NAME
sd-login - APIs for tracking loginsSYNOPSIS
#include <systemd/sd-login.h>
pkg-config
--cflags --libs libsystemd
DESCRIPTION
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.DEFINITION OF TERMS
seatA 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.
UDEV RULES
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.
NOTES
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.SEE ALSO
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.NOTES
- 1.
- Multi-Seat on Linux
systemd 252 |