Regarding the below point : c) The service prints to stdout/stderr, but systemd attaches the service's stdout/stderr to a pipe which is read by journald (using sd_journal_stream_fd(3) from libsystemd). See [Service] StandardOutput= in systemd.service(5).
I did not see StandardOutput field in [Service] sections of a service file, for example sshd.service, but its logs are visible in journalctl. Is it by default piped to journal and we need to explicitly mention it (StandardOutput=) only when we want to redirect it somewhere else? Even in my self made service I did not mention StandardOutput=, but I can see the logs in journalctl. Is there a place where I can see .service file defaults ? Nishant On Fri, 20 Aug 2021 at 15:20, Mantas Mikulėnas <graw...@gmail.com> wrote: > On Fri, Aug 20, 2021 at 12:31 PM Nishant Nayan < > nayan.nishant2...@gmail.com> wrote: > >> Hi, >> My query is how does systemd-journald talk to other services so that >> it stores their logs/output in journal files, which could be displayed >> using journalctl utlity. >> > > Journald doesn't talk to services, services talk to journald: > > a) The service uses the standard syslog(3) call to send basic messages > through the /dev/log socket, where journald (or a traditional syslogd) is > listening. > > b) The service uses sd_journal_print(3) from libsystemd to send structured > messages through /run/systemd/journal/socket (systemd-journald.socket). > Some frameworks, such as GLib, have their own implementations of this > protocol without needing libsystemd. > > c) The service prints to stdout/stderr, but systemd attaches the service's > stdout/stderr to a pipe which is read by journald (using > sd_journal_stream_fd(3) from libsystemd). See [Service] StandardOutput= in > systemd.service(5). > > d) Journald also reads kernel messages (dmesg) from the /dev/kmsg device. > Programs can actually write to /dev/kmsg to generate dmesg messages and > journald will capture them. > > -- > Mantas Mikulėnas >