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).

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 straightforward as you can simply compile systemd and run the relevant tool from the build directory.

For some components (most importantly, systemd/PID 1 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 install the mkosi package (if not packaged for your distro, it can be downloaded from the GitHub repository. mkosi will build an image for the host distro by default. mkosi-13 or newer version is required. 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:

$ mkosi boot


$ mkosi qemu

Every time you rerun the mkosi command a fresh image is built, incorporating all current changes you made to the project tree. To save time when rebuilding, you can use mkosi’s incremental mode (-i). This instructs mkosi to build a set of cache images that make future builds a lot faster. Note that the -i flag both instructs mkosi to build cached images if they don’t exist yet and to use cached images if they already exist so make sure to always specify -i if you want mkosi to use the cached images.

If you’re going to build mkosi images that use the same distribution and release that you’re currently using, you can speed up the initial mkosi run by having it reuse the host’s package cache. To do this, create a mkosi override file in mkosi.default.d/ (e.g 20-local.conf) and add the following contents:

Cache=<full-path-to-package-manager-cache> # (e.g. /var/cache/dnf)

If you want to do a local build without mkosi, most distributions also provide very simple and convenient ways to install all development packages necessary to build systemd:

# Fedora
$ sudo dnf builddep systemd
# Debian/Ubuntu
$ sudo apt-get build-dep systemd
# Arch
$ sudo pacman install asp
$ asp checkout systemd
$ cd systemd/trunk
$ makepkg -seoc

Putting this all together, here’s a series of commands for preparing a patch for systemd:

# Install build dependencies (see above)
# Install a recent version of mkosi (either via your distro's package manager if
# available there or from the github repository otherwise)
$ git clone https://github.com/systemd/systemd.git
$ cd systemd
$ git checkout -b <BRANCH>        # where BRANCH is the name of the branch
$ vim src/core/main.c             # or wherever you'd like to make your changes
$ meson build                     # configure the build
$ ninja -C build                  # build it locally, see if everything compiles fine
$ meson test -C build             # run some simple regression tests
$ sudo mkosi                      # mkosi-13 or newer required to build a test image
$ sudo mkosi boot                 # boot up the test image
$ git add -p                      # interactively put together your patch
$ git commit                      # commit it
$ git push -u <REMOTE>            # where REMOTE is your "fork" on GitHub

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

Happy hacking!

Templating engines in .in files

Some source files are generated during build. We use two templating engines:

Please note that files for both template engines use the .in extension.

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 distribution and can be disabled by setting -Dmode=release.

Sanitizers in mkosi

See Testing systemd using sanitizers for more information on how to build with sanitizers enabled in mkosi.


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/meson.build.

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/fuzz-foo_seed_corpus.zip in tools/oss-fuzz.sh.

The fuzzers can be built locally if you have libFuzzer installed by running tools/oss-fuzz.sh. You should also confirm that the fuzzers can be built and run using the OSS-Fuzz toolchain:


git clone --depth=1 https://github.com/google/oss-fuzz
cd oss-fuzz

for sanitizer in address undefined memory; do
  for engine in libfuzzer afl honggfuzz; do
    ./infra/helper.py build_fuzzers --sanitizer "$sanitizer" --engine "$engine" \
       --clean systemd "$path_to_systemd"

    ./infra/helper.py check_build --sanitizer "$sanitizer" --engine "$engine" \

./infra/helper.py build_fuzzers --clean --architecture i386 systemd "$path_to_systemd"
./infra/helper.py check_build --architecture i386 -e ALLOWED_BROKEN_TARGETS_PERCENTAGE=0 systemd

./infra/helper.py build_fuzzers --clean --sanitizer coverage systemd "$path_to_systemd"
./infra/helper.py coverage --no-corpus-download systemd

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

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

mkosi + clangd

clangd is a language server that provides code completion, diagnostics and more right in your editor of choice (with the right plugin installed). When using mkosi, we can run clangd in the mkosi build container to avoid needing to build systemd on the host machine just to make clangd work. To achieve this, create a script with the following contents in systemd’s project directory on the host:

#!/usr/bin/env sh
tee mkosi-clangd.build > /dev/null << EOF
#!/usr/bin/env sh
exec clangd \\
        --compile-commands-dir=/root/build \\
" \\
chmod +x mkosi-clangd.build
exec pkexec mkosi --source-file-transfer=mount --incremental --skip-final-phase --build-script mkosi-clangd.build build

Next, mark the script as executable and point your editor plugin to use this script to start clangd. For vscode’s clangd extension, this is done via setting the clangd.path option to the path of the mkosi-clangd.sh script.

To be able to navigate to include files of systemd’s dependencies, we need to make the /usr/include folder of the build image available on the host. mkosi supports this by setting the IncludeDirectory option in mkosi’s config. The easiest way to set the option is to create a file 20-local.conf in mkosi.default.d/ and add the following contents:


This will make the contents of /usr/include available in mkosi.includedir in the systemd project directory. We already configured clangd to map any paths in /usr/include in the build image to mkosi.includedir/ on the host in the mkosi-clangd.sh script.

We also need to make sure clangd is installed in the build image. To have mkosi install clangd in the build image, edit the 20-local.conf file we created earlier and add the following contents under the [Content] section:


Note that the exact package containing clangd will differ depending on the distribution used. Some distributions have a separate clangd package, others put the clangd binary in a clang-tools-extra package and some bundle clangd in the clang package.

Because mkosi needs to run as root, we also need to make sure we can enter the root password when the editor plugin tries to run the mkosi-clangd.sh script. To be able to enter the root password in non-interactive scripts, we use pkexec instead of sudo. pkexec will launch a graphical interface to let the user enter their password, so that the password can be entered by the user even when pkexec is executed from a non-interactive shell.

Due to a bug in btrfs, it’s currently impossible to mount two mkosi btrfs images at the same time. Because of this, trying to do a regular build while the clangd image is running will fail. To circumvent this, use ext4 instead of btrfs for the images by adding the following contents to 20-local.conf:


Finally, to ensure clangd starts up quickly in the editor, run an incremental build with mkosi to make sure the cached images are initialized (mkosi -i).

Now, your editor will start clangd in the mkosi build image and all of clangd’s features will work as expected.

Debugging binaries that need to run as root in vscode

When trying to debug binaries that need to run as root, we need to do some custom configuration in vscode to have it try to run the applications as root and to ask the user for the root password when trying to start the binary. To achieve this, we’ll use a custom debugger path which points to a script that starts gdb as root using pkexec. pkexec will prompt the user for their root password via a graphical interface. This guide assumes the C/C++ extension is used for debugging.

First, create a file sgdb in the root of the systemd repository with the following contents and make it executable:

exec pkexec gdb "$@"

Then, open launch.json in vscode, and set miDebuggerPath to ${workspaceFolder}/sgdb for the corresponding debug configuration. Now, whenever you try to debug the application, vscode will try to start gdb as root via pkexec which will prompt you for your password via a graphical interface. After entering your password, vscode should be able to start debugging the application.

For more information on how to set up a debug configuration for C binaries, please refer to the official vscode documentation here

Debugging systemd with mkosi + vscode

To simplify debugging systemd when testing changes using mkosi, we’re going to show how to attach VSCode’s debugger to an instance of systemd running in a mkosi image (either using QEMU or systemd-nspawn).

To allow VSCode’s debugger to attach to systemd running in a mkosi image, we have to make sure it can access the container/virtual machine spawned by mkosi where systemd is running. mkosi makes this possible via a handy SSH option that makes the generated image accessible via SSH when booted. The easiest way to set the option is to create a file 20-local.conf in mkosi.default.d/ and add the following contents:


Next, make sure systemd-networkd is running on the host system so that it can configure the network interface connecting the host system to the container/VM spawned by mkosi. Once systemd-networkd is running, you should be able to connect to a running mkosi image by executing mkosi ssh in the systemd repo directory.

Now we need to configure VSCode. First, make sure the C/C++ extension is installed. If you’re already using a different extension for code completion and other IDE features for C in VSCode, make sure to disable the corresponding parts of the C/C++ extension in your VSCode user settings by adding the following entries:

"C_Cpp.formatting": "Disabled",
"C_Cpp.intelliSenseEngine": "Disabled",
"C_Cpp.enhancedColorization": "Disabled",
"C_Cpp.suggestSnippets": false,

With the extension set up, we can create the launch.json file in the .vscode/ directory to tell the VSCode debugger how to attach to the systemd instance running in our mkosi container/VM. Create the file and add the following contents:

    "version": "0.2.0",
    "configurations": [
            "type": "cppdbg",
            "program": "/usr/lib/systemd/systemd",
            "processId": "${command:pickRemoteProcess}",
            "request": "attach",
            "name": "systemd",
            "pipeTransport": {
                "pipeProgram": "mkosi",
                "pipeArgs": [
                "debuggerPath": "/usr/bin/gdb"
            "MIMode": "gdb",
            "sourceFileMap": {
                "/root/build/../src": {
                    "editorPath": "${workspaceFolder}",
                    "useForBreakpoints": false
                "/root/build/*": {
                    "editorPath": "${workspaceFolder}/mkosi.builddir",
                    "useForBreakpoints": false

Now that the debugger knows how to connect to our process in the container/VM and we’ve set up the necessary source mappings, go to the “Run and Debug” window and run the “systemd” debug configuration. If everything goes well, the debugger should now be attached to the systemd instance running in the container/VM. You can attach breakpoints from the editor and enjoy all the other features of VSCode’s debugger.

To debug systemd components other than PID 1, set “program” to the full path of the component you want to debug and set “processId” to “${command:pickProcess}”. Now, when starting the debugger, VSCode will ask you the PID of the process you want to debug. Run systemctl show --property MainPID --value <component> in the container to figure out the PID and enter it when asked and VSCode will attach to that process instead.

Debugging systemd-boot

During boot, systemd-boot and the stub loader will output a message like systemd-boot@0x0A,0x0B, providing the location of the text and data sections. These location can then be used to attach to a QEMU session (provided it was run with -s) with these gdb commands:

    (gdb) file build/src/boot/efi/systemd-bootx64.efi
    (gdb) add-symbol-file build/src/boot/efi/systemd_boot.so 0x0A -s .data 0x0B
    (gdb) set architecture i386:x86-64
    (gdb) target remote :1234

This process can be automated by using the debug-sd-boot.sh script in the tools folder. If run without arguments it will provide usage information.

If the debugger is too slow to attach to examine an early boot code passage, we can uncomment the call to debug_break() inside of efi_main(). As soon as the debugger has control we can then run set variable wait = 0 or return to continue. Once the debugger has attached, setting breakpoints will work like usual.

To debug systemd-boot in an IDE such as VSCode we can use a launch configuration like this:

    "name": "systemd-boot",
    "type": "cppdbg",
    "request": "launch",
    "program": "${workspaceFolder}/build/src/boot/efi/systemd-bootx64.efi",
    "cwd": "${workspaceFolder}",
    "MIMode": "gdb",
    "miDebuggerServerAddress": ":1234",
    "setupCommands": [
        { "text": "shell mkfifo /tmp/sdboot.{in,out}" },
        { "text": "shell qemu-system-x86_64 [...] -s -serial pipe:/tmp/sdboot" },
        { "text": "shell ${workspaceFolder}/tools/debug-sd-boot.sh ${workspaceFolder}/build/src/boot/efi/systemd-bootx64.efi /tmp/sdboot.out systemd-boot.gdb" },
        { "text": "source /tmp/systemd-boot.gdb" },

Hacking on the kernel + systemd

If you’re hacking on the kernel in tandem with systemd, you can clone a kernel repository in mkosi.kernel/ in the systemd repository, and mkosi will automatically build that kernel and install it into the final image. To prevent the distribution’s kernel from being installed (which isn’t necessary since we’re building our own kernel), you can add the following snippets to mkosi.default.d/20-local.conf:

(This snippet is for Fedora, the list of packages will need to be changed for other distributions)