sd-login(3) — Linux manual page


SD-LOGIN(3)                       sd-login                       SD-LOGIN(3)

NAME         top

       sd-login - APIs for tracking logins

SYNOPSIS         top

       #include <systemd/sd-login.h>

       pkg-config --cflags --libs libsystemd

DESCRIPTION         top

       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

       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


           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.

           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         top

       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         top

       These APIs are implemented as a shared library, which can be compiled
       and linked to with the libsystemd pkg-config(1) file.

SEE ALSO         top

       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.

NOTES         top

        1. Multi-Seat on Linux

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug
       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ 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

systemd 246                                                      SD-LOGIN(3)

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