README.CodingStyle

Coding conventions in the libssh tree
======================================

===========
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 for C code, is the Linux kernel coding style (See
Documentation/CodingStyle in the kernel source tree). This closely matches what
libssh 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 4 Spaces to Indent

* No Trailing Whitespace
  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?


=============
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)))


Vim
----

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

    set ts=4 sw=4 et cindent

You can use the Vim gitmodline plugin to store this in the git config:

    https://git.cryptomilk.org/projects/vim-gitmodeline.git/

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, except if
the comment contains a summary of multiple following code blocks.

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 4 spaces. Do not use tabs!.  When
wrapping parameters for function calls, align the parameter list with the first
parameter on the previous line.  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 )

or

    if (x==1)

Yes we have a lot of code that uses the second and third 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 libssh 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 rc = 0;

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

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

    done:
        if (z != NULL) {
            free(z);
        }

        return rc;
    }

Initialize pointers
-------------------

All pointer variables MUST be initialized to NULL. History has
demonstrated that uninitialized pointer variables have lead to various
bugs and security issues.

Pointers MUST be initialized even if the assignment directly follows
the declaration, like pointer2 in the example below, because the
instructions sequence may change over time.

Good Example:

    char *pointer1 = NULL;
    char *pointer2 = NULL;

    pointer2 = some_func2();

    ...

    pointer1 = some_func1();

Typedefs
---------

libssh 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) {
        ...
    }

    rc = some_function_my_name(name);
    ...


Bad Example:

    rc = some_function_my_name(get_some_name());
    ...

Please try to avoid passing function return values to if- or while-conditions.
The reason for this is better handling of code under a debugger.

Good example:

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

Bad example:

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

There are exceptions to this rule. One example is walking a data structure in
an iterator style:

    while ((opt = poptGetNextOpt(pc)) != -1) {
        ... do something with opt ...
    }

But in general, please try to avoid this pattern.


Control-Flow changing macros
-----------------------------

Macros like STATUS_NOT_OK_RETURN that change control flow (return/goto/etc)
from within the macro are considered bad, because they look like function calls
that never change control flow. Please do not introduce them.