This document contains:
- our guidelines for bug reports
- general contribution guidelines
- a checklist for pull requests
- a developer guide that explains the development environment, project structure, and test suite
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¶
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
python3 -m noxwill run the linter, 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
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:
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 or 8 (other GCC versions are supported by gcovr,
but will cause spurious test failures) or clang version 10,
Please make sure that the tools are in the system
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)
Check your GCC installation, the binary directory must be added to the PATH environment. If the command gcc-5/g++-5/gcov-5, gcc-6/g++-6/gcov-6, gcc-8/g++-8/gcov-8, gcc-9/g++-9/gcov-9, clang-10/clang++-10/llvm-cov or clang-13/clang++-13/llvm-cov are available everything is OK. For gcc-6, gcc-8, gcc-9, clang-10 and clang-13 you should use the option
CC=...see run and filter tests. If this isn’t OK you can set the reference data to use by setting the environment
CC_REFERENCE=gcc-8or you have to 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:
- Create a directory somewhere, e.g. in your home directory:
- Create a symlink in that directory which points to GCC:
ln -s $(which gcc) ~/bin/gcc-8
- Add this directory to your PATH:
gcc-8 --versionto ensure everything worked.
- Create additional symlinks for g++ -> g++-8 and gcov -> gcov-8.
- Create a directory somewhere, e.g. in your home directory:
(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
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:
- for Windows:
||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
The coverage data is parsed in the
The HTML, XML, text, and summary reports
gcovr.generator.html and respective modules.
The QA process (
python3 -m nox) consists of multiple parts:
linting and checking format(
python3 -m nox --session lint)
python3 -m nox --session tests)
- unit tests in
- integration tests in
- documentation examples in
- unit tests in
documentation build (
python3 -m nox --session doc)
The tests are in the
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
test_gcovr.py integration test.
gcovr/tests/* directory is one such example project.
You can format files with
python3 -m nox --session black FileToFormat)
To get a list of all available sessions run
python3 -m nox -l.
Structure of integration tests¶
Each project in the corpus
Makefile and a
gcovr/tests/sometest/ reference/ Makefile README example.cpp
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
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.
# 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
#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
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.
# 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
diff.zip in the tests directory
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
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_qa_build
Then, run the container, which executes
python3 -m nox within the container:
python3 -m nox --session docker_qa_run
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, clang-10 or clang-13 or you can build and run the container with:
python3 -m nox --session 'docker_qa_compiler(gcc-9)'
You can also use the compiler ‘all’ to run the tests for all compiler versions. This is usefull to update the all reference files:
python3 -m nox --session 'docker_qa_compiler(all)' -- --update_reference
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.