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NAME | SYNOPSIS | DESCRIPTION | DEFINITION OF TERMS | UDEV RULES | NOTES | SEE ALSO | NOTES | COLOPHON |
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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 is part of libsystemd(3) and 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.
Added in version 235.
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.
Added in version 235.
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).
Added in version 235.
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.
Added in version 235.
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.
Added in version 235.
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.
Added in version 235.
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.
Added in version 235.
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.
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.
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] may also be of historical interest.
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 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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 258~rc2 SD-LOGIN(3)
Pages that refer to this page: libsystemd(3), sd_get_seats(3), sd_login_monitor_new(3), sd_machine_get_class(3), sd_pid_get_owner_uid(3), sd_seat_get_active(3), sd_session_is_active(3), sd_uid_get_state(3), org.freedesktop.login1(5), systemd.directives(7), systemd.index(7), systemd-logind.service(8), systemd-machined.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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