Skip to end of metadata
Go to start of metadata

Coding Style

This document described the coding style for the Apache Traffic Server project. The document is still evolving, but all contributors and committers are encourage to read this before contributing patches. And as usual, additions and edits to this document is appreciated, to assure that we all share the same coding style. For major changes to the style, a discussion should be opened on the dev@ mailing before changed here.


Code indentation and formatting was completely standardized prior to open sourcing the Apache Traffic Server code. Command line tool indent did most of the heavy lifting. This document will describe the indentation, formatting, doxygen comments, class naming, and naming conventions to use.

ATS v2.2 Prefix and Directory Structure changes

In the v2.2 release of Apache Traffic Server, we made some changes to prefixes, and directory structure. Some of this work is still undergoing, see this page for additional details. The most important part of these changes is the semantics of the prefix name spaces:

Public APIs (ts/ts.h, ts/experimental.h etc.)

The prefix to use for these include files and underlying code in e.g. is one of

  • TS or TS_
  • ts

In addition, none of these methods, struct names or definitions should be used in the core itself, other than the actual API implementations ( etc.).

Internal code

For all other code that's internal, the prefix should be one of

  • ATS or ATS_
  • ats or ats_
  • INK or INK_
  • ink or ink_

No new code / functionality should be added using the ink/INK prefix. Long term, we will migrate these into the ats/ATS prefix. Adhering to these rules is important, the goal is to be able to easily distinguish public from private APIs. In the past, we've had several cases where public APIs were used in the private code implementation, and this is a bad idea for both performance and functionality.

Header files


In most subsystems, header files are named with a P_ or I_ prefix. P_ files should contain any types and definitions that are private to the subsystem, while the public interface should be contained in a I_-prefixed header.


We have changed from the previous indentation rules, to rely completely on clang-format. The style is mostly the same as it was before, which means:

  • 2-space indentation (never tabs)
  • 132-character wide lines
  • A clang-format based primarily on the Mozilla formatting. But, note that we have our own .clang-format in the top-level of the source tree.

The easiest way to format the code is to simply run


$ make clang-format

You can also explicitly format one (or several) source files with clang-format directly. Make sure you run it from the top-level directory, which has the necessary .clang-format configuration file. E.g.

$ clang-format -i proxy/logging/

Vertical whitespace

No more than 1 adjacent empty line (clang-format enforces this). Leave a blank line after the closing branch when you have the same indentation level (clang-format sometimes messes this up).


You should have this :

  if (...) {
  if (...) {


clang-format binary and configuration

You must use the same clang-format binary as everyone else is. This is unfortunate, but is a side effect of how the clang-format team manages their code. You can download the current version, from April 15th 2016. Alternatively, you can build your own version from the clang / llvm source tree, but the tar-ball above includes binaries for both Linux and OS X. You have to copy either of these into somewhere your $PATH will locate, and rename it to just clang format. E.g.


$ tar xf clang-format-20150331.tar.bz2
$ sudo mv clang-format/clang-format.linux /usr/local/bin/clang-format


In addition to the binaries, there is a git script as well as en Emacs mode for clang-format.

Naming conventions

Classes and Structures

  • Upper case for the first character of the name
  • Use camel case (GoodClassName)


  • Use camel case for method names with the first charter being lower case (goodMethodName)
  • Prepend '_' to the beginning of private or protected methods

Member Variables

  • Prepend '_' to the beginning of the private or protected member variable to distinguish it from other variable


TODO comments

Example of comment TODO:

  // TODO handle case for negative config value

Notice the format:

  • the TODO is all capitalized
  • first letter of TODO message does not need to be upper-case
  • there is no colon or dash char after TODO
  • there is no period at the end
  • description is short (70 chars or less if possible) and helpful
  • the TODO message describes an action that needs to be performed in the future

XXX comments

Example of XXX:

  // XXX Hazardous code! We should find a way to make
  //     it more secure

Notice the format:

  • the XXX is all capitalized
  • there is no colon or dash char after XXX
  • there is no period at the end
  • description is short (70 chars or less if possible) and helpful
  • the XXX is used to warn other programmers of problematic or misguiding code.

no break comments

When your case will fall through, please add the no break comment with other more detialed comments at the end of the code block.
Example of no break:

    switch (*cur) {
    case ']' : // address close
      n_colon = MAX_COLON - 1;
      /* no break */
      /* fall through until ... */
    case ':' : // track colons, fail if too many.

That will help use understand the codes when someone may mistake that as bug. And be more nice to Eclipse CDT code analysis tool.

  • No labels