int sd_seat_get_active(const char *seat, char **session, uid_t *uid);
int sd_seat_get_sessions(const char *seat, char ***ret_sessions, uid_t **ret_uids, unsigned int *ret_n_uids);
int sd_seat_can_tty(const char *seat);
int sd_seat_can_graphical(const char *seat);
sd_seat_get_sessions() may be used to determine all sessions on the specified seat. Returns two arrays, one (NULL terminated) with the session identifiers of the sessions and one with the user identifiers of the Unix users the sessions belong to. An additional parameter may be used to return the number of entries in the latter array. This value is the same as the return value if the return value is nonnegative. The output parameters may be passed as NULL in case these output values are not needed. The arrays and the strings referenced by them 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.
sd_seat_can_tty() may be used to determine whether a specific seat provides TTY functionality, i.e. is useful as a text console.
sd_seat_can_graphical() may be used to determine whether a specific seat provides graphics functionality, i.e. is useful as a graphics display.
If the seat parameter of any of these functions is passed as NULL, the operation is executed for the seat of the session of the calling process, if there is any.