Coding Standard

This page documents the coding standard for libmaple. It’s intended as a guide for how you should structure any code you would like included into the LeafLabs releases of libmaple.

LeafLabs team members are required to follow these when producing new code. Community contributors to libmaple are strongly encouraged to do so; following these rules will greatly increase the probability that your patches will be folded in.

In general, follow this guide unless there’s a very good reason not to. Laziness doesn’t count as a good reason. Most, if not all, of these decisions are entirely arbitrary, but it’s important for readability that we be consistent. (If you notice an inconsistency, you should fix it).

Note that the file .dir-locals.el in the libmaple root directory already ensures that many of these standards are followed by default in Emacs (but not on Windows, where it would need to be named _dir_locals.el, and no way, man). There’s also some elisp scattered about this file which will provide you additional help.

Vim customizations to do the same thing would be nice!

License

Put an MIT license at the beginning of the file (look at any of our source files for an example). Copyright should go either to you or to LeafLabs, LLC.

Emacs: if you don’t like seeing the license, you should use elide-head (which will hide it for you). You can use the following:

(require 'elide-head)
(setq programming-mode-hooks '(c-mode-hook c++-mode-hook))
(add-to-list 'elide-head-headers-to-hide
             '("The MIT License" . "DEALINGS IN\n [*] THE SOFTWARE"))
(add-to-list 'elide-head-headers-to-hide
             '("The MIT License" . "DEALINGS IN THE\n...SOFTWARE"))
(dolist (hook programming-mode-hooks)
  (add-hook hook (lambda () (elide-head))))

Whitespace

TextMate users may find this bundle useful for automatically converting tabs to spaces and removing trailing whitespace at save time.

• 4 space indents (set in .dir-locals.el).
• Unix newlines.
• No tab characters (set in .dir-locals.el).
• No trailing whitespace. For help getting this (and no tab characters) done automatically in Emacs, you can use code-fascism.el.
• Files end in exactly one newline. The presence of a newline at EOF is already done by c-require-final-newline in recent versions of Emacs.
• Exactly two newlines separate source paragraphs (you do separate your code into paragraphs, don’t you?).
• The first line in a function is non-blank.

• Exactly one space after if, else, for, and while, before the following { or (. One space before else, after the preceding }. For example:

  // This is good; we like this:
  if (foo) {
      while (quux) {
          bar();
      }
  } else {
      baz();
  }
  
  // THIS IS BAD! DON'T DO THIS:
  if(foo){
      while(quux){
          bar();
      }
  }else{
      baz();
  }
  

• Exactly one space in between binary arithmetic, logical, and comparison operators and their operands. Examples:

  // This is good:
  int x = a + b * (c - d);
  if (x != 0 && a > 7) {
      SerialUSB.println(x);
  }
  
  // THIS IS BAD!
  int x = a+b*(c-d);
  if (x!=0 && a>7) {
      SerialUSB.println(x);
  }
  
  // This is good:
  uint32 adc_data = ADC1_BASE->DR;
  SerialUSB.println(adc_data);
  
  // THIS IS BAD!
  uint32 adc_data = ADC1_BASE -> DR;
  SerialUSB . println(adc_data);
  

• No space between a unary operator and its operand. Examples:

  // Good:
  x++;
  
  // BAD!
  x ++;
  
  // Good:
  y = -x;
  
  // BAD!
  y = - x;
  

• If you need to break up a long line:

  • Prefer to break up long expressions after a binary operator. Example:

    // Good:
    if (some_really_long_conditional_wow_this_really_goes_on_forever ||
        maybe_something_else_could_happen_too) {
        ...
    }
    
    // BAD!
    if (some_really_long_conditional_wow_this_really_goes_on_forever
        || maybe_something_else_could_happen_too) {
        ...
    }
    

  • When breaking up a function’s arguments over multiple lines, align the arguments on subsequent lines with the first argument. Example:

    // Good:
    return_type value_i_got = function_with_a_really_long_name(argument1,
                                                               argument2,
                                                               argument3);
    
    // BAD!
    return_type value_i_got = function_with_a_really_long_name(argument1,
        argument2,
        argument3);
    
    // BAD!
    return_type value_i_got = function_with_a_really_long_name(argument1,
                                                                  argument2,
                                                                  argument3);
    

• In function invocations, no space in between the function name and the opening parenthesis. Example:

  // Good:
  SerialUSB.println("Hello, world!");
  
  // BAD!
  SerialUSB.println ("Hello, world!");
  

• Don’t indent C code within a conditionally-compiled extern "C" block. Example:

  // Good:
  #ifdef __cplusplus
  extern "C"{
  #endif
  
  void some_c_function(void);
  
  #ifdef __cplusplus
  } // extern "C"
  #endif
  
  // BAD!
  #ifdef __cplusplus
  extern "C"{
  #endif
  
      void some_c_function(void);
  
  #ifdef __cplusplus
  } // extern "C"
  #endif
  

  Emacs does the “bad” behavior by default, which can be very annoying. You can turn this off with

  (defun c-mode-inextern-lang-hook ()
      (setcdr (assq 'inextern-lang c-offsets-alist) '-))
  (add-hook 'c-mode-hook c-mode-inextern-lang-hook)

Comments

• Multi-line comments are pretty flexible. Any of these is fine:

  /* Comment starts here.
   * Continued lines have a '*' before them.
   * The comment can end after the last line.
   */
  
  /* Comment starts here.
   * The comment can end on the same line. */
  
  /*
   * You can also place a newline after the opening "/*".
   */
  

• Doxygen comments are multi-line comments that begin with /** instead.

• Single-line comments are up to you.

Braces

• Mostly 1TBS. The only difference is that the opening brace of a function’s definition occurs exactly one space character after the closing parenthesis in that function’s parameter list. Example:

  void func(void) {
      ...
  }
  

Naming conventions

We’ll handle the usual casing/underscore debate as follows.

• First, Dont_Mix_Like_This, because It_Looks_Really_Ugly, ok? [There’s been some debate about this, and some exceptions are already grandfathered in, so in order to settle it, let’s call this a “recommendation” instead of “requirement”.]

• Variables: Use underscores to separate words in C identifiers:

  int some_example_name;
  

  User-facing C++ variables should be camel cased (thisIsAnExample, boardPWMPins, etc.), for consistency with the Arduino style. It’s probably a good idea for you to case non-user facing C++ variables in the C style; this will help disambiguate what’s part of the Wirish API and what’s not.

• Classes: Pascal case. So ThisIsAClassName, but thisIsNot, this_is_not, and Dont_You_DareTryANYTHING_STUPID.

• Functions: C functions are all lowercase, and words are separated by underscores. C++ method names are camel cased.

• Structs: Usually like variables (adc_dev, adc_reg_map, etc.), but it’s not crucial. Don’t feel obliged to put _t at the end of the type name; we don’t.

• Macros and constants: all caps, separated by underscores. C++ variables with the const qualifier generally aren’t considered “constants” for the purposes of this rule; i.e., they are cased according to the rules for variables. We make an exception for PIN_MAP, because it’s the central Wirish data structure.

• foo.h gets #ifdef‘ed to _FOO_H_.

• Acronyms: The case of letters in an acronym is determined by the case of the first letter in the acronym, which is determined by following the above rules. Examples:

  // Good:
  void usb_func() { ... }
  void frob_usb_disc() { ... }
  class SomethingUSB {
      void usbInit();
      void initUSB();
  };
  
  // BAD:
  class BadUsb { ... }; // say "GoodUSB" instead
  void swizzle_USB_disc() { ... } // say "swizzle_usb_disc" instead
  

Documentation

• Doxygen comments on every user-facing function and type. Additionally, individually document the fields and enumerator values of nontrivial user-facing structs and enums. See any register map type’s definition for an example.

• For libmaple proper, you don’t need comments for each register bit definition, since that’s just repeating information better obtained by reading ST RM0008.

• Doxygen comments generally only belong on types, functions, etc. that are part of the public user-facing API.

  This essentially means that if what you’re writing is going to be documented under http://leaflabs.com/docs/ (i.e., if there’s Sphinx documentation for it in the leaflabs-docs repository), then you need to write Doxygen comments. Further, those Sphinx docs should use Breathe to pull the Doxygen out. (For more information on this, see the leaflabs-docs README).

  Because Breathe isn’t totally mature yet, you won’t always be able to do this. In these cases, document the code “manually” using the Sphinx C and C++ domains. This should be avoided if at all possible, since it creates a maintenance burden of documenting things in two places at once, and makes it easier for documentation to go stale.

  If you do have to document something manually, put a comment in the source file informing future maintainers about it, so they’ll pay extra attention when making changes.

• When adding peripheral support, it would be nice if you put longer-form comments into the libmaple notes/ directory, with a comment in the corresponding .h file referring to it. See the dac.h source for an example.

  This lets us keep the source files relatively free of “introductory” material, while allowing new readers a convenient starting point. These longer-form notes also have a habit of turning into official, user-facing documentation (or wiki pages).

• For libmaple proper, the convention is to document any user-facing function at the point where it is defined. In particular, this means you should document an externally-linked function defined in a .c file in that .c file, not in the header file where it is declared to the user.

  For Wirish, the convention is to put the documentation in the header file where the function is declared.

General Formatting

• Keep it 80-column clean.

  Emacs users: this means that the largest column number is 79. You should turn on column number mode to help you out:

  (column-number-mode 1)
  

  You can get more help from lineker-mode. Just put lineker.el somewhere in your load-path, and:

  (require 'lineker)
  (dolist (hook '(c-mode-hook c++-mode-hook))
    (add-hook hook (lambda () (lineker-mode 1))))
  

Language Features

In libmaple proper, aim for C99 compatibility. Some GCC extensions are OK, but don’t get crazy.

Explicitly approved GCC extensions:

• asm volatile
• Nested functions

In Wirish, generally be very conservative when using C++ features that aren’t part of C. We are forced to use C++ for Arduino compatibility (and the general Arduino style of conflating objects and libraries), but it’s an angry beast, and we don’t want to provoke it. The mantra is “C with classes”.

Explicitly approved C++ features:

• Initializers that aren’t constant; e.g. the gpio_dev* values in a PIN_MAP.
• Default arguments: e.g., the timeout argument in waitForButtonPress().

Explicitly forbidden C++ features:

• Templates

Conditionally allowed C++ features:

• Operator overloading: Never allowed when it’s just for style. Probably fine when you’re implementing a class that models a mathematical structure, and you’d like to implement e.g. operator+().