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Re: Strange bug with /dev/console in mintty




On Wed, Aug 23, 2017 at 6:08 PM, Corinna Vinschen wrote:
> On Aug 23 10:33, Corinna Vinschen wrote:
>> 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.
>
> I checked in a patch removing the /dev/conin, /dev/conout and
> /dev/console files from /dev if the process is not running from
> a console.  Check out the latest developer snapshot from
> https://cygwin.com/snapshots/

Thanks Corinna, this patch makes sense.  Indeed, the existence check
just using GetConsoleCP() was a problem I had noted as well, and the
main problem for me was simply that /dev/con{in,out,sole} were being
listed under /dev/ at all when not attached to a Windows console (so
that code I had that was looping over /dev/ entries would break).

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