Syslinux uses Linux kernel coding style, except that we are "heretic" in the sense of using 4 spaces instead of 8 for indentation. This coding style will be applied after the 3.81 release. ------------------------------------------------- Linux kernel coding style This is a short document describing the preferred coding style for the linux kernel. Coding style is very personal, and I won't _force_ my views on anybody, but this is what goes for anything that I have to be able to maintain, and I'd prefer it for most other things too. Please at least consider the points made here. First off, I'd suggest printing out a copy of the GNU coding standards, and NOT read it. Burn them, it's a great symbolic gesture. Anyway, here goes: Chapter 1: Indentation Tabs are 8 characters, and thus indentations are also 8 characters. There are heretic movements that try to make indentations 4 (or even 2!) characters deep, and that is akin to trying to define the value of PI to be 3. Rationale: The whole idea behind indentation is to clearly define where a block of control starts and ends. Especially when you've been looking at your screen for 20 straight hours, you'll find it a lot easier to see how the indentation works if you have large indentations. Now, some people will claim that having 8-character indentations makes the code move too far to the right, and makes it hard to read on a 80-character terminal screen. The answer to that is that if you need more than 3 levels of indentation, you're screwed anyway, and should fix your program. 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; do_something_everytime; 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. Get a decent editor and don't leave whitespace at the end of lines. Chapter 2: Breaking long lines and strings Coding style is all about readability and maintainability using commonly available tools. The limit on the length of lines is 80 columns and this is a strongly preferred limit. Statements longer than 80 columns will be broken into sensible chunks. Descendants are always substantially shorter than the parent and are placed substantially to the right. The same applies to function headers with a long argument list. Long strings are as well broken into shorter strings. The only exception to this is where exceeding 80 columns significantly increases readability and does not hide information. void fun(int a, int b, int c) { if (condition) printk(KERN_WARNING "Warning this is a long printk with " "3 parameters a: %u b: %u " "c: %u \n", a, b, c); else next_statement; } 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 choose one placement strategy over the other, but the preferred way, as shown to us by the prophets Kernighan and Ritchie, is to put the opening brace last on the line, and put the closing brace first, thusly: if (x is true) { 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"; case KOBJ_REMOVE: return "remove"; case KOBJ_CHANGE: 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: int function(int x) { body of function } Heretic people all over the world have claimed that this inconsistency is ... well ... inconsistent, but all right-thinking people know that (a) K&R are _right_ and (b) K&R are right. Besides, functions are special anyway (you can't nest them in C). Note that the closing brace is empty on a line of its own, _except_ in the cases where it is followed by a continuation of the same statement, ie a "while" in a do-statement or an "else" in an if-statement, like this: do { body of do-loop } while (condition); and if (x == y) { .. } else if (x > y) { ... } else { .... } Rationale: K&R. Also, note that this brace-placement also minimizes the number of empty (or almost empty) lines, without any loss of readability. Thus, as the 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. Do not unnecessarily use braces where a single statement will do. if (condition) action(); This does not apply if one branch of a conditional statement is a single statement. Use braces in both branches. if (condition) { do_this(); do_that(); } else { otherwise(); } 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 *bad*: 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. Do not leave trailing whitespace at the ends of lines. Some editors with "smart" indentation will insert whitespace at the beginning of new lines as appropriate, so you can start typing the next line of code right away. However, some such editors do not remove the whitespace if you end up not putting a line of code there, such as if you leave a blank line. As a result, you end up with lines containing trailing whitespace. Git will warn you about patches that introduce trailing whitespace, and can optionally strip the trailing whitespace for you; however, if applying a series of patches, this may make later patches in the series fail by changing their context lines. Chapter 4: Naming C is a Spartan language, and so should your naming be. Unlike Modula-2 and Pascal programmers, C programmers do not use cute names like ThisVariableIsATemporaryCounter. A C programmer would call that variable "tmp", which is much easier to write, and not the least more difficult to understand. HOWEVER, while mixed-case names are frowned upon, descriptive names for global variables are a must. To call a global function "foo" is a shooting offense. GLOBAL variables (to be used only if you _really_ need them) need to have descriptive names, as do global functions. If you have a function that counts the number of active users, you should call that "count_active_users()" or similar, you should _not_ call it "cntusr()". Encoding the type of a function into the name (so-called Hungarian notation) is brain damaged - the compiler knows the types anyway and can check those, and it only confuses the programmer. No wonder MicroSoft makes buggy programs. LOCAL variable names should be short, and to the point. If you have some random integer loop counter, it should probably be called "i". Calling it "loop_counter" is non-productive, if there is no chance of it being mis-understood. Similarly, "tmp" can be just about any type of 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 chapter 6 (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 thin...
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