Systemd for Embedded Linux

Over the last few years, there has been a lot of controversy in the Linux world about systemd. As I understand it, systemd is intended to be a better-engineered, more powerful version of the motley collection of little programs and scripts which keeps the essential services on a Linux system running.

systemctl

The controversy arises because the original 1970s Unix way of doing things was to rely on a motley collection of little programs and scripts for everything, each of which was simple but well understood, and to knit them together to form a complete operating system. Systemd takes a different approach, using larger and more sophisticated components which are more dedicated to particular tasks, such as managing services or network connections. This is supposed to make it more efficient and easier to manage in the twenty-first century.

I’ve been doing some work recently on an embedded Linux system which runs on the latest version of Debian Linux, version 8 (‘Jessie’). Debian Jessie fully supports systemd to the extent that it seems to be the default way of doing things. I thought I’d experiment with it a bit.

When working on an embedded Linux system, I very frequently want to have a piece of my software run reliably at startup, get restarted if it fails, and be able to output logging information to an easily-managed place. In this case, my software provides a D-Bus interface to a piece of industrial electronics.

In the past I’ve relied on copying and pasting scripts from other pieces of software, and managing log files has always been a bit of a mess. It’s hard to do these things right, so re-inventing the wheel is too risky, which means that the best strategy is to copy somebody else’s scripts. I have never counted the hours of my time which have been wasted by dealing with awkward corner cases and peculiar bugs due to recycled scripts behaving in ways I hadn’t anticipated.

What does it look like with systemd? There are some helpful tutorials out there, including this one from Alexander Patrakov, so it didn’t take me too long to put together a service file which looks like this:

[Unit]
Description=My D-Bus Gateway
[Service]
Type=dbus
BusName=com.martin-jones.gateway
ExecStart=/usr/bin/my_dbus_gateway
Restart=always
[Install]
WantedBy=multi-user.target

I’ve changed the names to protect the innocent, but the contents of the file are pretty self-explanatory. The [Unit] section just includes a description which is readable to a human being. The [Service] section describes the service itself. In this case it’s of type  dbus, which means that systemd will check that the service name (com.martin-jones.gateway in this case) gets correctly published on to D-Bus. The Restart=always setting means that my software gets restarted if it exits. The [Install] section just indicates that this service should run when the system comes up in multi-user mode (like the old runlevel 5).

Having created this file, I simply copied it into /etc/systemd/system/my_dbus_gateway.service and, lo and behold, my new service worked. It was immediately possible to manage the service using commands like

systemctl start my_dbus_gateway.service
systemctl stop my_dbus_gateway.service
systemctl status my_dbus_gateway.service

Great! That’s exactly what I wanted.

Now for logging. I’d heard that systemd would log the stdout and stderr outputs of services into its own journal, and forward that to syslog as required. It does, but there’s a subtlety. Output from stderr appears in /var/log/syslog immediately line-by-line, but output from stdout gets aggressively buffered. This means that it gives the appearance of not working at all unless you explicitly flush the stdout buffer in your code using something like

fflush(stdout)

That’s the only wrinkle I came across, though.

In summary, using systemd’s facilities has made my life as an embedded Linux developer much, much easier and hopefully more reliable. That’s a good thing. My top tips for getting your software working under systemd are these:

  • Create your .service file using the recipe above and the documentation
  • Don’t forget to flush stdout if you want to see it in syslog.
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2 thoughts on “Systemd for Embedded Linux

  1. Jeff Thompson

    I’m starting a new embedded system product design and want to use systemd. Where can I go to find the latest information? Most of what I’ve found using Google is a couple of years old.

    Reply
    1. martinjonestechnology Post author

      I too have had this problem: I never found an authoritative source of information on the particular version of systemd delivered with whatever Linux distribution you’re using. I had to do lots of web searching and then compare what I found with the manpages on the Debian system. It all seems to develop faster than the documentation can keep up.

      Reply

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