sd_seat_get_active, sd_seat_get_sessions, sd_seat_can_tty, sd_seat_can_graphical - Determine state of a specific seat
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_active() may be used to determine which session is currently active on a seat, if there is any. Returns the session identifier and the user identifier of the Unix user the session is belonging to. Either the session or the user identifier parameter can be passed NULL, in case only one of the parameters shall be queried. The returned string needs to be freed with the libc free(3) call after use.
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.
On success, sd_seat_get_active() returns 0 or a positive integer. On success, sd_seat_get_sessions() returns the number of entries in the session identifier array. If the test succeeds, sd_seat_can_tty() and sd_seat_can_graphical() return a positive integer, if it fails 0. On failure, these calls return a negative errno-style error code.
Returned errors may indicate the following problems:
The given field is not specified for the described seat.
The specified seat is unknown.
An input parameter was invalid (out of range, or NULL, where that is not accepted).
Memory allocation failed.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
In the past, sd_seat_can_multi_session() was used to check whether the seat supports multiple sessions. All seats support that now, so that function has been deprecated and always returns true.