On Fri, Aug 20, 2021 at 12:31 PM Nishant Nayan <nayan.nishant2000@xxxxxxxxx> 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
