Coding style

General

This document overrides the original Baseflight style that was referenced before. This document has taken inspiration from that style, from Eclipse defaults and from Linux, as well as from some Cleanflight and Betaflight developers and existing code.

There are not so many changes from the old style, if you managed to find it.

Formatting style

Indentation

1TBS (based K&R) indent style with 4 space indent, NO hard tabs (all tabs replaced by spaces).

Tool support

Any of these tools can get you pretty close:

Eclipse built in "K&R" style, after changing the indent to 4 spaces and change Braces after function declarations to Next line.

astyle --style=kr --indent=spaces=4 --min-conditional-indent=0 --max-instatement-indent=80 --pad-header --pad-oper --align-pointer=name --align-reference=name --max-code-length=120 --convert-tabs --preserve-date --suffix=none --mode=c

indent -kr -i4 -nut

(the options for these commands can be tuned more to comply even better)

Note: These tools are not authoritative. Sometimes, for example, you may want other columns and line breaks so it looks like a matrix.

Note2: The Astyle settings have been tested and will produce a nice result. Many files will be changed, mostly to the better but maybe not always, so use with care.

Curly Braces

Functions

Functions shall have the opening and closing braces at the beginning of the next line, and followed by a line break.

int function(int x)
{
    body of function
}

Non-function statement blocks

Opening braces

All non-function statement blocks (i.e. if, switch, for, as well as any others) shall have the opening brace last on the same line, with the following statement on the next line.

Closing braces

Closing braces shall be but on the line after the last statement in the block.

if (x is true) {
    we do y
    ...
}

switch (action) {
case ADD:
    return "add";
case REMOVE:
    return "remove";
case CHANGE:
    return "change";
default:
    return NULL;
}

If the closing brace is followed by an else or else if that shall be on the same line, again with the opening brace on the same line.

if (x is true) {
    we do y
    ...
} else {
    we do z
    ...
}

Braces are required

Omission of "unnecessary" braces in cases where an if or else block consists only of a single statement is not permissible in any case. These "single statement blocks" are future bugs waiting to happen when more statements are added without enclosing the block in braces.

Spaces

Use a space after (most) keywords. The notable exceptions are sizeof, typeof, alignof, and attribute, which look somewhat like functions (and are usually used with parentheses). So use a space after these keywords:

if, switch, case, for, do, while

but not with sizeof, typeof, alignof, or attribute. E.g.,

s = sizeof(struct file);

When declaring pointer data or a function that returns a pointer type, the preferred use of '*' is adjacent to the data name or function name and not adjacent to the type name. Examples:

char *linux_banner;
memparse(char *ptr, char **retptr);
char *match_strdup(substring_t *s);

Use one space around (on each side of) most binary and ternary operators, such as any of these:

=  +  -  <  >  *  /  %  |  &  ^  \<=  >=  ==  !=  ?  :

but no space after unary operators:

&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined

no space before the postfix increment & decrement unary operators:

++  --

no space after the prefix increment & decrement unary operators:

++  --

and no space around the '.' and "->" structure member operators.

'*' and '&', when used for pointer and reference, shall have no space between it and the following variable name.

typedef

enums that do not have a count or some other form of terminator element shall have a comma after their last element:

typedef enum {
    MSP_RESULT_ACK = 1,
    MSP_RESULT_ERROR = -1,
    MSP_RESULT_NO_REPLY = 0,
    MSP_RESULT_CMD_UNKNOWN = -2,
} mspResult_e;

This ensures that, if more elements are added at a later stage, only the additional lines show up in the review, making it easier to review.

enums with a count should have that count declared as the last item in the enumeration list, so that it is automatically maintained, e.g.:

typedef enum {
    PID_CONTROLLER_MW23 = 0,
    PID_CONTROLLER_MWREWRITE,
    PID_CONTROLLER_LUX_FLOAT,
    PID_COUNT
} pidControllerType_e;

It shall not be calculated afterwards, e.g. using PID_CONTROLLER_LUX_FLOAT + 1;

typedef struct definitions should include the struct name, so that the type can be forward referenced, that is in:

typedef struct motorMixer_s {
    float throttle;
...
    float yaw;
} motorMixer_t;

the motorMixer_s name is required.

Variables

Naming

Generally, descriptive lowerCamelCase names are preferred for function names, variables, arguments, etc. For configuration variables that are user accessible via CLI or similar, all_lowercase with underscore is preferred.

Variable names should be nouns.

Simple temporary variables with a very small scope may be short where it aligns with common practice. Such as "i" as a temporary counter in a for loop, like for (int i = 0; i < 4; i++). Using "temporaryCounter" in that case would not improve readability.

Declarations

Avoid global variables.

Variables should be declared at the top of the smallest scope where the variable is used. Variable re-use should be avoided - use distinct variables when their use is unrelated. One blank line should follow the declaration(s).

Hint: Sometimes you can create a block, i.e. add curly braces, to reduce the scope further. For example to limit variable scope to a single case branch.

Variables with limited use may be declared at the point of first use. It makes PR-review easier (but that point is lost if the variable is used everywhere anyway).

Initialisation

The pattern with "lazy initialisation" may be advantageous in the Configurator to speed up the start when the initialisation is "expensive" in some way. In the FC, however, it’s always better to use some milliseconds extra before take-off than to use them while flying.

So don't use "lazy initialisation".

An explicit "init" function, is preferred.

Data types

Be aware of the data types you use and do not trust implicit type casting to be correct.

Angles are sometimes represented as degrees in a float. Sometimes as decidegrees in a uint8_t. You have been warned.

Avoid implicit double conversions and only use float-argument functions.

Check .map file to make sure no conversions sneak in, and use -Wdouble-promotion warning for the compiler

Instead of sin() and cos(), use sin_approx() and cos_approx() from common/math.h.

Float constants should be defined with "f" suffix, like 1.0f and 3.1415926f, otherwise double conversion might occur.

Functions

Naming

Methods that return a boolean should be named as a question, and should not change any state. e.g. 'isOkToArm()'.

Methods should have verb or verb-phrase names, like deletePage or save. Tell the system to 'do' something 'with' something. e.g. deleteAllPages(pageList).

Non-static functions should be prefixed by their class. Eg baroUpdate and not updateCompass .

Groups of functions acting on an 'object' should share the same prefix, e.g.

float biQuadFilterApply(...);
void biQuadFilterInit(...);
boolean biQuadIsReady();

rather than

float applyBiQuadFilter(...);
void newBiQuadLpf(...);
boolean isBiQuadReady();

Parameter order

Data should move from right to left, as in memcpy(void *dst, const void *src, size_t size). This also mimics the assignment operator (e.g. dst = src;)

When a group of functions act on an 'object' then that object should be the first parameter for all the functions, e.g.:

float biQuadFilterApply(biquad_t *state, float sample);
void biQuadNewLpf(biquad_t *state, float filterCutFreq, uint32_t refreshRate);

rather than

float biQuadFilterApply(float sample, biquad_t *state);
void biQuadNewLpf(float filterCutFreq, biquad_t *state, uint32_t refreshRate);

Declarations

Functions not used outside their containing .c file should be declared static (or STATIC_UNIT_TESTED so they can be used in unit tests).

Non-static functions should have their declaration in a single .h file.

Don't make more than necessary visible for other modules, not even types. Pre-processor macros may be used to declare module internal things that must be shared with the modules test code but otherwise hidden.

In the .h file:

#ifdef MODULENAME_INTERNALS_
… declarations …
#endif

In the module .c file, and in the test file but nowhere else, put #define MODULENAME_INTERNALS_ just before including the .h file.

Note: You can get the same effect by putting the internals in a separate .h file.

Implementation

Keep functions short and distinctive. Think about unit test when you define your functions. Ideally you should implement the test cases before implementing the function.

Never put multiple statements on a single line. Never put multiple assignments on a single line. Never put multiple assignments in a single statement.

Defining constants using pre-processor macros is not preferred. Const-correctness should be enforced. This allows some errors to be picked up at compile time (for example getting the order of the parameters wrong in a call to memcpy).

A function should only read data from the HW once in each call, and preferably all at one place. For example, if gyro angle or time is needed multiple times, read once and store in a local variable.

Use for loops (rather than do or while loops) for iteration.

The use of continue or goto should be avoided. Same for multiple return from a function and multiple break inside a case. In general, they reduce readability and maintainability. In rare cases such constructs can be justified but only when you have considered and understood the alternatives and still have a strong reason.

In expressions, parentheses should only be used where they are required, i.e. where operator precedence will not evaluate in the right order, or where a compiler warning is triggered without parentheses. This brings all expressions into a canonical form, and avoids the problem of different developers having different ideas of what 'easy to read' expressions are.

One exception to this rule is the ternary conditional operator

pidStabilisationEnabled = (pidControllerState == PID_STABILISATION_ON) ? true : false

Here, the condition shall be enclosed in braces, to make the ternary operator easier to spot when reading left to right.

Includes

All files must include their own dependencies and not rely on includes from the included files or that some other file was included first.

Do not include things you are not using.

#pragma once (see wiki) is preferred over #include guards to avoid multiple includes.

Documentation comments

All new code files, structs, enums, and functions should include a comment at its top describing its purpose. All PRs that modify existing items should add a comment if not already present, and update it if applicable due to changes made by the PR. These comments may be omitted if the item is trivial or self-explanatory.

Example file-level comment:

/* This file contains code used to send DSHOT commands using the STM timer's burst DMA functionality.
* It uses DMA to alter timer PWM duty cycle. For an alternative approach, see `drivers/dshot_bitbang.c`
for a DSHOT implementation that uses GPIO bitbanging.
*/

Example enum-level comment:

/*
* Describes allocation of ELRS telemetry packets, and is associated with time between telemetry
* packet transmission.
* See [ELRS: Telemetry Bandwidth](https://www.expresslrs.org/1.0/info/telem-bandwidth/) for more information.
*/
typedef enum {
    TLM_RATIO_NO_TLM = 0,
    // ..

Other details

No trailing whitespace at the end of lines or at blank lines.

Stay within 120 columns, unless exceeding 120 columns significantly increases readability and does not hide information. (Less is acceptable. More than 140 makes it difficult to read on Github so that shall never be exceeded.)

Take maximum possible advantage of compile time checking, so generally warnings should be as strict as possible.

Don't call or reference "upwards". That is don't call or use anything in a software layer that is above the current layer. The software layers are not that obvious in Betaflight, but we can certainly say that device drivers are the bottom layer and so should not call or use anything outside the device drivers.

Target specific code (e.g. #ifdef CC3D) is not permissible outside of the src/main/target directory.

typedef void handlerFunc(void); is easier to read than typedef void (*handlerFuncPtr)(void);.

Code should be spherical. That is its surface area (public interfaces) relative to its functionality should be minimised. Which is another way of saying that the public interfaces shall be easy to use, do something essential and all implementation should be hidden and unimportant to the user

Code should work in theory as well as in practice. It should be based on sound mathematical, physical or computer science principles rather than just heuristics. This is important for test code too. Tests shall be based on such principles and real-world properties so they don't just test the current implementation as it happens to be.

Guidelines not tramlines: guidelines are not totally rigid - they can be broken when there is good reason.