Who's using "CYGWIN=tty" and why?

Andy Koppe andy.koppe@gmail.com
Wed May 11 16:00:00 GMT 2011

On 11 May 2011 16:02, Edward Lam wrote:
> On 5/11/2011 2:34 AM, Corinna Vinschen wrote:
>> Kind of weird.  The difference is that in tty mode the stdio handles are
>> pipes, while in the notty case the stdio handles are console handles.
>> Usually native Windows applications shouldn't see a difference and even
>> work *better* in notty mode.
> One problem I ran into was with *Windows mode* applications (ie. MS link.exe
> option /SUBSYSTEM:windows) trying to detect stdout redirection. I apologize
> that this takes a bit of explaining first as to why we run into a problem
> with Cygwin.
> For Windows-mode applications, _isatty(_fileno(stdout)) will always return
> false. Due to a bug (in Windows and/or the CRT), the FILE *stdout object
> will be initialized to a black hole.

That's not a bug, at least not in either Windows or Cygwin. Linking
with /SUBSYSTEM:windows tells Windows that the program doesn't need a
console, so Windows does neither attach it to the console of its
parent process nor create a new console for it. This mean that there's
nowhere for the standard handles to point to.

(With CYGWIN=tty, the standard handles are connected to the pipes
underlying Cygwin's pty implementation, which aren't affected by the
/SUBSYSTEM:windows flag.)


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