How do I create a log entry for the systemd journal?

Question

I have a service, and I would like to have it create logs for the journald daemon in certain cases. So far, I have not been able to find any instructions on how to do so.

Am I misunderstanding the intended use of the journal? Or is it something really obvious that I am missing?

Answer 1

If you have a service, you write your log to standard error. (It's even available as a stream named std::clog in C++, with more "log-like" semantics than std::cerr.) That's it. It's a logging mechanism that works with systemd, daemontools, daemontools-encore, runit, s6, perp, nosh, freedt, and others.

There is an API for writing to the systemd journal. Make sure that you have an excellent reason for locking your softwares and your users in to that API, however. Writing to standard error is a mechanism that just works, pretty much everywhere. It's well understood and easy for administrators to control, tweak, and comprehend. It should be the first choice.

Further reading

• Jonathan de Boyne Pollard (2015). "Logging". The daemontools family. Frequently Given Answers.
• Jonathan de Boyne Pollard (2001). "Don't use `syslog().". Mistakes to avoid when designing Unix dæmon programs. Frequently Given Answers.
• Laurent Bercot (2011). "The Logging Chain" The s6-log program. s6.
• sd-journal. Freedesktop.org.
• https://superuser.com/a/868519/38062
• What is the difference between cout, cerr, clog of iostream header in c++? When to use which one?

Answer 2

Lennart Poettering's blog shows how this is done with the API if you want this functionality,

#include <systemd/sd-journal.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
        sd_journal_send("MESSAGE=Hello World!",
                        "MESSAGE_ID=52fb62f99e2c49d89cfbf9d6de5e3555",
                        "PRIORITY=5",
                        "HOME=%s", getenv("HOME"),
                        "TERM=%s", getenv("TERM"),
                        "PAGE_SIZE=%li", sysconf(_SC_PAGESIZE),
                        "N_CPUS=%li", sysconf(_SC_NPROCESSORS_ONLN),
                        NULL);
        return 0;
}

Note using sd_journal_send allows you to add additional fields to the log beyond just MESSAGE, and it allows you finer grain control over PRIORITY.