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Date: Wed, 10 Sep 2025 11:21:36 +0200
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Subject: dynamic conpty loading
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Precedence: list
From: Thomas Wolff via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Thomas Wolff <towo AT towo DOT net>
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Errors-To: cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

[Reposting by response from cygwin-developers]

I wrote:
> The ancient conhost and also the initial conpty implementation of 
> Windows were obstacles to using terminal feature from Windows 
> applications.
> There are nuisances like mangled or ignored character attributes, 
> terminal reports not passed through etc.
> While there is an updated Windows conpty layer being developed as part 
> of the Windows terminal project, it is not yet deployed with Windows 
> 11 and will probably not be deployed with Windows 10 at all anymore 
> (https://github.com/microsoft/terminal/issues/17452).
> Various terminals had to fight with these problems and some found a 
> solution.
> According to 
> https://github.com/saitoha/libsixel/issues/199#issuecomment-3181728693 
> it is possible to inject a specific version of conpty so to supersede 
> the Windows-installed conhost.exe.
>
> I've tried to apply that to mintty by just preloading a conpty.dll 
> before calling forkpty for the terminal child process but that has no 
> effect.
> The procedure seems to be:
> try LoadLibrary("conpty.dll"), (if not successful, fallback to 
> GetModuleHandle("kernel32") instead) to retrieve GetProcAddress for 
> CreatePseudoConsole, ResizePseudoConsole, ClosePseudoConsole, and then 
> somehow (?) use those while calling CreateProcess.
> The latter is woven into the cygwin library and I don't think it's a 
> good idea to clone that out of cygwin for a patched process creation 
> in mintty.
>
> My question/suggestion:
> Can a cygwin mode switch to a selected conpty library instead of the 
> default one please? My idea would be to configure this per process 
> with the environment variable CYGWIN. Like its error_start= setting, 
> it could have a conpty= setting to configure the filename to be used 
> for conpty when starting a subprocess.
> That would allow mintty to inject a modern conpty when calling a 
> Windows application (e.g. wsl.exe).
>
Thomas

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