Background
==========

Every project has its coding style, and Embedded Linux Library is not an
exception.  This document describes the preferred coding style for the
Embedded Linux Library code, in order to keep some level of consistency among
developers so that code can be easily understood and maintained, and also
to help your code survive under maintainer's fastidious eyes so that you
can get a passport for your patch ASAP.

First of all, the Embedded Linux Library coding style must follow every rule
for the Linux kernel
(https://www.kernel.org/doc/html/latest/process/coding-style.html).
There also exists a tool named checkpatch.pl to help you check the compliance
with it. Just type "checkpatch.pl --no-tree --no-signoff patch_name" to check
your patch.

Please note, that this in effect means that we also follow the Linux Kernel
'programming language' process guidelines.  Please see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/programming-language.rst

In theory, you need to clean up all the warnings and errors.  In
certain circumstances one can ignore the 80 character per line limit.
This is generally only allowed if the alternative would make the code
even less readable.

Besides the kernel coding style above, the ell has special
flavors for its own. Some of them are mandatory (marked as 'M'), while
some others are optional (marked as 'O'), but generally preferred.


M1: Blank line before and after an if/while/do/for statement
============================================================
There should be a blank line before if statement unless the if is nested and
not preceded by an expression or variable declaration.

Example:
1)
a = 1;
if (b) {  // wrong

2)
a = 1

if (b) {
}
a = 2;	// wrong

3)
if (a) {
	if (b)  // correct

4)
b = 2;

if (a) {	// correct

}

b = 3;

The only exception to this rule applies when a variable is being initialized:
foo = foo_construct(...);
if (foo)	// Correct
	...;


M2: Multiple line comment
=========================
If your comments have more then one line, please start it from the second line.

Example:
/*
 * first line comment	// correct
 * ...
 * last line comment
 */


M3: Space before and after operator
===================================
There should be a space before and after each operator.

Example:
a + b;  // correct


M4: Wrap long lines
===================
If your condition in if, while, for statement or a function declaration is too
long to fit in one line, the new line needs to be indented not aligned with the
body.

Example:
1)
if (call->status == CALL_STATUS_ACTIVE ||
	call->status == CALL_STATUS_HELD) {  // wrong
	ofono_dbus_dict_append();

2)
if (call->status == CALL_STATUS_ACTIVE ||
		call->status == CALL_STATUS_HELD) {  // correct
	ofono_dbus_dict_append();

3)
gboolean sim_ust_is_available(unsigned char *service_ust, unsigned char len,
	num sim_ust_service index) // wrong
{
	int a;
	...
}

4)
gboolean sim_ust_is_available(unsigned char *service_ust, unsigned char len,
					enum sim_ust_service index) // correct
{
	int a;
	...
}

If the line being wrapped is a function call or function declaration, the
preferred style is to indent at least past the opening parenthesis. Indenting
further is acceptable as well (as long as you don't hit the 80 character
limit).

If this is not possible due to hitting the 80 character limit, then indenting
as far as possible to the right without hitting the limit is preferred.

Example:

1)
gboolean sim_ust_is_available(unsigned char *service_ust, unsigned char len,
		enum sim_ust_service index); // worse

2)
gboolean sim_ust_is_available(unsigned char *service_ust, unsigned char len,
						enum sim_ust_service index);
						// better


M5: Git commit message 50/72 formatting
=======================================
The commit message header should be within 50 characters. And if you have
detailed explanatory text, wrap it to 72 character.


M6: Space when performing type casting
=================================
There should be a space between new type and variable.

Example:
1)
a = (int *)b;  // wrong
2)
a = (int *) b;  // correct


M7: Don't initialize variable unnecessarily
===========================================
When declaring a variable, try not to initialize it unless necessary.

Example:
int i = 1;  // wrong

for (i = 0; i < 3; i++) {
}


M8: Follow the order of include header elements
===============================================
When writing an include header the various elements should be in the following
order:
	- #includes
	- forward declarations
	- #defines
	- enums
	- typedefs
	- function declarations and inline function definitions


M9: Internal headers must not use include guards
=================================================
Any time when creating a new header file with non-public API, that header
must not contain include guards.


M10: Naming of enums
====================
Enums must have a descriptive name.  The enum type should be small caps and
it should not be typedef-ed.  Enum contents should be in CAPITAL letters and
prefixed by the enum type name.

Example:

enum animal_type {
	ANIMAL_TYPE_FOUR_LEGS,
	ANIMAL_TYPE_EIGHT_LEGS,
	ANIMAL_TYPE_TWO_LEGS,
};

If the enum contents have values (e.g. from specification) the formatting
should be as follows:

enum animal_type {
	ANIMAL_TYPE_FOUR_LEGS =		4,
	ANIMAL_TYPE_EIGHT_LEGS =	8,
	ANIMAL_TYPE_TWO_LEGS =		2,
};


M11: Enum as switch variable
====================

If the variable of a switch is an enum, you must not include a default in
switch body. The reason for this is: If later on you modify the enum by adding
a new type, and forget to change the switch accordingly, the compiler will
complain the new added type hasn't been handled.

Example:

enum animal_type {
	ANIMAL_TYPE_FOUR_LEGS =		4,
	ANIMAL_TYPE_EIGHT_LEGS =	8,
	ANIMAL_TYPE_TWO_LEGS =		2,
};

enum animal_type t;

switch (t) {
case ANIMAL_TYPE_FOUR_LEGS:
	...
	break;
case ANIMAL_TYPE_EIGHT_LEGS:
	...
	break;
case ANIMAL_TYPE_TWO_LEGS:
	...
	break;
default:  // wrong
	break;
}

However if the enum comes from an external header file outside ofono
we cannot make any assumption of how the enum is defined and this
rule might not apply.


M12: Check for pointer being NULL
=================================
When checking if a pointer or a return value is NULL, use the shorter check
with "!" operator rather than explicitly compare to NULL.

Example:
1)
foo = foo_constructor(...);
if (!foo)	// Correct
	...;

2)
foo = foo_constructor(...);
if (foo == NULL)	// Wrong
	return;


M13: Always use parenthesis with sizeof
=======================================
The expression argument to the sizeof operator should always be in
parenthesis, too.

Example:
1)
memset(stuff, 0, sizeof(*stuff));

2)
memset(stuff, 0, sizeof *stuff); // Wrong


M14: Use void if function has no parameters
===========================================================
A function with no parameters must use void in the parameter list.

Example:
1)
void foo(void)
{
}

2)
void foo()	// Wrong
{
}


M15: Don't use hex value with shift operators
==============================================
The expression argument to the shift operators should not be in hex.

Example:

1)
1 << y

2)
0x1 << y	// Wrong


O1: Shorten the name
====================
Better to use abbreviation, rather than full name, to name a variable,
function, struct, etc.

Example:
supplementary_service  // too long
ss  // better


O2: Try to avoid complex if body
================================
It's better not to have a complicated statement for if. You may judge its
contrary condition and return | break | continue | goto ASAP.

Example:
1)
if (a) {  // worse
	struct voicecall *v;
	call = synthesize_outgoing_call(vc, vc->pending);
	v = voicecall_create(vc, call);
	v->detect_time = time(NULL);
	DBG("Registering new call: %d", call->id);
	voicecall_dbus_register(v);
} else
	return;

2)
if (!a)
	return;

struct voicecall *v;
call = synthesize_outgoing_call(vc, vc->pending);
v = voicecall_create(vc, call);
v->detect_time = time(NULL);
DBG("Registering new call: %d", call->id);
voicecall_dbus_register(v);