README.Coding: add section about usage of helper variables

[Samba/cd1.git] / README.Coding

Coding conventions in the Samba tree
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. contents::

===========
Quick Start
===========

Coding style guidelines are about reducing the number of unnecessary
reformatting patches and making things easier for developers to work together.
You don't have to like them or even agree with them, but once put in place
we all have to abide by them (or vote to change them).  However, coding
style should never outweigh coding itself and so the guidelines
described here are hopefully easy enough to follow as they are very
common and supported by tools and editors.

The basic style, also mentioned in prog_guide4.txt, is the Linux kernel coding 
style (See Documentation/CodingStyle in the kernel source tree). This closely 
matches what most Samba developers use already anyways.

But to save you the trouble of reading the Linux kernel style guide, here
are the highlights.

* Maximum Line Width is 80 Characters
  The reason is not for people with low-res screens but rather sticking
  to 80 columns prevents you from easily nesting more than one level of
  if statements or other code blocks.  Use source3/script/count_80_col.pl
  to check your changes.

* Use 8 Space Tabs to Indent
  No whitespace filler.

* No Trailing Whitespace
  Use source3/script/strip_trail_ws.pl to clean you files before committing.

* Follow the K&R guidelines.  We won't go throw them all here.  You have
  a copy of "The C Programming Language" anyways right?  You can also use
  the format_indent.sh script found in source3/script/ if all else fails.



============
Editor Hints
============

Emacs
-----
Add the follow to your $HOME/.emacs file:

  (add-hook 'c-mode-hook
        (lambda ()
                (c-set-style "linux")
                (c-toggle-auto-state)))


Vi
--
(Thanks to SATOH Fumiyasu <fumiyas@osstech.jp> for these hints):

For the basic vi editor included with all variants of \*nix, add the
following to $HOME/.exrc:

  set tabstop=8
  set shiftwidth=8

For Vim, the following settings in $HOME/.vimrc will also deal with 
displaying trailing whitespace::

  if has("syntax") && (&t_Co > 2 || has("gui_running"))
        syntax on
        function! ActivateInvisibleCharIndicator()
                syntax match TrailingSpace "[ \t]\+$" display containedin=ALL
                highlight TrailingSpace ctermbg=Red
        endf
        autocmd BufNewFile,BufRead * call ActivateInvisibleCharIndicator()
  endif
  " Show tabs, trailing whitespace, and continued lines visually
  set list listchars=tab:»·,trail:·,extends:…

  " highlight overly long lines same as TODOs.
  set textwidth=80
  autocmd BufNewFile,BufRead *.c,*.h exec 'match Todo /\%>' . &textwidth . 'v.\+/'


=========================
FAQ & Statement Reference
=========================

Comments
--------

Comments should always use the standard C syntax.  C++ 
style comments are not currently allowed.


Indention & Whitespace & 80 columns
-----------------------------------

To avoid confusion, indentations are to be 8 character with tab (not 
8 ' ' characters.  When wrapping parameters for function calls, 
align the parameter list with the first parameter on the previous line.
Use tabs to get as close as possible and then fill in the final 7 
characters or less with whitespace.  For example,

        var1 = foo(arg1, arg2,
                   arg3);

The previous example is intended to illustrate alignment of function 
parameters across lines and not as encourage for gratuitous line 
splitting.  Never split a line before columns 70 - 79 unless you
have a really good reason.  Be smart about formatting.


If, switch, & Code blocks
-------------------------

Always follow an 'if' keyword with a space but don't include additional
spaces following or preceding the parentheses in the conditional.
This is good:

        if (x == 1)

This is bad:

        if ( x == 1 )

Yes we have a lot of code that uses the second form and we are trying 
to clean it up without being overly intrusive.

Note that this is a rule about parentheses following keywords and not
functions.  Don't insert a space between the name and left parentheses when 
invoking functions.

Braces for code blocks used by for, if, switch, while, do..while, etc.
should begin on the same line as the statement keyword and end on a line 
of their own.  NOTE: Functions are different and the beginning left brace
should begin on a line of its own.

If the beginning statement has to be broken across lines due to length,
the beginning brace should be on a line of its own.

The exception to the ending rule is when the closing brace is followed by 
another language keyword such as else or the closing while in a do..while 
loop.

Good examples::

        if (x == 1) {
                printf("good\n");
        }

        for (x=1;
             x<10;
             x++)
        {
                print("%d\n", x);
        }

        do {
                printf("also good\n");
        } while (1);

Bad examples::

        while (1)
        {
                print("I'm in a loop!\n"); }
        

Goto
----

While many people have been academically taught that goto's are fundamentally
evil, they can greatly enhance readability and reduce memory leaks when used
as the single exit point from a function.  But in no Samba world what so ever 
is a goto outside of a function or block of code a good idea.

Good Examples::

        int function foo(int y)
        {
                int *z = NULL;
                int ret = 0;

                if (y < 10) {
                        z = malloc(sizeof(int)*y);
                        if (!z) {
                                ret = 1;
                                goto done;
                        }
                }

                print("Allocated %d elements.\n", y);

         done: 
                if (z)
                        free(z);

                return ret;
        }


Checking Pointer Values
-----------------------

When invoking functions that return pointer values, either of the following 
are acceptable.  Use you best judgement and choose the more readable option.
Remember that many other people will review it.::

        if ((x = malloc(sizeof(short)*10)) == NULL ) {
                fprintf(stderr, "Unable to alloc memory!\n");
        }

or::

        x = malloc(sizeof(short)*10);
        if (!x) {
                fprintf(stderr, "Unable to alloc memory!\n");
        }


Primitive Data Types
--------------------

Samba has large amounts of historical code which makes use of data types 
commonly supported by the C99 standard. However, at the time such types 
as boolean and exact width integers did not exist and Samba developers 
were forced to provide their own.  Now that these types are guaranteed to 
be available either as part of the compiler C99 support or from lib/replace/, 
new code should adhere to the following conventions:

  * Booleans are of type "bool" (not BOOL)
  * Boolean values are "true" and "false" (not True or False)
  * Exact width integers are of type [u]int[8|16|32|64]_t


Typedefs
--------

Samba tries to avoid "typedef struct { .. } x_t;", we always use
"struct x { .. };". We know there are still those typedefs in the code,
but for new code, please don't do that.

Make use of helper variables
----------------------------

Please try to avoid passing function calls as function parameters
in new code. This makes the code much easier to read and
it's also easier to use the "step" command within gdb.

Good Example::

        char *name;

        name = get_some_name();
        if (name == NULL) {
                ...
        }

        ret = some_function_my_name(name);
        ...


Bad Example::

        ret = some_function_my_name(get_some_name());
        ...