1. Introduction

This document contains information for developers about LinuxCNC infrastructure, and describes the best practices for contributing code and documentation updates to the LinuxCNC project.

Throughout this document, "source" means both the source code to the programs and libraries, and the source text for the documentation.

2. Communication among LinuxCNC developers

The two main ways that project developers communicate with each other are:

3. The LinuxCNC Source Forge project

We use Source Forge for mailing lists: http://sourceforge.net/p/emc/mailman/

4. The git Revision Control System

All of the LinuxCNC source is maintained in the revision control system git

4.1. LinuxCNC official git repo

The official LinuxCNC git repo is at http://git.linuxcnc.org.

Anyone can get a read-only copy of the LinuxCNC source tree via git:

git clone git://git.linuxcnc.org/git/linuxcnc.git linuxcnc-dev

If you are a developer with push access, the ssh clone command is:

git clone ssh://$YOU@git.linuxcnc.org/git/linuxcnc.git linuxcnc-dev

Note that both the clone commands put the local LinuxCNC repo in a directory called linuxcnc-dev, instead of the default linuxcnc. This is because the LinuxCNC software by default expects configs and G-code programs in a directory called $HOME/linuxcnc, and having the git repo there too is confusing.

4.2. LinuxCNC on github

There is a regularly updated git clone on github: https://github.com/LinuxCNC/linuxcnc

The old sourceforge bug tracker continues to exist in read-only mode: http://sourceforge.net/p/emc/bugs/

You can submit pull requests here.

4.3. Use of git in the LinuxCNC project

We use the "merging upwards" and "topic branches" git workflows described here:

We have a development branch called master, and one or more stable branches with names like 2.6 and 2.7 indicating the version number of the releases we make from it.

Bugfixes go in the oldest applicable stable branch, and that branch gets merged into the next newer stable branch, and so on up to master. The committer of the bugfix may do the merges themselves, or they may leave the merges for someone else.

New features generally go in the master branch, but some kinds of features (specifically well isolated device drivers and documentation) may (at the discretion of the stable branch release managers) go into a stable branch and get merged up just like bugfixes do.

4.4. git tutorials

There are many excellent, free git tutorials on the internet.

The first place to look is probably the "gittutorial" manpage. This manpage is accessible by running "man gittutorial" in a terminal (if you have the git manpages installed). The gittutorial and its follow-on documentation are also available online here:

For a more thorough documentation of git see the "Pro Git" book: http://git-scm.com/book

Another online tutorial that has been recommended is "Git for the Lazy": http://wiki.spheredev.org/Git_for_the_lazy

5. Overview of the process

The high-level overview of how to contribute changes to the source goes like this:

  • Communicate with the project developers and let us know what you’re hacking on

  • Clone the git repo

  • Make your changes in a local branch, making sure you "sign off" your commits according to our signed-off-by policy (see below).

  • Adding documentation and tests is an important part of adding a new feature. Otherwise, others won’t know how to use your feature, and if other changes break your feature it can go unnoticed without a test.

  • Share your changes with the other project developers in one of these ways:

    • Push your branch to github and create a github pull request to https://github.com/linuxcnc/linuxcnc (this requires a github account)

    • Push your branch to a publicly visible git repo (such as github, bitbucket, your own publicly-accessible server, etc) and share that location on the emc-developers mailing list, or

    • Email your commits to the emc-developers mailing list (use git format-patch to create the patches)

  • Advocate for your patch

    • Explain what problem it addresses and why it should be included in LinuxCNC

    • Be receptive to questions and feedback from the developer community.

    • It is not uncommon for a patch to go through several revisions before it is accepted.

6. Signed-off-by policy

To improve tracking of who did what, we use the "sign-off" procedure introduced by the Linux kernel. The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the Developer’s Certificate of Origin 1.1 with GPLv2+ clause, then you just add a line saying

Signed-off-by: Random J Developer <random@developer.example.org>

This line can be automatically added by git if you run the git-commit command with the -s option.

For more information, see docs/SubmittingPatches and docs/developer-certificate-of-origin in the source tree. By policy, commits authored after 2014-09-29 must bear a signed-off-by line or they will be rejected by git.linuxcnc.org.

7. git configuration

In order to be considered for inclusion in the LinuxCNC source, commits must have correct Author fields identifying the author of the commit. A good way to ensure this is to set your global git config:

git config --global user.name "Your full name"

git config --global user.email "you@example.com"

Use your real name (not a handle), and use an unobfuscated e-mail address.

8. Effective use of git

8.1. Commit contents

Keep your commits small and to the point. Each commit should accomplish one logical change to the repo.

8.2. Write good commit messages

Keep commit messages around 72 columns wide (so that in a default-size terminal window, they don’t wrap when shown by git log).

Use the first line as a summary of the intent of the change (almost like the subject line of an e-mail). Follow it with a blank line, then a longer message explaining the change. Example:

Get rid of RTAPI_SUCCESS, use 0 instead
The test "retval < 0" should feel familiar; it's the same kind of
test you use in userspace (returns -1 for error) and in kernel space
(returns -ERRNO for error)

8.3. Commit to the proper branch

Bugfixes should go on the oldest applicable branch. New features should go in the master branch. If you’re not sure where a change belongs, ask on irc or on the mailing list.

8.4. Use multiple commits to organize changes

When appropriate, organize your changes into a branch (a series of commits) where each commit is a logical step towards your ultimate goal. For example, first factor out some complex code into a new function. Then, in a second commit, fix an underlying bug. Then, in the third commit, add a new feature which is made easier by the refactoring and which would not have worked without fixing that bug.

This is helpful to reviewers, because it is easier to see that the "factor out code into new function" step was right when there aren’t other edits mixed in; it’s easier to see that the bug is fixed when the change that fixes it is separate from the new feature; and so on.

8.5. Follow the style of the surrounding code

Make an effort to follow the prevailing indentation style of surrounding code. In particular, changes to whitespace make it harder for other developers to track changes over time. When reformatting code must be done, do it as a commit separate from any semantic changes.

8.6. Simplify complicated history before sharing with fellow developers

With git, it’s possible to record every edit and false start as a separate commit. This is very convenient as a way to create checkpoints during development, but often you don’t want to share these false starts with others.

Git provides two main ways to clean history, both of which can be done freely before you share the change:

git commit --amend lets you make additional changes to the last thing you committed, optionally modifying the commit message as well. Use this if you realized right away that you left something out of the commit, or if you typo’d the commit message.

git rebase --interactive upstream-branch lets you go back through each commit made since you forked your feature branch from the upstream branch, possibly editing commits, dropping commits, or squashing (combining) commits with others. Rebase can also be used to split individual commits into multiple new commits.

8.7. Make sure every commit builds

If your change consists of several patches, git rebase -i may be used to reorder these patches into a sequence of commits which more clearly lays out the steps of your work. A potential consequence of reordering patches is that one might get dependencies wrong - for instance, introducing a use of a variable, and the declaration of that variable only follows in a later patch.

While the branch HEAD will build, not every commit might build in such a case. That breaks git bisect - something somebody else might use later on to find the commit which introduced a bug. So beyond making sure your branch builds, it is important to assure every single commit builds as well.

There’s an automatic way to check a branch for each commit being buildable - see http://dustin.sallings.org/2010/03/28/git-test-sequence.html , and the code at https://github.com/dustin/bindir/blob/master/git-test-sequence . Use as follows (in this case testing every commit from origin/master to HEAD, including running regression tests):

cd linuxcnc-dev
git-test-sequence origin/master..  '(cd src && make && ../scripts/runtests)'

This will either report All’s well or Broke on <commit>

8.8. Renaming files

Please use the ability to rename files very cautiously. Like running indent on single files, renames still make it more difficult to follow changes over time. At a minimum, you should seek consensus on irc or the mailing list that the rename is an improvement.

8.9. Prefer "rebase"

Use git pull --rebase instead of bare git pull in order to keep a nice linear history. When you rebase, you always retain your work as revisions that are ahead of origin/master, so you can do things like git format-patch them to share with others without pushing to the central repository.

9. Other ways to contribute

There are many ways to contribute to LinuxCNC, that are not addressed by this document. These ways include:

  • Answering questions on the forum, mailing lists, and in IRC

  • Reporting bugs on the bug tracker, forum, mailing lists, or in IRC

  • Helping test experimental features