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

Christopher Faylor cgf-use-the-mailinglist-please@cygwin.com
Wed May 11 15:40:00 GMT 2011

On Wed, May 11, 2011 at 11:02:40AM -0400, 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. So if you want printf's to 
>make its way into the redirected file, you have to manually connect the 
>FILE *stdout object to the redirected file output handle.
>The usual method is to call GetStartupInfo(&info) and check if 
>info.dwFlags has the STARTF_USESTDHANDLES flag set. If it is set, then 
>assume that info.hStdOutput contains the redirected file output handle 
>and attach it with something like:
>    *stdout = _fdopen(_open_osfhandle(info.hStdOutput, _O_TEXT));
>So this brings us to Cygwin. When we spawn such a Windows mode app from 
>Cygwin, the method I describe above fails. The call to 
>_open_osfhandle(info.hStdOutput, _O_TEXT) returns with an error value of 
>-1. This is likely why jam reports "the handle is invalid".
>Personally, when I first ran into this problem, I never realized that 
>CYGWIN=tty would fix it. I did notice that there was a change in the 
>behavior between Cygwin B20 and the Cygwin 1.X releases but I only 
>realize now that this was probably the reason.

Just to go on mean record: I don't think the fact that jam works better
as side effect of setting CYGWIN=tty is a good enough reason to derail
any plans to get rid of CYGWIN=tty.

It seems like you might be able to get the same behavior by saying

jam </dev/null 2>&1 | cat


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