sd-daemon

This is sd-daemon, a reimplementation of the sd-daemon(3) routines from libsystemd as a portable static library for POSIX.1-2001 systems. It is not necessarily better than the original implementation (it is certainly less well-tested), but it is more compact, is organized as a proper static library with separately linkable objects, and only requires a POSIX libc and a C89 or C++98 compiler to build.

What is this for?

Talking to systemd if your program is a daemon running under it.

The sd_notify_*() and sd_pid_notify_*() functions notify systemd of the state of your daemon (for example, that it is ready to process requests, so its dependents can now be started). The sd_watchdog_enabled() function tells you if systemd is expecting periodic watchdog resets using sd_notify*().

The sd_listen_fds*() functions can be used to identify and retrieve file descriptors received in socket activation, and the sd_is_*() functions to ascertain which kind of file descriptor you are looking at.

Finally, the sd_booted() function tells you if the system is running systemd as the system-level service manager.

There is no detailed documentation available at the moment, but you can use the systemd man page sd-daemon(3) and its references instead.

Building

Running ./configure && make && sudo make install will install a a static library, a header, and a pkg-config file to /usr/local, with the usual configuration options available. If you don't like the configure script, make -f Makefile.in will build a libsd-daemon.a in the source directory, to be used alongside the header located in systemd/sd-daemon.h, so you only lose the pkg-config support. Finally, you can import the sources you need into your project and build them any way you want, as long as you remember to pass -D_GNU_SOURCE to get the Linux-specific VSOCK support.

Usage

In your source code, #include <systemd/sd-daemon.h> the same way you would for the libsystemd implementation. In your build system, include the output of pkg-config --cflags sd-daemon in your compiler's command line and the output of pkg-config --libs sd-daemon at the end of your linker's command line if you installed the library; otherwise, arrange for the compiler to find the include file as appropriate and put the libsd-daemon.a you built at the end of the linker's command line with the other libraries.

Compatibility

This library should be source-compatible with libsystemd release 255, except for the following:

The functions sd_is_mq() and sd_is_special() are absent. I am open to introducing the former if someone turns out to care about using socket activation with POSIX message queues, although it is fairly Linux-specific by its nature. The latter does not make any sense to me from the documentation; I have looked at its implementation in libsystemd out of sheer desperation, and turns out that does not make any sense either.

The functions sd_is_socket_sockaddr() and sd_pid_notify_with_fds() take a size_t argument where the original takes an unsigned one.

No attempt is made to mark transient file descriptors CLOEXEC.

Size

The source code currently weighs in at a bit under 800 lines, and the x86-64 machine code for the whole library is around 10K bytes with gcc -O2. This is more than I would really prefer, and the following reasons annoy me the most:

• Around 100 lines are in simple forwarder functions, like sd_notify_barrier() is for sd_pid_notify_barrier(). This is due to each of those being its own source file, with includes and such, following standard practices for static libraries.

• Around 100 lines are spent in sd_pid_notify_with_fds.c on parsing the notification socket address, and most of those on the Linux VSOCK (virtio socket) support that ad hoc reimplementations in other projects don't bother with.

• The rest of that file, about 100 more lines, is spent on file descriptor and credential passing, because sendmsg() is that cumbersome. Mainline systemd also has a pecularity where for setuid/setgid processes it will pass the real UID/GID instead of the effective one. Reproducing that means that even the simplest notification functions like sd_notify(), which would otherwise be a glorified send(), need to forward to this code. This would not be needed if we did not care about setuid/setgid, but I do not see a clean way to make this an option.

Suggestions for improvements are welcome, as is evidence that some of the pecularities mentioned above are not worth caring about.

License

Public domain, or as close as we can get to it. You can use either the CC0 deed in COPYING.CC0 in this distribution or the 0BSD/FPL license in COPYING.FPL, at your option. If you want to make a contribution to this repository, you need to grant others the right to use it under those terms, although you can always fork and relicense your own copy however you wish.

Maintainership

This library was created in response to the XZ backdoor and the accompanying realization that multiple distros patch important daemons to link to libsystemd just to be able to call sd_notify(0, "READY=1") once. I hope the existence of this library will help with that a bit.

On the other hand, the XZ Utils project is a low-profile, basically feature complete piece of foundational software maintained by a single guy, and this library would be no different even if it does not need binary test inputs. (Right now it has no tests at all!) Moreover, you likely do not know or trust me.

So I am, in no uncertain terms, giving this away. If an IBM employee or a Debian developer or even an Arch Linux TU wants to step up and maintain this, I will be happy to turn over this leaf and go do something else. My only aspiration is that this small bit of code will improve the situation.

Though maybe refrain from making it into a dynamic library? Please?