This document contains:

How to report bugs

When reporting a bug, first search our issues to avoid duplicates. In your bug report, please describe what you expected gcovr to do, and what it actually did. Also try to include the following details:

  • how you invoked gcovr, i.e. the exact flags and from which directory

  • your project layout

  • your gcovr version

  • your compiler version

  • your operating system

  • and any other relevant details.

Ideally, you can provide a short script and the smallest possible source file to reproduce the problem.

How to help

If you would like to help out, please take a look at our open issues and pull requests. The issues labeled help wanted and needs review would have the greatest impact.

There are many ways how you can help:

  • assist other users with their problems

  • share your perspective as a gcovr user in discussions

  • test proposed changes in your real-world projects

  • improve our documentation

  • submit pull requests with bug fixes and enhancements

How to submit a Pull Request

Thank you for helping with gcovr development! Please follow this checklist for your pull request:

  • Is this a good approach? Fixing open issues is always welcome! If you want to implement an enhancement, please discuss it first as a GitHub issue.

  • Does it work? Please run the tests locally:

    python3 -m nox

    (see also: Test suite)

    In any case, the tests will run automatically when you open the pull request. But please prevent unnecessary build failures and run the tests yourself first. If you cannot run the tests locally, you can activate GitHub for your fork, or run the tests with Docker. If there are differences the updated files will be available for download from the CI system (one ZIP for each test environment).

    If you add new features, please try to add a test case.

  • Does it conform to the style guide? The source code should conform to the PEP 8 standard. Please check your code:

    python3 -m nox --session lint
    # or:
    python3 -m flake8 doc gcovr

    The command python3 -m nox will run the linters, run the tests, and check that the docs can be built.

  • Add yourself as an author. If this is your first contribution to gcovr, please add yourself to the AUTHORS.txt file.

  • One change at a time. Please keep your commits and your whole pull request fairly small, so that the changes are easy to review. Each commit should only contain one kind of change, e.g. refactoring or new functionality.

  • Why is this change necessary? When you open the PR, please explain why we need this change and what your PR does. If this PR fixes an open issue, reference that issue in the pull request description. Add a reference to the issue in the CHANGELOG.rst, if the change should not be visible in the changelog (minor or not of interest), add the following string to a single line in the PR body:

    [no changelog]

Once you submit the PR, it will be automatically tested on Windows and Linux, and code coverage will be collected. Your code will be reviewed. This can take a week. Please fix any issues that are discovered during this process. Feel free to force-push your updates to the pull request branch.

If you need assistance for your pull request, you can

  • chat in our Gitter room

  • discuss your problem in an issue

  • open an unfinished pull request as a work in progress (WIP), and explain what you’ve like to get reviewed

How to set up a development environment

For working on gcovr, you will need a supported version of Python 3, GCC version 5, 6, 8, 9, 10 or 11 (other GCC versions are supported by gcovr, but will cause spurious test failures) or clang version 10, 13 or 14, make, cmake and ninja. Please make sure that the tools are in the system PATH. On Windows, you will need to install a GCC toolchain as the tests expect a Unix-like environment. You can use MinGW-W64 or MinGW. An easier way is to run tests with Docker, on Windows a Pro license or the WSL (Windows subsystem for Linux) is needed.

  • Check your GCC installation, the binary directory must be added to the PATH environment. If on of the following command groups are everything is OK.

    • gcc-5/g++-5/gcov-5

    • gcc-6/g++-6/gcov-6

    • gcc-8/g++-8/gcov-8

    • gcc-9/g++-9/gcov-9

    • gcc-10/g++-10/gcov-10

    • gcc-11/g++-11/gcov-11

    • gcc-12/g++-12/gcov-12

    • gcc-13/g++-13/gcov-13

    • clang-10/clang++-10/llvm-cov

    • clang-13/clang++-13/llvm-cov

    • clang-14/clang++-14/llvm-cov

    • clang-15/clang++-15/llvm-cov

    are available everything is OK. The test suite uses the newest GCC found in the PATH. To use another one you need to set the environment CC=... see run and filter tests. If you only have gcc in your path the version is detected to select the correct reference. You can also create symlinks for the gcc executables with the following steps. You can check the GCC version with gcc --version. If the output says version 8, you should also be able to run gcc-8 --version. Your Linux distribution should have set all of this up already. If you don’t have an alias like gcc-8, perform the following steps to create an alias for gcc, this should also work in the MSYS shell under Windows:

    1. Create a directory somewhere, e.g. in your home directory: mkdir ~/bin

    2. Create a symlink in that directory which points to GCC: ln -s $(which gcc) ~/bin/gcc-8

    3. Add this directory to your PATH: export PATH="$HOME/bin:$PATH"

    4. Re-test gcc-8 --version to ensure everything worked.

    5. Create additional symlinks for g++ -> g++-8 and gcov -> gcov-8.

  • (Optional) Fork the project on GitHub.

  • Clone the git repository.

  • (Optional) Set up a virtualenv (e.g. with python3 -m venv my-venv)

  • Install gcovr in development mode, and install nox:

    pip install -e .
    pip install nox

    You can then run gcovr as gcovr or python3 -m gcovr.

    Run the tests to verify that everything works (see Test suite).

  • (Optional) Activate GitHub Actions for your forked repository, so that the cross-platform compatibility tests get run whenever you push your work to your repository. These tests will also be run when you open a pull request to the main gcovr repository.

Tip: If you have problems getting everything set up, consider looking at these files:

  • for Linux: .github/workflows/test.yml and admin/

  • for Windows: .github/workflows/test.yml

Project Structure




project root


the gcovr source code (Python module)


command line interface + top-level behaviour


HTML report templates


unit tests + integration test corpus


Definition of tests tasks


Python package configuration




user guide + website


runnable examples for the user guide

The program entrypoint and command line interface is in gcovr/ The coverage data is parsed in the gcovr.gcov module. The HTML, XML, text, and summary reports are in gcovr.generator.html and respective modules.

Test suite

The QA process (python3 -m nox) consists of multiple parts:

  • linting and checking format(python3 -m nox --session lint)

  • tests (python3 -m nox --session tests)

    • unit tests in gcovr/tests

    • integration tests in gcovr/tests

    • documentation examples in doc/examples

  • documentation build (python3 -m nox --session doc)

The tests are in the gcovr/tests directory. You can run the tests with python3 -m nox --session tests for the default GCC version (specified via CC environment variable, defaults to gcc-5). You can also select the gcc version if you run the tests with e.g. python3 -m nox --session 'tests_compiler(gcc-8)'.

There are unit tests for some parts of gcovr, and a comprehensive corpus of example projects that are executed as the integration test. Each gcovr/tests/* directory is one such example project.

You can format files with python3 -m nox --session black)

To get a list of all available sessions run python3 -m nox -l.

The next sections discuss the structure of integration tests, how to run and filter tests, and how to run tests with Docker.

Changed in version 5.2: If black is called without arguments, all files are reformated instead of checked. To check the format use the session lint.

Structure of integration tests

Each project in the corpus contains a Makefile and a reference directory:


The Makefile controls how the project is built, and how gcovr should be invoked. The reference directory contains baseline files against which the gcovr output is compared. Tests can be executed even without baseline files.

Each Makefile contains the following targets:

  • all: builds the example project. Can be shared between gcovr invocations.

  • run: lists available targets which must be a subset of the available output formats.

  • clean: remove any generated files after all tests of the scenario have finished.

  • output formats (txt, html, json, sonarqube, …): invoke gcovr to produce output files of the correct format. The test runner automatically finds the generated files (if any) and compares them to the baseline files in the reference directory. All formats are optional, but using at least JSON is recommended.

  • clean-each: if provided, will be invoked by the test runner after testing each format.

Run and filter tests

To run all tests, use python3 -m nox. The tests currently assume that you are using GCC 5 and have set up a development environment. You can select a different GCC version by setting the CC environment variable. Supported versions are CC=gcc-5, CC=gcc-6, CC=gcc-8, CC=gcc-9, gcc-10, gcc-11, gcc-12, gcc-13, clang-10, clang-13, clang-14 and clang-15.

You can run the tests with additional options by adding -- and then the options to the test invocation. Run all tests after each change is a bit slow, therefore you can limit the tests to a specific test file, example project, or output format. For example:

# run only XML tests
python3 -m nox --session tests -- -k 'xml'

# run the simple1 tests
python3 -m nox --session tests -- -k 'simple1'

# run the simple1 tests only for XML
python3 -m nox --session tests -- -k 'xml and simple1'

To see which tests would be run, add the --collect-only option:

#see which tests would be run
python3 -m nox --session tests -- --collect-only

Sometimes during development you need to create reference files for new test or update the current reference files. To do this you have to add --generate_reference or --update-reference option to the test invocation. By default generated output files are automatically removed after test run. To skip this process you can add --skip_clean option the test invocation. For example:

# run tests and generate references for simple1 example
python3 -m nox --session tests -- -k 'simple1' --generate_reference

# run tests and update xml references for simple1 example
python3 -m nox --session tests -- -k 'xml and simple1' --update_reference

# run only XML tests and do not remove generated files
python3 -m nox --session tests -- -k 'xml' --skip_clean

To update the refernce data for all compiler in one call see run tests with Docker.

When the currently generated output reports differ to the reference files you can create a ZIP archive named in the tests directory by using --archive_differences option. Currently in gcovr it is used by GitHub CI to create a ZIP file with the differences as an artifact.

# run tests and generate a ZIP archive when there were differences
python3 -m nox --session tests -- --archive_differences

Changed in version 5.1: Change how to start test from make test to python3 -m nox --session tests

New in version 5.0: Added test options --generate_reference, --update_reference, --skip_clean, ‘--archive_differences’ and changed way to call tests only by make test.

Run tests with Docker

If you can’t set up a toolchain locally, you can run the QA process via Docker. First, build the container image:

python3 -m nox --session docker_build

Then, run the container, which executes nox within the container:

python3 -m nox --session docker_run -s qa

Or to build and run the container in one step:

python3 -m nox --session docker_qa

You can select the gcc version to use inside the docker by setting the environment variable CC to gcc-5 (default), gcc-6, gcc-8, gcc-9, gcc-10, gcc-11, gcc-12, gcc-13, clang-10, clang-13, or clang-14 or you can build and run the container with:

python3 -m nox --session 'docker_qa_compiler(gcc-9)'

To run a specific session you can use the session docker_compiler and give the arguments to the nox executed inside the container after a -- :

python3 -m nox --session 'docker_compiler(gcc-9)' -- -s tests

You can also use the compiler ‘all’ to run the tests for all compiler versions, ‘gcc’ to only use the gcc versions, or ‘clang’ to use clang versions. A useful command to update all the reference files is :

python3 -m nox --session 'docker_compiler(all)' -- -s tests -- --update_reference

Use a devcontainer

For developing gcovr you can use whatever editor you want. If the editor supports Devcontainers (e.g. VS Code) you do not need to install the needed tools on your local system. You can also use GitHub Codespaces to contribute to the project.

Become a gcovr developer

After you’ve contributed a bit (whether with discussions, documentation, or code), consider becoming a gcovr developer. As a developer, you can:

  • manage issues and pull requests (label and close them)

  • review pull requests (a developer must approve each PR before it can be merged)

  • participate in votes

Just open an issue that you’re interested, and we’ll have a quick vote.