sd_get_seats, sd_get_sessions, sd_get_uids, sd_get_machine_names - Determine available seats, sessions, logged in users and virtual machines/containers
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 the number of seat identifiers and if the input pointer is non-NULL, a NULL-terminated array of seat identifiers is stored at the address. 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.
Returned errors may indicate the following problems:
Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.