ifdef CONFIG_SOMETHING
...
#else /* CONFIG_SOMETHING */
...
#endif /* !CONFIG_SOMETHING */
It will greatly help the reading of the code.