Re: Strange bug with /dev/console in mintty

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Aug 22 18:49, Erik Bray wrote:
> Hi folks,
> 
> I noticed a strange discrepancy when running Cygwin through mintty, vs
> through a normal cmd.exe console.  This is on a build from the latest
> git master, but also on Cygwin 2.8.0.  When run from a cmd.exe
> console, we can see the following output from fhandler_dev::readdir:
> 
> $ ls -l /dev/con*
> crw-rw-rw- 4 Erik M. Bray Erik M. Bray 3, 0 Aug 22 18:29 /dev/conin
> crw-rw-rw- 4 Erik M. Bray Erik M. Bray 3, 0 Aug 22 18:29 /dev/conout
> crw-rw-rw- 4 Erik M. Bray Erik M. Bray 3, 0 Aug 22 18:29 /dev/cons0
> crw-rw-rw- 4 Erik M. Bray Erik M. Bray 3, 0 Aug 22 18:29 /dev/console
> 
> The same command when run in mintty returns:
> 
> crw-rw-rw- 1 Erik M. Bray Erik M. Bray 5, 255 Aug 22 18:29 /dev/conin
> crw-rw-rw- 1 Erik M. Bray Erik M. Bray 5, 254 Aug 22 18:29 /dev/conout
> crw-rw-rw- 1 Erik M. Bray Erik M. Bray 5,   1 Aug 22 18:29 /dev/console
> 
> In the latter case, the device numbers are the default device numbers
> for /dev/console, etc.  In the former case, the inodes for
> /dev/console, /dev/conin, and /dev/conout are set to the same as
> /dev/cons0.  This logic can be found in fhandler_dev::readdir:
> 
> 203       if (cdev->get_major () == DEV_TTY_MAJOR
> 204       && (cdev->is_device (FH_CONIN)
> 205           || cdev->is_device (FH_CONOUT)
> 206           || cdev->is_device (FH_CONSOLE)))
> 207     {
> 208       /* Make sure conin, conout, and console have the same inode number
> 209          as the current consX. */
> 210       de->d_ino = myself->ctty;
> 211     }
> 
> where myself->ctty seems to depend largely on what file types the
> stdio handles are attached to.
> 
> When running from cmd.exe, GetStdHandle(...) returns handles to a
> character stream--i.e. the console itself.  However, when running from
> mintty it returns pipes (specifically, to a pty, probably related to
> the call to forkpty in mintty).  In Cygwin (particularly, in
> dtable::init_std_file_from_handle) the result ends up being that when
> the stdio handles are pipes, the /dev/cons0 device never gets created.
> And thus /dev/console and friends never get rerouted to a real device.
> This results in errors when trying to access /dev/console directly:
> 
> $ ls -l /dev/console
> ls: cannot access '/dev/console': No such device or address

/dev/console is an old concept in Cygwin and it was always attached to
the current console window, *if* you were running inside a windows
console.  It was never implemented for pseudo ttys as used by mintty.

The fact that the /dev emulation shows the con devices is a bit
disappointing, given that they are basically unusable.  The existence
check only checks if GetConsoleCP() returns successfully, but that's
true for a pseudo tty as well.  A console is attached to the process
anyway.

I'll think about the existence check.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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