This chapter describes the source code style preferred by the LinuxCNC team.

1. Do no harm

When making small edits to code in a style different than the one described below, observe the local coding style. Rapid changes from one coding style to another decrease code readability.

Never check in code after running “indent” on it. The whitespace changes introduced by indent make it more difficult to follow the revision history of the file.

Do not use an editor that makes unneeded changes to whitespace (e.g., which replaces 8 spaces with a tabstop on a line not otherwise modified, or word-wraps lines not otherwise modified)

2. Tab Stops

A tab stop always corresponds to 8 spaces. Do not write code that displays correctly only with a differing tab stop setting.

3. Indentation

Use 4 spaces per level of indentation. Combining 8 spaces into one tab is acceptable but not required.

4. Placing Braces

Put the opening brace last on the line, and put the closing brace first:

if (x) {
    // do something appropriate

The closing brace is on a line of its own, except in the cases where it is followed by a continuation of the same statement, i.e. a while in a do-statement or an else in an if-statement, like this:

do {
    // something important
} while (x > 0);


if (x == y) {
    // do one thing
} else if (x < y) {
    // do another thing
} else {
    // do a third thing

This brace-placement also minimizes the number of empty (or almost empty) lines, which allows a greater amount of code or comments to be visible at once in a terminal of a fixed size.

5. Naming

C is a Spartan language, and so should your naming be. Unlike Modula-2 and Pascal programmers, C programmers do not use cute names like ThisVariableIsATemporaryCounter. A C programmer would call that variable tmp, which is much easier to write, and not the least more difficult to understand.

However, descriptive names for global variables are a must. To call a global function foo is a shooting offense.

GLOBAL variables (to be used only if you really need them) need to have descriptive names, as do global functions. If you have a function that counts the number of active users, you should call that count_active_users() or similar, you should not call it cntusr().

Encoding the type of a function into the name (so-called Hungarian notation) is brain damaged - the compiler knows the types anyway and can check those, and it only confuses the programmer. No wonder Microsoft makes buggy programs.

LOCAL variable names should be short, and to the point. If you have some random integer loop counter, it should probably be called i. Calling it loop_counter is non-productive, if there is no chance of it being misunderstood. Similarly, tmp can be just about any type of variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another problem, which is called the function-growth-hormone-imbalance syndrome. See next chapter.

6. Functions

Functions should be short and sweet, and do just one thing. They should fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24, as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the complexity and indentation level of that function. So, if you have a conceptually simple function that is just one long (but simple) case-statement, where you have to do lots of small things for a lot of different cases, it’s OK to have a longer function.

However, if you have a complex function, and you suspect that a less-than-gifted first-year high-school student might not even understand what the function is all about, you should adhere to the maximum limits all the more closely. Use helper functions with descriptive names (you can ask the compiler to in-line them if you think it’s performance-critical, and it will probably do a better job of it that you would have done).

Another measure of the function is the number of local variables. They shouldn’t exceed 5-10, or you’re doing something wrong. Re-think the function, and split it into smaller pieces. A human brain can generally easily keep track of about 7 different things, anything more and it gets confused. You know you’re brilliant, but maybe you’d like to understand what you did 2 weeks from now.

7. Commenting

Comments are good, but there is also a danger of over-commenting. NEVER try to explain HOW your code works in a comment: it’s much better to write the code so that the working is obvious, and it’s a waste of time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW. A boxed comment describing the function, return value, and who calls it placed above the body is good. Also, try to avoid putting comments inside a function body: if the function is so complex that you need to separately comment parts of it, you should probably re-read the Functions section again. You can make small comments to note or warn about something particularly clever (or ugly), but try to avoid excess. Instead, put the comments at the head of the function, telling people what it does, and possibly WHY it does it.

If comments along the lines of /* Fix me */ are used, please, please, say why something needs fixing. When a change has been made to the affected portion of code, either remove the comment, or amend it to indicate a change has been made and needs testing.

8. Shell Scripts & Makefiles

Not everyone has the same tools and packages installed. Some people use vi, others emacs - A few even avoid having either package installed, preferring a lightweight text editor such as nano or the one built in to Midnight Commander.

gawk versus mawk - Again, not everyone will have gawk installed, mawk is nearly a tenth of the size and yet conforms to the Posix AWK standard. If some obscure gawk specific command is needed that mawk does not provide, than the script will break for some users. The same would apply to mawk. In short, use the generic awk invocation in preference to gawk or mawk.

9. C++ Conventions

C++ coding styles are always likely to end up in heated debates (a bit like the emacs versus vi arguments). One thing is certain however, a common style used by everyone working on a project leads to uniform and readable code.

Naming conventions: Constants either from #defines or enumerations should be in upper case through out. Rationale: Makes it easier to spot compile time constants in the source code. e.g. EMC_MESSAGE_TYPE

Classes and Namespaces should capitalize the first letter of each word and avoid underscores. Rationale: Identifies classes, constructors and destructors. e.g. GtkWidget

Methods (or function names) should follow the C recommendations above and should not include the class name. Rationale: Maintains a common style across C and C++ sources. e.g. get_foo_bar()

However, boolean methods are easier to read if they avoid underscores and use an is prefix (not to be confused with methods that manipulate a boolean). Rationale: Identifies the return value as TRUE or FALSE and nothing else. e.g. isOpen, isHomed

Do NOT use Not in a boolean name, it leads only leads to confusion when doing logical tests. e.g. isNotOnLimit or is_not_on_limit are BAD.

Variable names should avoid the use of upper case and underscores except for local or private names. The use of global variables should be avoided as much as possible. Rationale: Clarifies which are variables and which are methods. Public: e.g. axislimit Private: e.g. maxvelocity_

Specific method naming conventions

The terms get and set should be used where an attribute is accessed directly. Rationale: Indicates the purpose of the function or method. e.g. get_foo set_bar

For methods involving boolean attributes, set & reset is preferred. Rationale: As above. e.g. set_amp_enable reset_amp_fault

Math intensive methods should use compute as a prefix. Rationale: Shows that it is computationally intensive and will hog the CPU. e.g. compute_PID

Abbreviations in names should be avoided where possible - The exception is for local variable names. Rationale: Clarity of code. e.g. pointer is preferred over ptr compute is preferred over cmp compare is again preferred over cmp.

Enumerates and other constants can be prefixed by a common type name e.g. enum COLOR { COLOR_RED, COLOR_BLUE };

Excessive use of macros and defines should be avoided - Using simple methods or functions is preferred. Rationale: Improves the debugging process.

Include Statements Header files must be included at the top of a source file and not scattered throughout the body. They should be sorted and grouped by their hierarchical position within the system with the low level files included first. Include file paths should NEVER be absolute - Use the compiler -I flag instead. Rationale: Headers may not be in the same place on all systems.

Pointers and references should have their reference symbol next to the variable name rather than the type name. Rationale: Reduces confusion. e.g. float *x or int &i

Implicit tests for zero should not be used except for boolean variables. e.g. if (spindle_speed != 0) NOT if (spindle_speed)

Only loop control statements must be included in a for() construct. e.g. sum = 0; for (i = 0; i < 10; i++) { sum += value[i]; }

NOT for (i = 0, sum =0; i < 10; i++) sum += value[i];

Likewise, executable statements in conditionals must be avoided. e.g. if (fd = open(file_name) is bad.

Complex conditional statements should be avoided - Introduce temporary boolean variables instead.

Parentheses should be used in plenty in mathematical expressions - Do not rely on operator precedence when an extra parentheses would clarify things.

File names: C++ sources and headers use .cc and .hh extension. The use of .c and .h are reserved for plain C. Headers are for class, method, and structure declarations, not code (unless the functions are declared inline).

10. Python coding standards

Use the PEP 8 style for Python code.

11. Comp coding standards

In the declaration portion of a .comp file, begin each declaration at the first column. Insert extra blank lines when they help group related items.

In the code portion of a .comp file, follow normal C coding style.