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NAME | SYNOPSIS | DESCRIPTION | DEFINITION OF TERMS | UDEV RULES | NOTES | SEE ALSO | NOTES | COLOPHON |
SD-LOGIN(3) sd-login SD-LOGIN(3)
sd-login - APIs for tracking logins
#include <systemd/sd-login.h>
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.
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 /proc, /sys/fs/cgroup
and /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 each other.
If the functions return string arrays, these are generally NULL
terminated and need to be freed by the caller with the libc free(3)
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 string array NULL may be
returned, which should be treated equivalent to an empty string
array.
See sd_pid_get_session(3), sd_uid_get_state(3),
sd_session_is_active(3), sd_seat_get_active(3), sd_get_seats(3),
sd_login_monitor_new(3) for more information about the functions
implemented.
seat
A seat consists of all hardware devices assigned to a specific
workplace. It consists of at least one graphics device, and
usually also includes keyboard, mouse. It can also include video
cameras, sound cards and more. Seats are identified by seat
names, which are strings (<= 255 characters), that start with the
four characters "seat" followed by at least one character from
the range [a-zA-Z0-9], "_" and "-". They are suitable for use as
file names. Seat names may or may not be stable and may be reused
if a seat becomes available again.
session
A session is defined by the time a user is logged in until they
log out. A session is bound to one or no seats (the latter for
'virtual' ssh logins). Multiple sessions can be attached to the
same seat, but only one of them can be active, the others are in
the background. A session is identified by a short string.
systemd(1) ensures that audit sessions are identical to systemd
sessions, and uses the audit session ID as session ID in systemd
(if auditing is enabled). In general the session identifier is a
short string consisting only of [a-zA-Z0-9], "_" and "-",
suitable for use as a file name. Session IDs are unique on the
local machine and are never reused as long as the machine is
online. A user (the way we know it on UNIX) corresponds to the
person using a computer. A single user can have multiple sessions
open at the same time. A user is identified by a numeric user id
(UID) or a user name (a string). A multi-session system allows
multiple user sessions on the same seat at the same time. A
multi-seat system allows multiple independent seats that can be
individually and simultaneously used by different users.
All hardware devices that are eligible to being assigned to a seat,
are assigned to one. A device can be assigned to only one seat at a
time. If a device is not assigned to any particular other seat it is
implicitly assigned to the special default seat called "seat0".
Note that hardware like printers, hard disks or network cards is
generally not assigned to a specific seat. They are available to all
seats equally. (Well, with one exception: USB sticks can be assigned
to a seat.)
"seat0" always exists.
Assignment of hardware devices to seats is managed inside the udev
database, via settings on the devices:
Tag "seat"
When set, a device is eligible to be assigned to a seat. This tag
is set for graphics devices, mice, keyboards, video cards, sound
cards and more. Note that some devices like sound cards consist
of multiple subdevices (i.e. a PCM for input and another one for
output). This tag will be set only for the originating device,
not for the individual subdevices. A UI for configuring
assignment of devices to seats should enumerate and subscribe to
all devices with this tag set and show them in the UI. Note that
USB hubs can be assigned to a seat as well, in which case all
(current and future) devices plugged into it will also be
assigned to the same seat (unless they are explicitly assigned to
another seat).
Tag "master-of-seat"
When set, this device is enough for a seat to be considered
existent. This tag is usually set for the framebuffer device of
graphics cards. A seat hence consists of an arbitrary number of
devices marked with the "seat" tag, but (at least) one of these
devices needs to be tagged with "master-of-seat" before the seat
is actually considered to be around.
Property ID_SEAT
This property specifies the name of the seat a specific device is
assigned to. If not set the device is assigned to "seat0". Also,
to speed up enumeration of hardware belonging to a specific seat,
the seat is also set as tag on the device. I.e. if the property
ID_SEAT=seat-waldo is set for a device, the tag "seat-waldo" will
be set as well. Note that if a device is assigned to "seat0", it
will usually not carry such a tag and you need to enumerate all
devices and check the ID_SEAT property manually. Again, if a
device is assigned to seat0 this is visible on the device in two
ways: with a property ID_SEAT=seat0 and with no property ID_SEAT
set for it at all.
Property ID_AUTOSEAT
When set to "1", this device automatically generates a new and
independent seat, which is named after the path of the device.
This is set for specialized USB hubs like the Pluggable devices,
which when plugged in should create a hotplug seat without
further configuration.
Property ID_FOR_SEAT
When creating additional (manual) seats starting from a graphics
device this is a good choice to name the seat after. It is
created from the path of the device. This is useful in UIs for
configuring seats: as soon as you create a new seat from a
graphics device, read this property and prefix it with "seat-"
and use it as name for the seat.
A seat exists only and exclusively because a properly tagged device
with the right ID_SEAT property exists. Besides udev rules there is
no persistent data about seats stored on disk.
Note that systemd-logind(8) manages ACLs on a number of device
classes, to allow user code to access the device nodes attached to a
seat as long as the user has an active session on it. This is mostly
transparent to applications. As mentioned above, for certain user
software it might be a good idea to watch whether they can access
device nodes instead of thinking about seats.
These APIs are implemented as a shared library, which can be compiled
and linked to with the libsystemd pkg-config(1) file.
systemd(1), sd_pid_get_session(3), sd_uid_get_state(3),
sd_session_is_active(3), sd_seat_get_active(3), sd_get_seats(3),
sd_login_monitor_new(3), sd-daemon(3), pkg-config(1)
Multi-Seat on Linux[1] for an introduction to multi-seat support on
Linux and the background for this set of APIs.
1. Multi-Seat on Linux
https://www.freedesktop.org/wiki/Software/systemd/multiseat
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-11.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
systemd 246 SD-LOGIN(3)
Pages that refer to this page: sd_get_machine_names(3) , sd_get_seats(3) , sd_get_sessions(3) , sd_get_uids(3) , sd_login_monitor(3) , sd_login_monitor_flush(3) , sd_login_monitor_get_events(3) , sd_login_monitor_get_fd(3) , sd_login_monitor_get_timeout(3) , sd_login_monitor_new(3) , sd_login_monitor_unref(3) , sd_login_monitor_unrefp(3) , sd_machine_get_class(3) , sd_machine_get_ifindices(3) , sd_peer_get_cgroup(3) , sd_peer_get_machine_name(3) , sd_peer_get_owner_uid(3) , sd_peer_get_session(3) , sd_peer_get_slice(3) , sd_peer_get_unit(3) , sd_peer_get_user_slice(3) , sd_peer_get_user_unit(3) , sd_pid_get_cgroup(3) , sd_pid_get_machine_name(3) , sd_pid_get_owner_uid(3) , sd_pid_get_session(3) , sd_pid_get_slice(3) , sd_pid_get_unit(3) , sd_pid_get_user_slice(3) , sd_pid_get_user_unit(3) , sd_seat_can_graphical(3) , sd_seat_can_tty(3) , sd_seat_get_active(3) , sd_seat_get_sessions(3) , sd_session_get_class(3) , sd_session_get_desktop(3) , sd_session_get_display(3) , sd_session_get_remote_host(3) , sd_session_get_remote_user(3) , sd_session_get_seat(3) , sd_session_get_service(3) , sd_session_get_state(3) , sd_session_get_tty(3) , sd_session_get_type(3) , sd_session_get_uid(3) , sd_session_get_vt(3) , sd_session_is_active(3) , sd_session_is_remote(3) , sd_uid_get_display(3) , sd_uid_get_seats(3) , sd_uid_get_sessions(3) , sd_uid_get_state(3) , sd_uid_is_on_seat(3) , org.freedesktop.login1(5) , 30-systemd-environment-d-generator(7) , systemd.directives(7) , systemd.index(7) , systemd-logind(8) , systemd-logind.service(8) , systemd-machined(8) , systemd-machined.service(8)