/dev/tty does not work from shell

Eric Blake eblake@redhat.com
Tue Feb 10 16:28:00 GMT 2015

On 02/09/2015 01:53 PM, Eric Blake wrote:
> On 02/09/2015 01:38 PM, Thomas Wolff wrote:
>>> bash prints that message when it has the noclobber-option set. Maybe
>>> you check it?
>> Yes, indeed, the effect shows up after set -o noclobber;
>> however, this shouldn’t apply to device files (and doesn’t on other
>> systems) -
>> man bash: “redirection will fail if the file ... is a regular file.”
> Huh. I can reproduce it, even with the new experimental bash 4.3.  I'll
> see if I can find the bug in the sources.

The bug is in cygwin1.dll, not bash.  Here's a STC; opening a file
should NOT change its st_ino number, but that's exactly what cygwin is
doing to /dev/tty.  As such, bash thinks that because the ino changed
between stat() and open() that there is a race of someone trying to do
bad things behind bash's back, and noclobber restrictions kick in to
prevent bash from using the file.

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
int main(void) {
  struct stat st1, st2;
  if (stat"/dev/tty", &st1))
    return 1;
  int fd = open("/dev/tty", O_CREAT|O_WRONLY, 0666);
  if (fd < 0)
    return 2;
  if (fstat(fd, &st2))
    return 3;
  printf ("%lld %c= %lld\n", (long long) st1.st_ino,
          st1.st_ino == st2.st_ino ? '=' : '!',
          (long long) st2.st_ino);
  return 0;

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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