[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: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/CodingStyle

@@ -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.