sd_get_seats, sd_get_sessions, sd_get_uids, sd_get_machine_names — Determine available seats, sessions, logged in users and virtual machines/containers
#include <systemd/sd-login.h>
| int sd_get_seats( | char ***seats ); | 
| int sd_get_sessions( | char ***sessions ); | 
| int sd_get_uids( | uid_t **users ); | 
| int sd_get_machine_names( | char ***machines ); | 
sd_get_seats() may be used to determine
    all currently available local seats. Returns a
    NULL terminated array of seat identifiers.
    The returned array and all strings it references need to be freed
    with the libc
    free(3)
    call after use. Note that instead of an empty array
    NULL may be returned and should be considered
    equivalent to an empty array.
Similarly, sd_get_sessions() may be
    used to determine all current login sessions.
Similarly, sd_get_uids() may be used to
    determine all Unix users who currently have login sessions.
Similarly, sd_get_machine_names() may
    be used to determine all current virtual machines and containers
    on the system.
Note that the returned lists are not sorted and in an undefined order.
On success, sd_get_seats(),
    sd_get_sessions(),
    sd_get_uids() and
    sd_get_machine_names() return the number of
    entries in the arrays. On failure, these calls return a negative
    errno-style error code.
The sd_get_seats(),
    sd_get_sessions(),
    sd_get_uids() and
    sd_get_machine_names() interfaces are
    available as a shared library, which can be compiled and linked to
    with the
    libsystemd pkg-config(1)
    file.