Who's using "CYGWIN=tty" and why?
Wed May 11 15:02:00 GMT 2011
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.
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