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Date: Fri, 21 Nov 2025 16:46:24 +0100
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Subject: Re: 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
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Am 21.11.2025 um 11:04 schrieb Takashi Yano via Cygwin:
> Hi Thomas,
>
> I'm very sorry for replying so late.
>
> On Sun, 5 Oct 2025 10:15:55 +0200
> Thomas Wolff wrote:
>> Am 10.09.2025 um 11:21 schrieb Thomas Wolff via Cygwin:
>>> [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
>> It would also help if someone writes a small wrapper that loads a local
>> conpty.dll version, runs CreatePseudoConsole from it, and invokes
>> wsl.exe (likely with CreateProcess). I somehow fail to get this working,
>> being umfamiliar with Windows intrinsics...
> I'm not sure what is the advantage of dynamic conpty loading.
> Does this allow use newer functionality than using conhost.exe?
Yes, it allows to use the conpty.dll version retrieved from the Windows 
Terminal project which fixes a lot of interaction problems for pty-based 
programs.
In Windows 10, for example, the current conhost version does not allow 
terminal mouse modes so mouse in editor, mc etc would not work.

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