where was mention of what creates NUL files?

Ken Brown kbrown@cornell.edu
Thu Jun 27 19:00:00 GMT 2013

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.


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