README.Coding: add examples for good and bad comments

[samba.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, with a few
exceptions as mentioned below.

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 about 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 fillers.

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

* Follow the K&R guidelines.  We won't go through all of them here. Do 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. The lines before
a comment should be empty. If the comment directly belongs
to the following code, there should be no empty line after
the comment. In case the comment contains a summary of
mutliple following code blogs.

This is good:

        ...
        int i;

        /*
         * This is a multi line comment,
         * which explains the logical steps we have to do:
         *
         * 1. We need to set i=5, because...
         * 2. We need to call complex_fn1
         */

        /* This is a one line comment about i = 5. */
        i = 5;

        /*
         * This is a multi line comment,
         * explaining the call to complex_fn1()
         */
        ret = complex_fn1();
        if (ret != 0) {
        ...

        /**
         * @brief This is a doxygen comment.
         *
         * This is a more detailed explanation of
         * this simple function.
         *
         * @param[in]   param1     The parameter value of the function.
         *
         * @param[out]  result1    The result value of the function.
         *
         * @return              0 on success and -1 on error.
         */
        int example(int param1, int *result1);

This is bad:

        ...
        int i;
        /*
         * This is a multi line comment,
         * which explains the logical steps we have to do:
         *
         * 1. We need to set i=5, because...
         * 2. We need to call complex_fn1
         */
        /* This is a one line comment about i = 5. */
        i = 5;
        /*
         * This is a multi line comment,
         * explaining the call to complex_fn1()
         */
        ret = complex_fn1();
        if (ret != 0) {
        ...

        /*This is a one line comment.*/

        /* This is a multi line comment,
           with some more words...*/

        /*
         * This is a multi line comment,
         * with some more words...*/

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

To avoid confusion, indentations have to be tabs with length 8 (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. You should always include braces, even if the block only
contains one statement.  NOTE: Functions are different and the beginning left
brace should be located in the first column on the next line.

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);
        }

        for (really_really_really_really_long_var_name=0;
             really_really_really_really_long_var_name<10;
             really_really_really_really_long_var_name++)
        {
                print("%d\n", really_really_really_really_long_var_name);
        }

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

Bad examples:

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

        for (x=1;
             x<10;
             x++)
        {
                print("no good\n");
        }

        if (i < 10)
                print("I should be in braces.\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 your best judgement and choose the more readable option.
Remember that many other persons 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;" so we do always try to use
"struct x { .. };". We know there are still such typedefs in the code,
but for new code, please don't do that anymore.

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());
        ...