Re: Help porting the XLaunch feature to autoselect the display number

[Jon TURNEY <jon dot turney at dronecode dot org dot uk>]

On 24/02/2015 03:57, Michael DePaulo wrote:
> I am trying to port this feature of VcXsrv (and XMing also I think) to
> Cygwin XLaunch:
> https://sourceforge.net/p/vcxsrv/code/ci/460182676a960385dff96c1563f781213060f6fc/
>
> Attached is my WIP patch. (I know it needs the comments updated for main.cc).
>
> There's a bug in main.cc that is causing this to happen when -1 is specified:
> http://imgur.com/Jv4tpip

Protip: Ctrl-c works on MessageBox dialogs to copy their contents.

Thanks for looking into this.

Unfortunately, this turns into a non-trivial amount of work.

The upstream design is that -displayfd introduces a file descriptor that 
the child X server process inherits, to which it will write it's display 
number.

(This fd will be one end of a pipe which the parent process has opened, 
and it will read the display number from the fd for the other end)

In the VcXsrv implementation, instead a handle to some shared memory is 
used to pass the display number back. (I guess a Windows anonymous pipe 
would have worked just as well, but possibly this is simpler to implement).

So, you will need to re-write the changes to main.cc to create a pipe 
(using pipe()), pass the write fd using -displayfd, and read the diplay 
number from the read fd.

Even after you've done this, I'm not 100% sure that cygwin pipes are 
successfully propagated across CreateProcess().  I have a vague memory 
this is where I got stuck when I last looked at this, but I'm not sure 
I'd noticed that CreateProcess() is being invoked with 
bInheritHandles=FALSE.

I found my old (non-working) attempt at implementing this [1], which 
might help you a bit.

[1] 
https://github.com/jon-turney/xlaunch/commit/b3fc02fcc9ac43224137963e2aba39abb88608da

--
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

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