Contribute Code


Interested in developing new features, fixing bugs, or contributing code you have laying around? Read on to find out how.

Development process

We follow a development process designed to reduce errors, encourage collaboration, and make high quality code. The process may seem rigid and tedious, but every step is worth the effort (especially if you like applications that work).

Steps to follow

We will use the Gazebo repository as an example, but the step apply equally well to other repositories maintained by OSRF.

  1. Are you sure?

    Run through this mental checklist before getting started.

    1. Has your idea already been done, or maybe someone is already working on it?

      Check and the issue tracker.

    2. Get feedback from the Gazebo core team. Send an email to the mailing list, post a question on, or use the issue tracker to get feedback from Gazebo developers.

  2. Fork Gazebo

    This will create your own personal copy of Gazebo. All of your development should take place in your fork.

  3. Work out of a branch: hg branch my_new_branch_name

    Always work out of a new branch, never off of default. This is a good habit to get in, and will make your life easier.

  4. Choose a base branch.

    If your changes will break API or ABI, then base your new branch off of default. If your changes don't break API/ABI and you would like them to be released to an existing gazebo release with major version N, then use branch gazeboN as the base.

  5. Write your code.

    This is the fun part.

  6. Write tests.

    A pull request will only be accepted if it has tests. See the Test coverage section below for more information.

  7. Compiler warnings.

    Code must have zero compile warnings. This currently only applies to Linux.

  8. Style

    A tool is provided in Gazebo (and other repositories) to check for correct style. Your code must have no errors after running the following command from the root of the source tree:

    sh tools/

    The tool does not catch all style errors. See the Style section below for more information.

  9. Tests pass

    There must be no failing tests. You can check by running make test in your build directory.

  10. Documentation.

    Document all your code. Every class, function, member variable must have doxygen comments. All code in source files must have documentation that describes the functionality. This will help reviewers, and future developers.

  11. Review your code.

    Before submitting your code through a pull request, take some time to review everything line-by-line. The review process will go much faster if you make sure everything is perfect before other people look at your code. There is a bit of the human-condition involved here. Folks are less likely to spend time reviewing your code if it's bad.

  12. Small pull requests

    A large pull request is hard to review, and will take a long time. It is worth your time to split a large pull request into multiple smaller pull requests. For reference, here are a few examples:

    Small, very nice

    Medium, still okay

    Too large

  13. Pull request

    Submit a pull request from your Bitbucket fork when you are ready.

  14. Review

    At least two other people have to approve your pull request before it can be merged. Please be responsive to any questions and comments.

  15. Dashboard

    We have a dashboard that lists all open pull requests. Only pull requests above the green line can be merged, and they must also have at least two approvals. We do this to encourage the review of pull requests. If you submit a pull request, then try to review other open pull requests. This will reduce the time it takes for your code to get accepted, and also gets more eyes on more code.

  16. Done, phew.

    Once you have met all the requirements, your code will be merged. Thanks for improving Gazebo!

Internal Developers

This section is targeted mostly for people who have commit access to the main repositories.

In addition to the general development process, please follow these steps before submitting a pull request. Each step is pass/fail, where the test or check must pass before continuing to the next step.

  1. Test on Windows.
  2. Run the style checker on your personal computer.
  3. Run all, or only relevant, tests on your personal computer.
  4. Run your branch through a jenkins no-gpu build.
  5. Run your branch through a jenkins nvidia build.
  6. Run your branch through the ABI/API checker, if targeting a release branch.
  7. Submit the pull request, and include the following:
    1. Link to a coverage report.
    2. Link to a passing homebrew build.
    3. Link to a passing no-gpu build.
    4. Link to a passing nvidia build.
    5. Link to a passing ABI/API report if the pull request is targeted to a release branch.
    6. A statement that confirms you have tried the code on Windows.
  8. A set of jenkins jobs will run automatically once the pull request is created. Reviewers can reference these automatic jobs and the jenkins jobs listed in your pull request.


In general, we follow Google's style guide. However, we add in some extras.

  1. This pointer

    All class attributes and member functions must be accessed using the this-> pointer. Here is an example.

  2. Underscore function parameters

    All function parameters must start with an underscore. Here is an example.

  3. Do not cuddle braces

    All braces must be on their own line. Here is an example.

  4. Multi-line code blocks

    If a block of code spans multiple lines and is part of a flow control statement, such as an if, then it must be wrapped in braces. Here is an example

  5. ++ operator

    This occurs mostly in for loops. Prefix the ++ operator, which is slightly more efficient than postfix in some cases.

  6. PIMPL/Opaque pointer

    If you are writing a new class, it must use a private data pointer. Here is an example, and you can read more here.

  7. const functions

    Any class function that does not change a member variable should be marked as const. Here is an example.

  8. const parameters

    All parameters that are not modified by a function should be marked as const. This applies to parameters that are passed by reference, pointer, and value. Here is an example.

  9. Pointer and reference variables

    Place the * and & next to the varaible name, not next to the type. For example: int &variable is good, but int& variable is not. Here is an example.

  10. Camel case

    In general, everything should use camel case. Exceptions include SDF element names, and protobuf variable names. Here is an example.

  11. Class function names

    Class functions must start with a capital letter, and capitalize every word.

    void MyFunction(); : Good

    void myFunction(); : Bad

    void my_function(); : Bad

  12. Variable names

    Variables must start with a lower case letter, and capitalize every word thereafter.

    int myVariable; : Good

    int myvariable; : Bad

    int my_variable; : Bad

  13. No inline comments

    // style comments may not be placed on the same line as code.

    speed *= 0.44704; // miles per hour to meters per second : Bad

Reduce Code Duplication

Check to make sure someone else is not currently working on the same feature, before embarking on a project to add something to Gazebo. Simply send a quick email to the Gazebo mailing list expressing your interest and idea. Someone will get back to you shortly about your idea.

Write Tests

All code should have a corresponding unit test. Gazebo uses GTest for unit testing. All regression test should be placed in <gazebo_sources>/test/regresssion/.

Before creating a new regressions test file, check the current test files. If one closely matches the topic of your new code, simply add a new test function to the file. Otherwise, create a new test file, and write your test.

Test coverage

The goal is to achieve 100% line and branch coverage. However, this is not always possible due to complexity issues, analysis tools misreporting coverage, and time constraints. Try to write as complete of a test suite as possible, and use the coverage analysis tools as guide. If you have trouble writing a test please ask for help in your pull request.

Gazebo has a build target called make coverage that will produce a code coverage report. You'll need lcov and gcov installed.

  1. In your build folder, compile Gazebo and the tests with -DCMAKE_BUILD_TYPE=Coverage

    cmake -DCMAKE_BUILD_TYPE=Coverage ..\
    make tests
  2. Run a single test, or all the tests

    make test
  3. Make the coverage report

    make coverage
  4. View the coverage report

    firefox coverage/index.html

Gazebo assertions

What is an assertion?

An assertion is a check, which always produce a boolean result, that developers place in the code when want to be sure that check is always true. They are aimed to detect programming errors and should check for impossible situations in the code. If the assertion check failed, the assertion will stop the program immediately.

 Object * p = some_crazy_function()
 GZ_ASSERT(p != NULL, "Object from some_crazy_function should never point to NULL")

Gazebo runtime assertions: GZ_ASSERT

In Gazebo, the GZ_ASSERT macro is designed to handle all our runtime assertions

 GZ_ASSERT(<condition to check>,<fail msg>) 
  • '''condition-to-check:''' anything returning a boolean value that should always be true.
  • '''fail msg:''' message displayed when assertion is thrown

Benefits of the assertions

Some of the benefits of using the assertions:

  • They are really useful for not having to debug all kinds of weird and unexpected errors, especially in runtime. The exact failure point appears when an assertion is triggered.
  • The developer can be sure that some conditions are met at a given code point. This makes the code more reliable.
  • They help to detect when not-so-obvious errors are happening (affecting performance for example).

Difference between Assertion and Exception

While assertions are intended for impossible situations resulting from programming errors, exceptions handle all kinds of expected errors and unusual but logically possible code situations.

Let's review an example: suppose we are writing a math library and created a really fast method to calculate square roots, but it only works for positive numbers. Something declared as:

 double sqrt_for_positives(double number)

What could be an assertion and what would be an exception for our revolutionary function?

  • Exception: If the incoming number is negative (our function only accepts positive numbers), then we will throw an exception. It was an error by the user, but we should consider it a possible scenario since we are offering a public interface.

  • Assertion: Our square root should never return a negative number. This is not a logical error; it is a completely unexpected error which should be impossible if the function is written correctly.

Debugging Gazebo

Meaningful backtraces

In order to provide meaningful backtraces when using a debugger, such as GDB, Gazebo should be compiled with debugging support enabled. When using the ubuntu packages, specially the ''-dbg'' package, this support is limited but could be enough in most situations. There are three level of traces which can be obtained:

'''Maximum level of debugging support''' : This can only be obtained by compiling Gazebo from source and setting the CMAKE_BUILD_TYPE to DEBUG. This will set up debugging symbols with no optimizations. It may be required by developers for problems that are especially difficult to track down.

'''Medium level of debugging support''' : This can be obtained installing the ''gazebo-dbg'' package (since 1.4 version) or compiling Gazebo from source using the RELWITHDEBINFO CMAKE_BUILD_TYPE mode (which is the default if no mode is provided). This will set up ''-O2'' optimization level but provide debugging symbols. This should be the default when firing up gdb to explore errors and submit traces.

'''Minimum level of debugging support''' : This one is present in package versions previous to 1.4 (no ''-dbg'' package present) or compiling Gazebo from source using the RELEASE CMAKE_BUILD_TYPE option. This will set up the maximum level of optimizations and does not provide any debugging symbol information. These traces are particularly difficult to follow.

Code Check

Code pushed into the Gazebo repository should pass a few simple tests. It is also helpful if patches submitted through bitbucket pass these tests. Passing these tests is defined as generating no error or warning messages for each of the following tests.

Regression Tests

In your Gazebo build directory run make test:

    make test

All the tests should pass. If they do not, you can run and debug the tests individually. For example, to run the transport test from your build directory:


Static Code Check

Static code checking analyzes your code for bugs, such as potential memory leaks, and style. The Gazebo static code checker uses cppcheck, and a modified cpplint. You'll need to install cppcheck on your system. Ubuntu users can install via:

    sudo apt-get install cppcheck

To check your code, run the following script from the root of the Gazebo sources:

    sh tools/

It takes a few minutes to run. Fix all errors and warnings until the output looks like:

    Total errors found: 0

CMAKE_BUILD_TYPE=Check compiles with no warnings

This test compiles Gazebo with numerous warning flags enabled. The source code for Gazebo should compile cleanly. This does not include code in the deps directory. As a rule of thumb, start looking for compilation warnings after the proto messages are built which appear as a series of blue text:

    Linking CXX executable gazebomsgs_out
    Running C++ protocol buffer compiler on axis.proto
    Running C++ protocol buffer compiler on boxgeom.proto
    Running C++ protocol buffer compiler on camerasensor.proto
    Running C++ protocol buffer compiler on collision.proto
    Running C++ protocol buffer compiler on color.proto
    Running C++ protocol buffer compiler on contact.proto
    Running C++ protocol buffer compiler on contacts.proto