path: root/Documentation/CodingStyle
diff options
authorRandy Dunlap <>2006-12-10 02:18:56 -0800
committerLinus Torvalds <>2006-12-10 09:55:40 -0800
commitb3fc9941fbc6efe5cb77728adb0fb12be363e73e (patch)
tree3155c06708f90c662e9863c8b8555dde669499c8 /Documentation/CodingStyle
parent8d94cc50aa4f1448a6483975097805eb8d6be0e0 (diff)
[PATCH] CodingStyle updates
Add some kernel coding style comments, mostly pulled from emails by Andrew Morton, Jesper Juhl, and Randy Dunlap. - add paragraph on switch/case indentation (with fixes) - add paragraph on multiple-assignments - add more on Braces - add section on Spaces; add typeof, alignof, & __attribute__ with sizeof; add more on postfix/prefix increment/decrement operators - add paragraph on function breaks in source files; add info on function prototype parameter names - add paragraph on EXPORT_SYMBOL placement - add section on /*-comment style, long-comment style, and data declarations and comments - correct some chapter number references that were missed when chapters were renumbered Signed-off-by: Randy Dunlap <> Acked-by: Jesper Juhl <> Acked-by: Jan Engelhardt <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation/CodingStyle')
1 files changed, 121 insertions, 5 deletions
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 29c18966b050..0ad6dcb5d45f 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -35,12 +35,37 @@ 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;
+Don't put multiple assignments on a single line either. Kernel coding style
+is super simple. Avoid tricky expressions.
Outside of comments, documentation and except in Kconfig, spaces are never
used for indentation, and the above example is deliberately broken.
@@ -69,7 +94,7 @@ void fun(int a, int b, int c)
- Chapter 3: Placing Braces
+ 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
@@ -81,6 +106,20 @@ brace last on the line, and put the closing brace first, thusly:
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";
+ return "remove";
+ return "change";
+ default:
+ return NULL;
+ }
However, there is one special case, namely functions: they have the
opening brace at the beginning of the next line, thus:
@@ -121,6 +160,49 @@ 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.
+ 3.1: Spaces
+Linux kernel style for use of spaces depends (mostly) on
+function-versus-keyword usage. Use a space after (most) keywords. The
+notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
+somewhat like functions (and are usually used with parentheses in Linux,
+although they are not required in the language, as in: "sizeof info" after
+"struct fileinfo info;" is declared).
+So use a space after these keywords:
+ if, switch, case, for, do, while
+but not with sizeof, typeof, alignof, or __attribute__. E.g.,
+ s = sizeof(struct file);
+Do not add spaces around (inside) parenthesized expressions. This example is
+ 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.
Chapter 4: Naming
@@ -152,7 +234,7 @@ variable that is used to hold a temporary value.
If you are afraid to mix up your local variable names, you have another
problem, which is called the function-growth-hormone-imbalance syndrome.
-See next chapter.
+See chapter 6 (Functions).
Chapter 5: Typedefs
@@ -258,6 +340,20 @@ 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;
+In function prototypes, include parameter names with their data types.
+Although this is not required by the C language, it is preferred in Linux
+because it is a simple way to add valuable information for the reader.
Chapter 7: Centralized exiting of functions
@@ -306,16 +402,36 @@ time to explain badly written code.
Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
-you should probably go back to chapter 5 for a while. You can make
+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
-When commenting the kernel API functions, please use the kerneldoc format.
+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 "/* ... */" style.
+Don't use C99-style "// ..." comments.
+The preferred style for long (multi-line) comments is:
+ /*
+ * This is the preferred style for multi-line
+ * comments in the Linux kernel source code.
+ * Please use it consistently.
+ *
+ * Description: A column of asterisks on the left side,
+ * with beginning and ending almost-blank lines.
+ */
+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
@@ -591,4 +707,4 @@ Kernel CodingStyle, by at OLS 2002:
-Last updated on 30 April 2006.
+Last updated on 2006-December-06.