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Cygwin 3.1.2: Every call of exec(2) in the session starts to create a console window after some TTY outputs from programs compiled with "-mwindows"
- From: Koichi Murase <myoga dot murase at gmail dot com>
- To: cygwin at cygwin dot com
- Date: Mon, 20 Jan 2020 14:13:17 +0800
- Subject: Cygwin 3.1.2: Every call of exec(2) in the session starts to create a console window after some TTY outputs from programs compiled with "-mwindows"
Hi, I updated Cygwin from 3.0.7 to 3.1.2 and have two problems. This
is the report for one of them. (For the other one, read(2)
occasionally fails on the new cygwin console for some settings, but
I'm still trying to pin down the issue.)
Cygwin Version:
Cygwin 3.1.2 x86_64 on Windows 10. I also confirmed that the
problem is not fixed with the latest commit 85aff2830 of
newlib-cygwin. Note that Cygwin 3.0.7 doesn't have this problem.
Brief Description:
As in the subject, this problem is caused after some TTY outputs from
programs compiled with "-mwindows". For example, after running the
command "mintty.exe --version" in a terminal, every "exec" called in
the processes in the same session starts to create a console window
which will be closed instantaneously. As results, the display is
always flashing when some scripts are running, the shell scripts
becomes very slow, some of user inputs to the terminal go to the
created window and get lost, and CPU usage extremely increases.
This problem was not present in Cygwin 3.0.7.
Repeat-By:
The following is one example to reproduce it:
1. Open Cygwin Terminal (mintty)
2. Run "mintty --version"
3. Run some shell scripts, e.g., "for i in {0..99}; do date; done | uniq -c"
Details:
* The terminal
This phenomena is not reproduced with every terminal. For example,
it is reproduced with xterm, lxterminal and mintty, but not with the
cygwin console (which can be started, e.g., by directly calling
C:\cygwin64\bin\bash.exe from Windows) and GNU Screen. It appears
that it is caused when the terminal process that holds the PTY
master also has GUI windows.
* The trigger command
This phenomena is started when a program with
IMAGE_SUBSYSTEM_WINDOWS_GUI in its PE header (which can be created,
for example, by passing the option "-mwindows" to "gcc") outputs
something to TTY. One example for such a program is "mintty
--version". But this is not a specific problem to "mintty". The
simplest example "test1.exe" that causes the phenomena can be
created by the following command:
$ echo 'main(){puts("A");}' | gcc -xc - -mwindows -o test1.exe
The above program looks useless, but of course it is a reduced case
just to reproduce the phenomena. Actually, in one of my programs, I
need to output something to TTY in some case, and need to create a
GUI window (based on Windows API) in the other cases. Specifically,
my program is basically a GUI program (terminal emulator) and does
not want to show a console window when it is launched from, e.g.,
Explorer, so I need to specify "-mwindows" to tell Windows not to
create a console. But when my program is called inside terminal
with a special option, it runs in a TUI mode, so still I want to
output something to TTY when some conditions are satisfied.
* exec?
A new console window appears to be created when a new program is
loaded with "exec". A console window is not generated when a
subshell is created in the shell (which implies "fork" does not create
a console window) while the command "exec some-command" in the shell
creates a new console window.
Takashi: Thank you very much for your hard work on recent PTY issues!
Today I checked cygwin-patches mailing list to check possible related
fixes and found your recent patch on "disable_pcon", which is the very
feature I wanted for testing my terminal application with the legacy
console mode. Thank you!
Best regards,
Koichi
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