The Asterisk Coding Guidelines

The Asterisk Coding Guidelines attempts to explain the use, basic styles and formatting of code used in Asterisk. In order to easily maintain the existing code, new code (and new developers) should conform to these common set of coding standards. Before requesting a review and testing of your code, be sure to read these guidelines to make certain that your code meets these standards.

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 the Bugs Guidelines.

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 toa 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

In verbose, the above means:

• -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)

Code examples

The following are examples of how code should be formatted.

If statements

if (foo) {
	bar();
} else {
	blah();
}

Case statements

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

No nested statements without braces

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

Instead do nested statements with braces

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;
}

Build code like this

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


not

*CLI> show iax2 peer


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 functionality, 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 SVN.

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.