Usage

The CVS repository copy of nALFS does not include an executable configure script nor a Makefile. This is by design, as there is a script to create them, which ensures that their contents will always be up to date with the source tree contents and not require manual editing.

nALFS has been configured to use the GNU autotools suite for its build process. The build system was created using autoconf-2.57, automake-1.7.7 and libtool-1.5. If you use older versions than those, you may experience warnings and/or outright failures (automake-1.6 is known to be unable to handle the nALFS Makefile.am).

When you checkout the nALFS source tree, you will need to execute:

sh ./bootstrap

in the nALFS directory itself. This should result output similar to the following:

       $ sh ./bootstrap
You should update your 'aclocal.m4' by running aclocal.
Putting files in AC_CONFIG_AUX_DIR, `gnubuild'.
configure.ac: installing 'gnubuild/install-sh'
configure.ac: installing 'gnubuild/mkinstalldirs'
configure.ac: installing 'gnubuild/missing'
Makefile.am: installing 'gnubuild/compile'
Makefile.am: installing 'gnubuild/depcomp'
patching file ltmain.sh
Hunk #1 succeeded at 160 with fuzz 2 (offset -78 lines).
Hunk #2 succeeded at 269 with fuzz 1.
Hunk #3 succeeded at 4483 (offset -1172 lines).

This output comes from the autoreconf tool, which is part of autoconf, that automatically runs libtoolize, autoheader, aclocal and other parts of the autotools suite. The warning about "update your 'aclocal.m4'" can be ignored, as aclocal is already executed later by autoreconf. The patch offset and/or fuzz messages about ltmain.sh are caused by the bootstrap script applying a patch created against libtool-1.5; if your libtool version is different, the patch should still work correctly, but you may see messages like this from the patch command. In the worst case, the patch fails to apply, the nALFS build will still be functional, but libtool will generate more output messages than necessary.

Once bootstrap has been run, you can execute ./configure like any other GNU autoconf-based package to configure nALFS for your system.

Editing source files

The nALFS build system, because it uses automake, has full dependency tracking on all files used to build the binaries. This will reduce your rebuilding time as you edit header files and source files, as the make system will know exactly what must be rebuilt.

In addition, if you are going to make changes to bootstrap, you should add the "--enable-maintainer-mode" parameter to your configure command. With this done, each time you edit and re-run bootstrap, the Makefile will automatically re-run configure to make your changes take effect. Be warned though, that if you manually edit Makefile.am or configure.ac, you must modify the bootstrap script to incorporate your changes, because these files are not stored in the CVS repository.

If you add any C source files to the tree, you will need to rerun the bootstrap script to get them included in your build. If you add header files, it is not necessary to rerun the bootstrap script unless you plan on using "make dist" from that same tree, in which case those added headers would not get included into the tarball.

If you add or remove "syntax versions" in any of the handlers, you must re-run bootstrap to get the version lists in Makefile.am and configure.ac to incorporate your changes, or you will experience unusual build behavior.

If you add or remove any program options in src/options.h, you must re-run bootstrap to regenerate src/option-list.h. Without that file being regenerated, you may experience compile errors or find that your newly added option does not work properly.

All other types of files fall into two categories:

those that should be present only in the CVS repository

Nothing needs to be done for these files, other than the relevant "cvs add" commands.

those that should be added to the distribution tarball

The bootstrap.Makefile script will need to be edited, specifically the line that sets EXTRA_DIST near the beginning of the script. Add the path(s) to the new files to this line, or add an additional line starting with "EXTRA_DIST +=" (standard GNU makefile syntax). If the files are not listed on this line, they will not be included in the tarball created by make dist.

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.

Chapter 2: Placing Braces

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
}

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.

Chapter 3: 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 İ 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 next chapter.

Chapter 4: 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, as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the complexity and indentation level of that function. So, if you have a conceptually simple function that is just one long (but simple) case- statement, where you have to do lots of small things for a lot of different cases, it's OK to have a longer function.

However, if you have a complex function, and you suspect that a less- than-gifted first-year high-school student might not even understand what the function is all about, you should adhere to the maximum limits all the more closely. Use helper functions with descriptive names (you can ask the compiler to in-line them if you think it's performance-critical, and it will probably do a better job of it that you would have done).

Another measure of the function is the number of local variables. They shouldn't exceed 5-10, or you're doing something wrong. Re-think the function, and split it into smaller pieces. A human brain can generally easily keep track of about 7 different things, anything more and it gets confused. You know you're brilliant, but maybe you'd like to understand what you did 2 weeks from now.

Chapter 5: 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 write the code so that the working is obvious, and it's a waste of time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably go back to chapter 4 for a while. You can make small comments to note or warn about something particularly clever (or 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.

Chapter 6: 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 you, and you've noticed that yes, it does do that, but the defaults it uses are less than desirable (in fact, they are worse than random typing İ a infinite number of monkeys typing into GNU emacs would never make a good program).

So, you can either get rid of GNU emacs, or change it to use saner values. To do the latter, you can stick the following in your .emacs file:

(defun linux-c-mode ()
  "C mode with adjusted defaults for use with the Linux kernel."
  (interactive)
  (c-mode)
  (c-set-style "K&R")
  (setq c-basic-offset 8))

This will define the M-x linux-c-mode command. When hacking on a module, if you put the string -*- linux-c -*- somewhere on the first two lines, this mode will be automatically invoked. Also, you may want to add

(setq auto-mode-alist (cons '("/usr/src/linux.*/.*\\.[ch]$" . linux-c-mode)
      auto-mode-alist))

to your .emacs file if you want to have linux-c- mode switched on automagically when you edit source files under /usr/src/linux.

But even if you fail in getting emacs to do sane formatting, not everything is lost: use indent.

Now, again, GNU indent has the same brain dead settings that GNU emacs has, which is why you need to give it a few command line options. However, that's not too bad, because even the makers of GNU indent recognize the authority of K&R (the GNU people aren't evil, they are just severely misguided in this matter), so you just give indent the options -kr - i8 (stands for "K&R, 8 character indents").

indent has a lot of options, and especially when it comes to comment re-formatting you may want to take a look at the manual page. But remember: indent is not a fix for bad programming.

Chapter 7: Configuration-files

For configuration options (arch/xxx/config.in, and all the Config.in files), somewhat different indentation is used.

An indention level of 3 is used in the code, while the text in the config- options should have an indention-level of 2 to indicate dependencies. The latter only applies to bool/tristate options. For other options, just use common sense. An example:

if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then
   tristate 'Apply nitroglycerine inside the keyboard (DANGEROUS)' CONFIG_BOOM
   if [ "$CONFIG_BOOM" != "n" ]; then
      bool '  Output nice messages when you explode' CONFIG_CHEER
   fi
fi

Generally, CONFIG_EXPERIMENTAL should surround all options not considered stable. All options that are known to trash data (experimental write- support for file-systems, for instance) should be denoted (DANGEROUS), other Experimental options should be denoted (EXPERIMENTAL).

Chapter 8: Data structures

Data structures that have visibility outside the single-threaded environment they are created and destroyed in should always have reference counts. In the kernel, garbage collection doesn't exist (and outside the kernel garbage collection is slow and inefficient), which means that you absolutely have to reference count all your uses.

Reference counting means that you can avoid locking, and allows multiple users to have access to the data structure in parallel İ and not having to worry about the structure suddenly going away from under them just because they slept or did something else for a while.

Note that locking is not a replacement for reference counting. Locking is used to keep data structures coherent, while reference counting is a memory management technique. Usually both are needed, and they are not to be confused with each other.

Many data structures can indeed have two levels of reference counting, when there are users of different "classes". The subclass count counts the number of subclass users, and decrements the global count just once when the subclass count goes to zero.

Examples of this kind of "multi-reference-counting" can be found in memory management (struct mm_struct: mm_users and mm_count), and in filesystem code (struct super_block: s_count and s_active).

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.