sd-login — APIs for tracking logins
pkg-config --cflags --libs libsystemd
sd-login.h provides APIs to
introspect and monitor seat, login session and user
status information on the local system.
See Multi-Seat on Linux for an introduction into multi-seat support on Linux, the background for this set of APIs.
Note that these APIs only allow purely passive access and monitoring of seats, sessions and users. To actively make changes to the seat configuration, terminate login sessions, or switch session on a seat you need to utilize the D-Bus API of systemd-logind, instead.
These functions synchronously access data in
/run. All of these are virtual
file systems, hence the runtime cost of the accesses
is relatively cheap.
It is possible (and often a very good choice) to
mix calls to the synchronous interface of
sd-login.h with the asynchronous
D-Bus interface of systemd-logind. However, if this is
done you need to think a bit about possible races
since the stream of events from D-Bus and from
sd-login.h interfaces such as the
login monitor are asynchronous and not ordered against
If the functions return string arrays, these are
NULL terminated and need to be freed by the
caller with the libc
call after use, including the strings referenced
therein. Similarly, individual strings returned need to
be freed, as well.
As a special exception, instead of an empty
NULL may be returned, which should be
treated equivalent to an empty string array.
These APIs are implemented as a shared
library, which can be compiled and linked to with the