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docs/WRITING_DISPLAY_MANAGERS: update links

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to desktop environments and logind manpage
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docs/WRITING_DISPLAY_MANAGERS.md
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@ -11,13 +11,13 @@ _Or: How to hook up your favorite X11 display manager with systemd_
systemd's logind service obsoletes ConsoleKit which was previously widely used on Linux distributions. For X11 display managers the switch to logind requires a minimal amount of porting, however brings a couple of new features: true automatic multi-seat support, proper tracking of session processes, (optional) automatic killing of user processes on logout, a synchronous low-level C API and much simplification.
This document should be read together with [Writing Desktop Environments](http://www.freedesktop.org/wiki/Software/systemd/writing-desktop-environments) which focuses on the porting work necessary for desktop environments.
This document should be read together with [Writing Desktop Environments](WRITING_DESKTOP_ENVIRONMENTS) which focuses on the porting work necessary for desktop environments.
If required it is possible to implement ConsoleKit and systemd-logind support in the same display manager, detecting at runtime which interface is needed. The [sd_booted()](http://www.freedesktop.org/software/systemd/man/sd_booted.html) call may be used to determine at runtime whether systemd is used.
To a certain level ConsoleKit and systemd-logind may be used side-by-side, but a number of features are not available if ConsoleKit is used, for example automatic multi-seat support.
Please have a look at the [Bus API of logind](http://www.freedesktop.org/wiki/Software/systemd/logind) and the C API as documented in [sd-login(7)](http://www.freedesktop.org/software/systemd/man/sd-login.html). (Also see below)
Please have a look at the [Bus API of logind](https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.login1.html) and the C API as documented in [sd-login(7)](http://www.freedesktop.org/software/systemd/man/sd-login.html). (Also see below)
Minimal porting (without multi-seat) requires the following:
@ -27,7 +27,7 @@ Minimal porting (without multi-seat) requires the following:
4. Optionally, use pam_misc_setenv() to set the environment variables XDG_SEAT and XDG_VTNR. The former should contain "seat0", the latter the VT number your session runs on. pam_systemd can determine these values automatically but it's nice to pass these variables anyway.
In summary: porting a display manager from ConsoleKit to systemd primarily means removing code, not necessarily adding any new code. Here, a cheers to simplicity!
Complete porting (with multi-seat) requires the following (Before you continue, make sure to read up on [Multi-Seat on Linux](http://www.freedesktop.org/wiki/Software/systemd/multiseat) first.):
Complete porting (with multi-seat) requires the following (Before you continue, make sure to read up on [Multi-Seat on Linux](https://www.freedesktop.org/wiki/Software/systemd/multiseat) first.):
1. Subscribe to seats showing up and going away, via the systemd-logind D-Bus interface's SeatAdded and SeatRemoved signals. Take possession of each seat by spawning your greeter on it. However, do so exclusively for seats where the boolean CanGraphical property is true. Note that there are seats that cannot do graphical, and there are seats that are text-only first, and gain graphical support later on. Most prominently this is actually seat0 which comes up in text mode, and where the graphics driver is then loaded and probed during boot. This means display managers must watch PropertyChanged events on all seats, to see if they gain (or lose) the CanGraphical field.
2. Use ListSeats() on the D-Bus interface to acquire a list of already available seats and also take possession of them.
@ -36,4 +36,4 @@ Complete porting (with multi-seat) requires the following (Before you continue,
5. At this time X interprets the -seat parameter natively only for input devices, not for graphics devices. To work around this limitation we provide a tiny wrapper /lib/systemd/systemd-multi-seat-x which emulates the enumeration for graphics devices too. This wrapper will eventually go away, as soon as X learns udev-based graphics device enumeration natively, instead of the current PCI based one. Hence it is a good idea to fall back to the real X when this wrapper is not found. You may use this wrapper exactly like the real X server, and internally it will just exec() it after putting together a minimal multi-seat configuration.
And that's already it.
While most information about seats, sessions and users is available on systemd-logind's D-Bus interface, this is not the only API. The synchronous [sd-login(7)](http://www.freedesktop.org/software/systemd/man/sd-login.html) C interface is often easier to use and much faster too. In fact it is possible to implement the scheme above entirely without D-Bus relying only on this API. Note however, that this C API is purely passive, and if you want to execute an actually state changing operation you need to use the bus interface (for example, to switch sessions, or to kill sessions and suchlike). Also have a look at the [logind Bus API](http://www.freedesktop.org/wiki/Software/systemd/logind).
While most information about seats, sessions and users is available on systemd-logind's D-Bus interface, this is not the only API. The synchronous [sd-login(7)](http://www.freedesktop.org/software/systemd/man/sd-login.html) C interface is often easier to use and much faster too. In fact it is possible to implement the scheme above entirely without D-Bus relying only on this API. Note however, that this C API is purely passive, and if you want to execute an actually state changing operation you need to use the bus interface (for example, to switch sessions, or to kill sessions and suchlike). Also have a look at the [logind Bus API](https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.login1.html).