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Hacking on systemd Contributing default

Hacking on systemd

We welcome all contributions to systemd. If you notice a bug or a missing feature, please feel invited to fix it, and submit your work as a GitHub Pull Request (PR) at

Please make sure to follow our Coding Style when submitting patches. Also have a look at our Contribution Guidelines.

When adding new functionality, tests should be added. For shared functionality (in src/basic/ and src/shared/) unit tests should be sufficient. The general policy is to keep tests in matching files underneath src/test/, e.g. src/test/test-path-util.c contains tests for any functions in src/basic/path-util.c. If adding a new source file, consider adding a matching test executable. For features at a higher level, tests in src/test/ are very strongly recommended. If that is not possible, integration tests in test/ are encouraged.

Please also have a look at our list of code quality tools we have setup for systemd, to ensure our codebase stays in good shape.

Please always test your work before submitting a PR. For many of the components of systemd testing is straight-forward as you can simply compile systemd and run the relevant tool from the build directory.

For some components (most importantly, systemd/PID1 itself) this is not possible, however. In order to simplify testing for cases like this we provide a set of mkosi build files directly in the source tree. mkosi is a tool for building clean OS images from an upstream distribution in combination with a fresh build of the project in the local working directory. To make use of this, please acquire mkosi from first, unless your distribution has packaged it already and you can get it from there. After the tool is installed, symlink the settings file for your distribution of choice from .mkosi/ to mkosi.default in the project root directory (note that the package manager for this distro needs to be installed on your host system). After doing that, it is sufficient to type mkosi in the systemd project directory to generate a disk image image.raw you can boot either in systemd-nspawn or in an UEFI-capable VM:

# systemd-nspawn -bi image.raw


# qemu-system-x86_64 -enable-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -hda image.raw

Every time you rerun the mkosi command a fresh image is built, incorporating all current changes you made to the project tree.

Alternatively, you may install the systemd version from your git check-out directly on top of your host system's directory tree. This mostly works fine, but of course you should know what you are doing as you might make your system unbootable in case of a bug in your changes. Also, you might step into your package manager's territory with this. Be careful!

And never forget: most distributions provide very simple and convenient ways to install all development packages necessary to build systemd. For example, on Fedora the following command line should be sufficient to install all of systemd's build dependencies:

# dnf builddep systemd

Putting this all together, here's a series of commands for preparing a patch for systemd (this example is for Fedora):

$ sudo dnf builddep systemd               # install build dependencies
$ sudo dnf install mkosi                  # install tool to quickly build images
$ git clone
$ cd systemd
$ vim src/core/main.c                     # or wherever you'd like to make your changes
$ meson build                             # configure the build
$ meson compile -C build                  # build it locally, see if everything compiles fine
$ meson test -C build                     # run some simple regression tests
$ ln -s .mkosi/mkosi.fedora mkosi.default # Configure mkosi to build a fedora image
$ (umask 077; echo 123 > mkosi.rootpw)    # set root password used by mkosi
$ sudo mkosi                              # build a test image
$ sudo systemd-nspawn -bi image.raw       # boot up the test image
$ git add -p                              # interactively put together your patch
$ git commit                              # commit it
$ git push REMOTE HEAD:refs/heads/BRANCH
                                          # where REMOTE is your "fork" on GitHub
                                          # and BRANCH is a branch name.

And after that, head over to your repo on GitHub and click "Compare & pull request"

Happy hacking!

Developer and release modes

In the default meson configuration (-Dmode=developer), certain checks are enabled that are suitable when hacking on systemd (such as internal documentation consistency checks). Those are not useful when compiling for code for distribution and can be disabled by setting -Dmode=release.


systemd includes fuzzers in src/fuzz/ that use libFuzzer and are automatically run by OSS-Fuzz with sanitizers. To add a fuzz target, create a new src/fuzz/fuzz-foo.c file with a LLVMFuzzerTestOneInput function and add it to the list in src/fuzz/

Whenever possible, a seed corpus and a dictionary should also be added with new fuzz targets. The dictionary should be named src/fuzz/fuzz-foo.dict and the seed corpus should be built and exported as $OUT/ in tools/

The fuzzers can be built locally if you have libFuzzer installed by running tools/ You should also confirm that the fuzzer runs in the OSS-Fuzz environment by checking out the OSS-Fuzz repo, and then running commands like this:

python infra/ build_image systemd
python infra/ build_fuzzers --sanitizer memory systemd ../systemd
python infra/ run_fuzzer systemd fuzz-foo

If you find a bug that impacts the security of systemd, please follow the guidance in on how to report a security vulnerability.

For more details on building fuzzers and integrating with OSS-Fuzz, visit: