where was mention of what creates NUL files?

where was mention of what creates NUL files?

[Daniel Barclay]

Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
files with a simple name of "NUL"?

Thanks,
Daniel

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Re: where was mention of what creates NUL files?

[Eric Blake]

On 09/17/2010 11:12 AM, Daniel Barclay wrote:
> Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
> files with a simple name of "NUL"?

Windows automagically maps the file named "NUL", in any directory, to 
the equivalent of Unix' /dev/null.  Cygwin doesn't create it, but all 
the same, portable programs should never name a file that 
case-insensitively matches 'nul', 'aux', or a host of other 
windows-magic names:

http://www.gnu.org/software/autoconf/manual/autoconf.html#File-System-Conventions

Meanwhile, cygwin 1.7 has added some magic to use native NT calls to 
work around these limitations, so that you can have a file that appears 
to be named "NUL" from within cygwin, but which is really exploiting 
some 16-bit values outside of Unicode.  But various windows programs 
that use windows API (rather than lower-level NT API), including your 
file Explorer, have a hard time figuring out what cygwin did.

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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Re: where was mention of what creates NUL files?

[Corinna Vinschen]

On Sep 17 11:22, Eric Blake wrote:
> On 09/17/2010 11:12 AM, Daniel Barclay wrote:
> >Does anyone recall a mention of what in CygWin (or possibly Emacs) creates

<finicking>
It's Cygwin, not CygWin.
</finicking>

files with a simple name of "NUL"?
> 
> Windows automagically maps the file named "NUL", in any directory,

Meep!  The Win32 API, not Windows per se.

> to the equivalent of Unix' /dev/null.  Cygwin doesn't create it, but
> all the same, portable programs should never name a file that
> case-insensitively matches 'nul', 'aux', or a host of other
> windows-magic names:
> 
> http://www.gnu.org/software/autoconf/manual/autoconf.html#File-System-Conventions
> 
> Meanwhile, cygwin 1.7 has added some magic to use native NT calls to
> work around these limitations, so that you can have a file that
> appears to be named "NUL" from within cygwin, but which is really
> exploiting some 16-bit values outside of Unicode.

Sorry, but that's not entirely correct.  There isn't any magic involved
and the resulting filename is actually "nul".  No mapping to the Unicode
private use area.

The terrible DOS device name hack, which maps filenames containing
substring named like the the old DOS device names (NUL, AUX, PRN, etc)
to the actual Windows device, only exists in the Win32 API.  Cygwin
doesn't use the Win32 API to access files, rather it uses the underlying
native NT API.  This API allows to create and access actual files like
"nul" or "aux.c", just as on any other OS.  The DOS device name hack
simply doesn't affect us.

So, any Cygwin application can create files like "nul".  It happens, for
instance, if you call something like:

  $ echo foo > NUL
  $ ls -l NUL
  -rw-r--r-- 1 corinna vinschen 4 Sep 17 21:25 NUL

Note:  Don't use DOS device names in Cygwin!

  Wrong:

    $ echo foo > NUL
    $ echo foo > nul
    $ echo foo > nul:

  Right:

    $ echo foo > /dev/null

See here:
http://cygwin.com/cygwin-ug-net/using-specialnames.html#pathnames-dosdevices
and here:
http://cygwin.com/cygwin-ug-net/using-specialnames.html#pathnames-posixdevices


Corinna

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

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Re: where was mention of what creates NUL files?

[Daniel Barclay]

Corinna Vinschen wrote:
...
>
> Note:  Don't use DOS device names in Cygwin!
>
>    Wrong:
>
>      $ echo foo>  NUL
>      $ echo foo>  nul
>      $ echo foo>  nul:
>
>    Right:
>
>      $ echo foo>  /dev/null

Yes, I know.  I'm not using NUL (or nul or nul:, etc.), but something
is.

(Now I'm thinking that it's an NTEmacs problem (perhaps thinking it's
running commands in a Windows/DOS shell rather than knowing it's
running them in Cygwin bash.).)



Daniel



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Re: where was mention of what creates NUL files?

[Oleksandr Gavenko]

On 17.09.2010 20:12, Daniel Barclay wrote:
> Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
> files with a simple name of "NUL"?
>
I also use native Emacs and after some operation I got file with name NUL.

I have a lot NUL files in many dirs, which can be removed only from Cygwin:

   $ rm NUL

I don't fully  debug but think this come from 'null-device' variable
(which has 'NUL' value by default).

To friend native GNU Emacs with Cygwin (and stop creating NUL) I use:

(when (eq system-type 'windows-nt)
   (setq shell-file-name "bash")
   ;; Workaround for Cygwin shell, when set 'CYGWIN=noglob'. By default 
'shell-quote-argument'
   ;; quoted by double '\' chars this cause failure.
   (defun shell-quote-argument (argument)
     (concat "'" argument "'")
     )
   ;; Workaround for Cygwin when 'shell-file-name' is 'bash'.
   (setq null-device "/dev/null")
   )

--
Happy hacking!


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Re: where was mention of what creates NUL files?

[g]

Daniel Barclay <daniel <at> fgm.com> writes:

> 
> Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
> files with a simple name of "NUL"?
> 
> Thanks,
> Daniel
> 
> 


This has been driving me nuts for years. Finally tracked it down.
These are created by emacs' man.el code when you get a man page.
Reproduce: 
In emacs, do:
    M-x man <ret>
    <enter anything, valid ('ls') or not>
Now, you'll see a NUL file in the directory.

Root cause:
construction of the 'man' command that is passed to the shell includes:
(concat " %s 2>" null-device)

The variable `null-device' is platform specific and defaults to a pure-copy
of "/dev/null" which, apparently, becomes "NUL" on windows.

It is a defvar in files.el and 'set' again in dos-w32.el.

Resolution:
In your .emacs file, do:

   (require 'dos-w32) ;; load this first to avoid it undo'ing the next line
   (setq null-device "c:/tmp/emacs-dev-null.txt") ;; set to anything

Happy days!




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Re: where was mention of what creates NUL files?

[Ken Brown]

On 6/26/2013 2:53 PM, g wrote:
>
>
>
> Daniel Barclay <daniel <at> fgm.com> writes:
>
>>
>> Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
>> files with a simple name of "NUL"?
>>
>> Thanks,
>> Daniel
>>
>>
>
>
> This has been driving me nuts for years. Finally tracked it down.
> These are created by emacs' man.el code when you get a man page.
> Reproduce:
> In emacs, do:
>      M-x man <ret>
>      <enter anything, valid ('ls') or not>
> Now, you'll see a NUL file in the directory.

I can't reproduce this with Cygwin emacs.  You must be using native 
Windows emacs.

> Root cause:
> construction of the 'man' command that is passed to the shell includes:
> (concat " %s 2>" null-device)
>
> The variable `null-device' is platform specific and defaults to a pure-copy
> of "/dev/null" which, apparently, becomes "NUL" on windows.
>
> It is a defvar in files.el and 'set' again in dos-w32.el.
>
> Resolution:
> In your .emacs file, do:
>
>     (require 'dos-w32) ;; load this first to avoid it undo'ing the next line
>     (setq null-device "c:/tmp/emacs-dev-null.txt") ;; set to anything

Just to be clear, users of Cygwin emacs should *not* do this.

Ken

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Re: where was mention of what creates NUL files?

[wynfield]

I do not have the problem using emacs built on cygwin either.


> On 6/26/2013 2:53 PM, g wrote:
> >
> >
> >
> > Daniel Barclay <daniel <at> fgm.com> writes:
> >
> >>
> >> Does anyone recall a mention of what in CygWin (or possibly Emacs) creates
> >> files with a simple name of "NUL"?
> >>
> >> Thanks,
> >> Daniel
> >>
> >>
> >
> >
> > This has been driving me nuts for years. Finally tracked it down.
> > These are created by emacs' man.el code when you get a man page.
> > Reproduce:
> > In emacs, do:
> >      M-x man <ret>
> >      <enter anything, valid ('ls') or not>
> > Now, you'll see a NUL file in the directory.
> 
> I can't reproduce this with Cygwin emacs.  You must be using native
> Windows emacs.
> 
> > Root cause:
> > construction of the 'man' command that is passed to the shell includes:
> > (concat " %s 2>" null-device)
> >
> > The variable `null-device' is platform specific and defaults to a pure-copy
> > of "/dev/null" which, apparently, becomes "NUL" on windows.
> >
> > It is a defvar in files.el and 'set' again in dos-w32.el.
> >
> > Resolution:
> > In your .emacs file, do:
> >
> >     (require 'dos-w32) ;; load this first to avoid it undo'ing the next line
> >     (setq null-device "c:/tmp/emacs-dev-null.txt") ;; set to anything
> 
> Just to be clear, users of Cygwin emacs should *not* do this.
> 
> Ken
> 
> 

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