[asterisk/asterisk.git] / doc / CODING-GUIDELINES

== Asterisk Patch/Coding Guidelines ==
--------------------------------------

We are looking forward to your contributions to Asterisk - the 
Open Source PBX! As Asterisk is a large and in some parts very
time-sensitive application, the code base needs to conform to
a common set of coding rules so that many developers can enhance
and maintain the code. Code also needs to be reviewed and tested
so that it works and follows the general architecture and guide-
lines, and is well documented.

Asterisk is published under a dual-licensing scheme by Digium.
To be accepted into the codebase, all non-trivial changes must be
disclaimed to Digium or placed in the public domain. For more information
see http://bugs.digium.com

Patches should be in the form of a unified (-u) diff, made from the directory
above the top-level Asterisk source directory. For example:

- the base code you are working from is in ~/work/asterisk-base
- the changes are in ~/work/asterisk-new

~/work$ diff -urN asterisk-base asterisk-new

If you are changing within a fresh CVS/SVN repository, you can create
a patch with
$ cvs diff -urN <mycodefile>.c

* General rules
---------------

- All code, filenames, function names and comments must be in ENGLISH.

- Don't annotate your changes with comments like "/* JMG 4/20/04 */";
  Comments should explain what the code does, not when something was changed
  or who changed it. If you have done a larger contribution, make sure
  that you are added to the CREDITS file.

- Don't make unnecessary whitespace changes throughout the code.
  If you make changes, submit them to the tracker as separate patches
  that only include whitespace and formatting changes.

- Don't use C++ type (//) comments.

- Try to match the existing formatting of the file you are working on.

- Use spaces instead of tabs when aligning in-line comments or #defines (this makes 
  your comments aligned even if the code is viewed with another tabsize)

* Declaration of functions and variables
----------------------------------------

- Do not declare variables mid-block (e.g. like recent GNU compilers support)
  since it is harder to read and not portable to GCC 2.95 and others.

- Functions and variables that are not intended to be used outside the module
  must be declared static.

- When reading integer numeric input with scanf (or variants), do _NOT_ use '%i'
  unless you specifically want to allow non-base-10 input; '%d' is always a better
  choice, since it will not silently turn numbers with leading zeros into base-8.

- Strings that are coming from input should not be used as a first argument to
  a formatted *printf function.

* Use the internal API
----------------------

- Make sure you are aware of the string and data handling functions that exist
  within Asterisk to enhance portability and in some cases to produce more
  secure and thread-safe code. Check utils.c/utils.h for these.


* Code formatting
-----------------

Roughly, Asterisk code formatting guidelines are generally equivalent to the 
following:

# indent -i4 -ts4 -br -brs -cdw -lp -ce -nbfda -npcs -nprs -npsl -nbbo -saf -sai -saw -cs -l90 foo.c

this means in verbose:
 -i4:    indent level 4
 -ts4:   tab size 4
 -br:    braces on if line
 -brs:   braces on struct decl line
 -cdw:   cuddle do while
 -lp:    line up continuation below parenthesis
 -ce:    cuddle else
 -nbfda: dont break function decl args
 -npcs:  no space after function call names
 -nprs:  no space after parentheses
 -npsl:  dont break procedure type
 -saf:   space after for
 -sai:   space after if
 -saw:   space after while
 -cs:    space after cast
 -ln90:  line length 90 columns

Function calls and arguments should be spaced in a consistent way across
the codebase.
        GOOD: foo(arg1, arg2);
        GOOD: foo(arg1,arg2);   /* Acceptable but not preferred */
        BAD: foo (arg1, arg2);
        BAD: foo( arg1, arg2 );
        BAD: foo(arg1, arg2,arg3);

Don't treat keywords (if, while, do, return) as if they were functions;
leave space between the keyword and the expression used (if any). For 'return',
don't even put parentheses around the expression, since they are not
required.

There is no shortage of whitespace characters :-) Use them when they make
the code easier to read. For example:

        for (str=foo;str;str=str->next)

is harder to read than

        for (str = foo; str; str = str->next)

Following are examples of how code should be formatted.

- Functions:
int foo(int a, char *s)
{
        return 0;
}

- If statements:
if (foo) {
        bar();
} else {
        blah();
}

- Case statements:
switch (foo) {
case BAR:
        blah();
        break;
case OTHER:
        other();
        break;
}

- No nested statements without braces, e.g.:

for (x = 0; x < 5; x++)
        if (foo) 
                if (bar)
                        baz();

instead do:
for (x = 0; x < 5; x++) {
        if (foo) {
                if (bar)
                        baz();
        }
}

- Don't build code like this:

if (foo) {
        /* .... 50 lines of code ... */
} else {
        result = 0;
        return;
}

Instead, try to minimize the number of lines of code that need to be
indented, by only indenting the shortest case of the 'if'
statement, like so:

if !(foo) {
        result = 0;
        return;
}

.... 50 lines of code ....

When this technique is used properly, it makes functions much easier to read
and follow, especially those with more than one or two 'setup' operations
that must succeed for the rest of the function to be able to execute.

- Labels/goto are acceptable
Proper use of this technique may occasionally result in the need for a
label/goto combination so that error/failure conditions can exit the
function while still performing proper cleanup. This is not a bad thing!
Use of goto in this situation is encouraged, since it removes the need
for excess code indenting without requiring duplication of cleanup code.
       
- Never use an uninitialized variable
Make sure you never use an uninitialized variable.  The compiler will 
usually warn you if you do so. However, do not go too far the other way,
and needlessly initialize variables that do not require it. If the first
time you use a variable in a function is to store a value there, then
initializing it at declaration is pointless, and will generate extra
object code and data in the resulting binary with no purpose. When in doubt,
trust the compiler to tell you when you need to initialize a variable;
if it does not warn you, initialization is not needed.

- Do not cast 'void *'
Do not explicitly cast 'void *' into any other type, nor should you cast any
other type into 'void *'. Implicit casts to/from 'void *' are explicitly
allowed by the C specification. This means the results of malloc(), calloc(),
alloca(), and similar functions do not _ever_ need to be cast to a specific
type, and when you are passing a pointer to (for example) a callback function
that accepts a 'void *' you do not need to cast into that type.

* Variable naming
-----------------

- Global variables
Name global variables (or local variables when you have a lot of them or
are in a long function) something that will make sense to aliens who
find your code in 100 years.  All variable names should be in lower 
case, except when following external APIs or specifications that normally
use upper- or mixed-case variable names; in that situation, it is
preferable to follow the external API/specification for ease of
understanding.

Make some indication in the name of global variables which represent
options that they are in fact intended to be global.
 e.g.: static char global_something[80]

- Don't use un-necessary typedef's
Don't use 'typedef' just to shorten the amount of typing; there is no substantial
benefit in this:

struct foo {
        int bar;
};
typedef foo_t struct foo;

In fact, don't use 'variable type' suffixes at all; it's much preferable to
just type 'struct foo' rather than 'foo_s'.

- Use enums instead of #define where possible
Use enums rather than long lists of #define-d numeric constants when possible;
this allows structure members, local variables and function arguments to
be declared as using the enum's type. For example:

enum option {
  OPT_FOO = 1
  OPT_BAR = 2
  OPT_BAZ = 4
};

static enum option global_option;

static handle_option(const enum option opt)
{
  ...
}

Note: The compiler will _not_ force you to pass an entry from the enum
as an argument to this function; this recommendation serves only to make
the code clearer and somewhat self-documenting. In addition, when using
switch/case blocks that switch on enum values, the compiler will warn
you if you forget to handle one or more of the enum values, which can be
handy.

* String handling
-----------------

Don't use strncpy for copying whole strings; it does not guarantee that the
output buffer will be null-terminated. Use ast_copy_string instead, which
is also slightly more efficient (and allows passing the actual buffer
size, which makes the code clearer).

Don't use ast_copy_string (or any length-limited copy function) for copying
fixed (known at compile time) strings into buffers, if the buffer is something
that has been allocated in the function doing the copying. In that case, you
know at the time you are writing the code whether the buffer is large enough
for the fixed string or not, and if it's not, your code won't work anyway!
Use strcpy() for this operation, or directly set the first two characters
of the buffer if you are just trying to store a one-character string in the
buffer. If you are trying to 'empty' the buffer, just store a single
NULL character ('\0') in the first byte of the buffer; nothing else is
needed, and any other method is wasteful.

In addition, if the previous operations in the function have already
determined that the buffer in use is adequately sized to hold the string
you wish to put into it (even if you did not allocate the buffer yourself),
use a direct strcpy(), as it can be inlined and optimized to simple
processor operations, unlike ast_copy_string().

* Use of functions
------------------

When making applications, always ast_strdupa(data) to a local pointer if
you intend to parse the incoming data string.

        if (data)
                mydata = ast_strdupa(data);


- Separating arguments to dialplan applications and functions
Use ast_app_separate_args() to separate the arguments to your application
once you have made a local copy of the string.

- Parsing strings with strsep
Use strsep() for parsing strings when possible; there is no worry about
're-entrancy' as with strtok(), and even though it modifies the original
string (which the man page warns about), in many cases that is exactly
what you want!

- Create generic code!
If you do the same or a similar operation more than one time, make it a
function or macro.

Make sure you are not duplicating any functionality already found in an
API call somewhere.  If you are duplicating functionality found in 
another static function, consider the value of creating a new API call 
which can be shared.

* Handling of pointers and allocations
--------------------------------------

- Dereference or localize pointers
Always dereference or localize pointers to things that are not yours like
channel members in a channel that is not associated with the current 
thread and for which you do not have a lock.
        channame = ast_strdupa(otherchan->name);

- Use const on pointer arguments if possible
Use const on pointer arguments which your function will not be modifying, as this 
allows the compiler to make certain optimizations. In general, use 'const'
on any argument that you have no direct intention of modifying, as it can
catch logic/typing errors in your code when you use the argument variable
in a way that you did not intend.

- Do not create your own linked list code - reuse!
As a common example of this point, make an effort to use the lockable
linked-list macros found in include/asterisk/linkedlists.h. They are
efficient, easy to use and provide every operation that should be
necessary for managing a singly-linked list (if something is missing,
let us know!). Just because you see other open-coded list implementations
in the source tree is no reason to continue making new copies of
that code... There are also a number of common string manipulation
and timeval manipulation functions in asterisk/strings.h and asterisk/time.h;
use them when possible.

- Avoid needless allocations!
Avoid needless malloc(), strdup() calls. If you only need the value in
the scope of your function try ast_strdupa() or declare structs on the
stack and pass a pointer to them. However, be careful to _never_ call
alloca(), ast_strdupa() or similar functions in the argument list
of a function you are calling; this can cause very strange stack
arrangements and produce unexpected behavior.

-Allocations for structures
When allocating/zeroing memory for a structure, use code like this:

struct foo *tmp;

...

tmp = ast_calloc(1, sizeof(*tmp));

Avoid the combination of ast_malloc() and memset().  Instead, always use
ast_calloc(). This will allocate and zero the memory in a single operation. 
In the case that uninitialized memory is acceptable, there should be a comment
in the code that states why this is the case.

Using sizeof(*tmp) instead of sizeof(struct foo) eliminates duplication of the 
'struct foo' identifier, which makes the code easier to read and also ensures 
that if it is copy-and-pasted it won't require as much editing.

The ast_* family of functions for memory allocation are functionally the same.
They just add an Asterisk log error message in the case that the allocation
fails for some reason. This eliminates the need to generate custom messages
throughout the code to log that this has occurred.

-String Duplications

The functions strdup and strndup can *not* accept a NULL argument. This results
in having code like this:

        if (str)
                newstr = strdup(str);
        else
                newstr = NULL;

However, the ast_strdup and ast_strdup functions will happily accept a NULL
argument without generating an error.  The same code can be written as:
        
        newstr = ast_strdup(str);

Furthermore, it is unnecessary to have code that malloc/calloc's for the length
of a string (+1 for the terminating '\0') and then using strncpy to copy the
copy the string into the resulting buffer.  This is the exact same thing as
using ast_strdup.

* CLI Commands
--------------

New CLI commands should be named using the module's name, followed by a verb
and then any parameters that the command needs. For example:

*CLI> iax2 show peer <peername>

not

*CLI> show iax2 peer <peername>

* New dialplan applications/functions
-------------------------------------

There are two methods of adding functionality to the Asterisk
dialplan: applications and functions. Applications (found generally in
the apps/ directory) should be collections of code that interact with
a channel and/or user in some significant way. Functions (which can be
provided by any type of module) are used when the provided
functionality is simple... getting/retrieving a value, for
example. Functions should also be used when the operation is in no way
related to a channel (a computation or string operation, for example).

Applications are registered and invoked using the
ast_register_application function; see the apps/app_skel.c file for an
example.

Functions are registered using 'struct ast_custom_function'
structures and the ast_custom_function_register function.

* Doxygen API Documentation Guidelines
--------------------------------------

When writing Asterisk API documentation the following format should be
followed. Do not use the javadoc style.

/*!
 * \brief Do interesting stuff.
 * \param thing1 interesting parameter 1.
 * \param thing2 interesting parameter 2.
 *
 * This function does some interesting stuff.
 *
 * \return zero on success, -1 on error.
 */
int ast_interesting_stuff(int thing1, int thing2)
{
        return 0;
}

Notice the use of the \param, \brief, and \return constructs.  These should be
used to describe the corresponding pieces of the function being documented.
Also notice the blank line after the last \param directive.  All doxygen
comments must be in one /*! */ block.  If the function or struct does not need
an extended description it can be left out.

Please make sure to review the doxygen manual and make liberal use of the \a,
\code, \c, \b, \note, \li and \e modifiers as appropriate.

When documenting a 'static' function or an internal structure in a module,
use the \internal modifier to ensure that the resulting documentation
explicitly says 'for internal use only'.

Structures should be documented as follows.

/*!
 * \brief A very interesting structure.
 */
struct interesting_struct
{
        /*! \brief A data member. */
        int member1;

        int member2; /*!< \brief Another data member. */
}

Note that /*! */ blocks document the construct immediately following them
unless they are written, /*!< */, in which case they document the construct
preceding them.

* Finishing up before you submit your code
------------------------------------------

- Look at the code once more
When you achieve your desired functionalty, make another few refactor
passes over the code to optimize it.

- Read the patch
Before submitting a patch, *read* the actual patch file to be sure that 
all the changes you expect to be there are, and that there are no 
surprising changes you did not expect. During your development, that
part of Asterisk may have changed, so make sure you compare with the
latest CVS.

- Listen to advice
If you are asked to make changes to your patch, there is a good chance
the changes will introduce bugs, check it even more at this stage.
Also remember that the bug marshal or co-developer that adds comments 
is only human, they may be in error :-)

- Optimize, optimize, optimize
If you are going to reuse a computed value, save it in a variable
instead of recomputing it over and over.  This can prevent you from 
making a mistake in subsequent computations, making it easier to correct
if the formula has an error and may or may not help optimization but 
will at least help readability.

Just an example (so don't over analyze it, that'd be a shame):

const char *prefix = "pre";     
const char *postfix = "post";
char *newname;
char *name = "data";

if (name && (newname = alloca(strlen(name) + strlen(prefix) + strlen(postfix) + 3)))
        snprintf(newname, strlen(name) + strlen(prefix) + strlen(postfix) + 3, "%s/%s/%s", prefix, name, postfix);

...vs this alternative:

const char *prefix = "pre";
const char *postfix = "post";
char *newname;
char *name = "data";
int len = 0;

if (name && (len = strlen(name) + strlen(prefix) + strlen(postfix) + 3) && (newname = alloca(len)))
        snprintf(newname, len, "%s/%s/%s", prefix, name, postfix);

* Creating new manager events?
------------------------------
If you create new AMI events, please read manager.txt. Do not re-use
existing headers for new purposes, but please re-use existing headers
for the same type of data.

Manager events that signal a status are required to have one
event name, with a status header that shows the status.
The old style, with one event named "ThisEventOn" and another named
"ThisEventOff", is no longer approved.

Check manager.txt for more information on manager and existing
headers. Please update this file if you add new headers.

-----------------------------------------------
Welcome to the Asterisk development community!
Meet you on the asterisk-dev mailing list. 
Subscribe at http://lists.digium.com!

Mark Spencer, Kevin P. Fleming and 
the Asterisk.org Development Team