mjhjmtu

1. 
   

   https://unix.stackexchange.com/a/503874/674 says

   
   
   

   
   

   The display is effectively the X server; there is exactly one display per X server. So multiple X servers can’t run simultaneously
   on the same display, and an X server can’t run simultaneously on
   multiple displays. (Strictly speaking, the latter point isn’t
   correct, but I don’t think there’s an X server which can serve
   multiple displays.)

   
   

   
   
   

   https://www.x.org/archive/X11R6.8.0/doc/X.7.html#sect4 says a
   display can have multiple screens/monitors. $DISPLAY specifies a
   screen, not just a display, and is used in starting a X server or a
   X client. So does a X server start in a display or a screen? So
   does a X server start in a display or a screen?

   
   


2. 
   

   https://unix.stackexchange.com/a/503884/674 has a diagram that

   
   
   

   • distinguishes screen and monitor, while https://www.x.org/archive/X11R6.8.0/doc/X.7.html#sect4 seems to say
     they are the same concept when explaining screen number. Which one is correct?

   
   

   • shows a X server covers all the screens in a display. So does a display server start in a display or a screen or a monitor?

   
   


3. 
   

   Can I specify an arbitrary `$DISPLAY`?
   says:

   
   
   

   
   

   An xserver can use a hardware framebuffer, a dummy framebuffer (Xvfb) or a window on another xserver (Xephyr). The latter two are
   examples of "virtual" xserver/display

   
   

   
   
   

   Is a framebuffer associated with a display or a screen or a monitor?

   
   

Sorry I am still confused by the multiple concepts. Thanks.

So does a X server start in a display or a screen?

I’m not sure how to say this in a different way than I did previously; for all intents and purposes, the X server is a display (“display” as the X Window concept, which I understand is what we’re discussing here). An X server doesn’t start in a display, it is a display. You can think of this as “an X server starts a display”, and “a display contains one or more screens”.

The DISPLAY variable can be confusing since, as you say, it can specify more than the X display.

Which one is correct?

The diagram; see the explanation below.

Does a display server start in a display or a screen or a monitor?

In the X Window documentation, “display server” is synonymous with X server, so the above applies.

It may help to consider that the X Window documentation was written a long time ago, at a time when virtual displays weren’t used (much, if at all), and when multi-monitor setups were complex and often involved multiple X screens, and sometimes even multiple X servers. So in the X documentation, a screen is usually a monitor. However it quickly became obvious that it was annoying to split multiple monitors into multiple screens, and once graphics cards became capable of handling multiple monitors as a single unit, usage patterns changed so that X screens tended to cover multiple monitors.

Is a framebuffer associated with a display or a screen or a monitor?

“Framebuffer” is a somewhat nebulous term, with multiple definitions. In the context of the comment you’re quoting, it’s associated with a screen, and you can see this with Xvfb: if you tell it to use memory-mapped files for its framebuffers, and define multiple screens, you’ll see it use one framebuffer file per screen.

As you been told the 1000th time, the display IS the X server. So "X server run in a display" makes no sense.

X document use "display" to refer a bunch of hardware because in the old time, X servers usually(if not always) take control of and render to real hardware, but nowadays, many modern servers are able to run on and render to non-real(virtual) hardware-based target, e.g. Xephyr or Xvfb or Xorg with dummy video driver. The document don't get update very often, but it's not proper to say "A display is a bunch of hardware" on modern systems, it would be much better to say "A display is a running X server process".

An X server listens on an address(es), this address(es) can be connected using X11 protocol. How the X11 protocol data is transported is purely a platform specific thing, it could be a pair of TCP sockets, a TCP socket on the posix server side and a magic object on the non-posix client side(basically any two connected TCP endpoint), a pair of locally connected UDS(basically any two connected IPC endpoint, in which case X window system may become more powerful and efficient because the client and server run on the same machine, things like DRI becomes possible).

An X server might run multiple X screens(not to be confused with real-life monitors), and might handle multiple framebuffer (no matter real GPU framebuffer or malloc() buffer or mmap() disk-file memory space region), frambuffers doesn't have strict mapping with X screens, depends on your driver, settings, and which kind of X server you use.