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Re: Strange bug with /dev/console in mintty




Am 22.08.2017 um 18:49 schrieb Erik Bray:
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
The device to refer to one's own session is /dev/tty, not /dev/console.
It is not quite obvious how to map the concept of a unique /dev/console to modern environments, esp. Cygwin.
Maybe it would help if you describe your intended use case.

which is the error message for a ENXIO.  This is coming specifically
from the function build_fh_pc in dtable.cc.  The reason is that trying
to access /dev/console results in trying to create an fhandler_console
for a non-existent console.

I'm not really sure what the correct behavior should be here though,
and if it should be fixed on the mintty side or the Cygwin side.
This is certainly not related to mintty, but likely to pty handling, as your own code findings already suggest.

Thomas

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