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Date: Sun, 5 Oct 2025 10:15:55 +0200
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Subject: Re: dynamic conpty loading
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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 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...

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