X-Git-Url: http://git.onelab.eu/?a=blobdiff_plain;f=Documentation%2FCodingStyle;h=6d2412ec91edb21ebab0c13e581951bbe352b610;hb=d3ec2a4ce4b4743ecf56c2ed7b32d74fd58210dc;hp=f25b3953f51398a023afd8b633aa5168313ccc4a;hpb=1a69f36a78895dad9a6b97ffa2d6d2b7c3975b5d;p=linux-2.6.git diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle index f25b3953f..6d2412ec9 100644 --- a/Documentation/CodingStyle +++ b/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. @@ -199,7 +275,7 @@ The rationale is: modifications are prevented - saves the compiler work to optimize redundant code away ;) -int fun(int ) +int fun(int a) { int result = 0; char *buffer = kmalloc(SIZE); @@ -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 @@ -236,8 +312,11 @@ 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. +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 @@ -285,7 +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. @@ -310,7 +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 @@ -341,7 +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, Inline functions and RTL + Chapter 12: Macros, Enums and RTL Names of macros defining constants and labels in enums are capitalized. @@ -396,7 +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 @@ -407,7 +486,54 @@ 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: References + Chapter 14: Allocating memory + +The kernel provides the following general purpose memory allocators: +kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API +documentation for further information about them. + +The preferred form for passing a size of a struct is the following: + + p = kmalloc(sizeof(*p), ...); + +The alternative form where struct name is spelled out hurts readability and +introduces an opportunity for a bug when the pointer variable type is changed +but the corresponding sizeof that is passed to a memory allocator is not. + +Casting the return value which is a void pointer is redundant. The conversion +from void pointer to any other pointer type is guaranteed by the C programming +language. + + + 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 +appropriate (for example as a means of replacing macros, see Chapter 11), it +very often is not. Abundant use of the inline keyword leads to a much bigger +kernel, which in turn slows the system as a whole down, due to a bigger +icache footprint for the CPU and simply because there is less memory +available for the pagecache. Just think about it; a pagecache miss causes a +disk seek, which easily takes 5 miliseconds. There are a LOT of cpu cycles +that can go into these 5 miliseconds. + +A reasonable rule of thumb is to not put inline at functions that have more +than 3 lines of code in them. An exception to this rule are the cases where +a parameter is known to be a compiletime constant, and as a result of this +constantness you *know* the compiler will be able to optimize most of your +function away at compile time. For a good example of this later case, see +the kmalloc() inline function. + +Often people argue that adding inline to functions that are static and used +only once is always a win since there is no space tradeoff. While this is +technically correct, gcc is capable of inlining these automatically without +help, and the maintenance issue of removing the inline when a second user +appears outweighs the potential value of the hint that tells gcc to do +something it would have done anyway. + + + + Appendix I: References The C Programming Language, Second Edition by Brian W. Kernighan and Dennis M. Ritchie. @@ -422,10 +548,13 @@ ISBN 0-201-61586-X. URL: http://cm.bell-labs.com/cm/cs/tpop/ GNU manuals - where in compliance with K&R and this text - for cpp, gcc, -gcc internals and indent, all available from http://www.gnu.org +gcc internals and indent, all available from http://www.gnu.org/manual/ WG14 is the international standardization working group for the programming -language C, URL: http://std.dkuug.dk/JTC1/SC22/WG14/ +language C, URL: http://www.open-std.org/JTC1/SC22/WG14/ + +Kernel CodingStyle, by greg@kroah.com at OLS 2002: +http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/ -- -Last updated on 16 February 2004 by a community effort on LKML. +Last updated on 30 April 2006.