Commit 226a6b84 authored by Randy Dunlap's avatar Randy Dunlap Committed by Linus Torvalds

[PATCH] CodingStyle: add typedefs chapter

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)
Signed-off-by: default avatarRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 3439dd86
......@@ -155,7 +155,83 @@ problem, which is called the function-growth-hormone-imbalance syndrome.
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 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
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 +259,7 @@ and it gets confused. You know you're brilliant, but maybe you'd like
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 +296,7 @@ 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 +316,7 @@ When commenting the kernel API functions, please use the kerneldoc format.
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 +364,7 @@ re-formatting you may want to take a look at the man page. But
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 +389,7 @@ support for file-systems, for instance) should be denoted (DANGEROUS), other
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 +420,7 @@ 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.
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 +475,7 @@ The cpp manual deals with macros exhaustively. The gcc internals manual also
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 +486,7 @@ Kernel messages do not have to be terminated with a period.
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 +505,7 @@ from void pointer to any other pointer type is guaranteed by the C programming
language.
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 +533,7 @@ 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 +557,4 @@ Kernel CodingStyle, by greg@kroah.com at OLS 2002:
http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
--
Last updated on 30 December 2005 by a community effort on LKML.
Last updated on 30 April 2006.
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