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SubjectRe: [PATCH] CodingStyle: add typedefs chapter
On Mon, 01 May 2006 18:06:06 +0100 David Woodhouse wrote:

> On Sun, 2006-04-30 at 17:44 -0700, Randy.Dunlap wrote:
> > + (b) Clear integer types, where the abstraction _helps_ avoid confusion
> > + whether it is "int" or "long".
> > +
> > + u8/u16/u32 are perfectly fine typedefs.
> No, u8/u16/u32 are fall into category (d):

Thanks. Here's an updated version.

From: Randy Dunlap <>

Add a chapter on typedefs, copied from an email from Linus
to lkml on Feb. 3, 2006:
(Subject: Re: [RFC][PATCH 1/5] Virtualization/containers: startup)
with added lkml feedback, esp. David Woodhouse.

Signed-off-by: Randy Dunlap <>
Documentation/CodingStyle | 96 ++++++++++++++++++++++++++++++++++++++++------
1 files changed, 84 insertions(+), 12 deletions(-)

--- linux-2617-rc3.orig/Documentation/CodingStyle
+++ linux-2617-rc3/Documentation/CodingStyle
@@ -155,7 +155,79 @@ problem, which is called the function-gr
See next chapter.

- Chapter 5: Functions
+ Chapter 5: Typedefs
+Please don't use things like "vps_t".
+It's a _mistake_ to use typedef for structures and pointers. When you see a
+ 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 gratuitous 'u8/u16/u32/u64' types and their signed
+ equivalents which are identical to standard types are permitted --
+ although they are not mandatory.
+ (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

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,
@@ -183,7 +255,7 @@ and it gets confused. You know you're b
to understand what you did 2 weeks from now.

- Chapter 6: Centralized exiting of functions
+ 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.
@@ -220,7 +292,7 @@ out:
return result;

- Chapter 7: Commenting
+ 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
@@ -240,7 +312,7 @@ When commenting the kernel API functions
See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
for details.

- Chapter 8: You've made a mess of it
+ 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
@@ -288,7 +360,7 @@ re-formatting you may want to take a loo
remember: "indent" is not a fix for bad programming.

- Chapter 9: Configuration-files
+ Chapter 10: Configuration-files

For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
somewhat different indentation is used.
@@ -313,7 +385,7 @@ support for file-systems, for instance)
experimental options should be denoted (EXPERIMENTAL).

- Chapter 10: Data structures
+ Chapter 11: Data structures

Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
@@ -344,7 +416,7 @@ Remember: if another thread can find you
have a reference count on it, you almost certainly have a bug.

- Chapter 11: Macros, Enums and RTL
+ Chapter 12: Macros, Enums and RTL

Names of macros defining constants and labels in enums are capitalized.

@@ -399,7 +471,7 @@ The cpp manual deals with macros exhaust
covers RTL which is used frequently with assembly language in the kernel.

- Chapter 12: Printing kernel messages
+ Chapter 13: Printing kernel messages

Kernel developers like to be seen as literate. Do mind the spelling
of kernel messages to make a good impression. Do not use crippled
@@ -410,7 +482,7 @@ Kernel messages do not have to be termin
Printing numbers in parentheses (%d) adds no value and should be avoided.

- Chapter 13: Allocating memory
+ Chapter 14: Allocating memory

The kernel provides the following general purpose memory allocators:
kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API
@@ -429,7 +501,7 @@ from void pointer to any other pointer t

- Chapter 14: The inline disease
+ Chapter 15: 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
@@ -457,7 +529,7 @@ something it would have done anyway.

- Chapter 15: References
+ Appendix I: References

The C Programming Language, Second Edition
by Brian W. Kernighan and Dennis M. Ritchie.
@@ -481,4 +553,4 @@ Kernel CodingStyle, by at

-Last updated on 30 December 2005 by a community effort on LKML.
+Last updated on 30 April 2006.
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