Re: How can I use unqualified names? (Possibly hard or impossible?)

"Alf P. Steinbach" <>
Fri, 31 Jul 2009 11:40:33 +0200
* James Kanze:

Actually, [Microsoft] probably are right in forbidding the usual
meta-characters ('<', '>', ':', '"', '/', '\\', '|', '?' and
'*') in filenames. And doing so explicitly, and expecting all

Actually ':' isn't forbidden. :-)

But it has very special meaning so can't be used anywhere you please (see [1]).

But this is completely off-topic, and so I'm setting follow-ups to Windows
programming group.


- Alf

     src="From intro chapter to Alf's never-completes book manuscript">
Some names cant or shouldnt be used as filenames because they have special
meanings. They act as named data streams that you can send data to and/or
receive data from, and since they stem from old DOS theyre called DOS devices,
even at the Windows API level. DOS had at least one that doesnt exist in
Windows, namely a device representing the clock, and at least in early DOS you
wrote a colon after any device name, which isnt necessary in Windows, but
otherwise its the same:

Name(s): Short for: Represents:
nul Always empty for reading, and a black hole for writing.
con console The console window.
com1 & com9 communication Serial communication port n (sort of like old times USB).
aux auxiliary One of the comn ports, connected to a terminal.
lpt1 & lpt9 line printer Parallel port n, typically printer connections (in old
prn printer One of the lptn ports, the default printer. This name
is defined/configured by the print command (obsolete).

Hackers sometimes have fun breaking into others computers and e.g. creating
directories with such names, which ordinary users find impossible to remove. The
hacker who is more than just a script-kiddie will then typically also ensure
that the path is longer than the maximum 260 characters that most Windows
programs (including Windows Explorer and the command interpreter) can handle&

E.g. you risk this if you open your machine for FTP access with Microsofts IIS
as FTP server.

   C:\test> md con
   The directory name is invalid. //Nah, cant call a directory (or file) con.

   C:\test> md \\?\c:\test\con //But, well, if one is a hacker then one can
do that after all&
   C:\test> dir
    Volume in drive C is maindisk
    Volume Serial Number is C469-4FA2

    Directory of C:\test

   05.07.2009 17:22 <DIR> .
   05.07.2009 17:22 <DIR> ..
   05.07.2009 17:22 <DIR> con // He he.
                  0 File(s) 0 bytes
                  3 Dir(s) 15 264 759 808 bytes free

   C:\test> rd con
   The system cannot find the file specified.

   C:\test> rd \\?\c:\test\con

   C:\test> _

You may perhaps recognize the path [\\?\c:\test\con] as being very similar to
the notation used to refer to a network share to establish a drive mapping with
net use. And its the same notation family, a very Windows-specific notation
where various special kinds of paths start with a double backslash, \\.

Technically the \\?\ prefix tells a file- or directory-handling API routine to
support paths that break the usual Windows limitations both regarding length and
what can be in them, and since the command interpreter evidently doesnt detect
and reject the \\, the hack works.

Apart from avoiding using the DOS device names for files and directories, in
practical work its mostly only the nul device that is of practical interest.
First and foremost, nul is a convenient trash can for unwanted error messages,
like &

   C:\test> del nosuch
   Could Not Find C:\test\nosuch

   C:\test> del nosuch 2>nul //Getting rid of unwanted output.

   C:\test> _

Second, since nul is always empty for reading, copying nul is a simple way to
create an empty file:

   C:\test> copy nul alfa
           1 file(s) copied.

   C:\test> dir | find "alfa"
   05.07.2009 17:57 0 alfa

   C:\test> _

A more idiomatic to do the same is to type the (non-existent) contents of nul,
and redirect that non-existent output to the file one wants to create:

   C:\test> type nul >beta //Idiomatic way to create an empty file.

   C:\test> dir | find "beta"
   05.07.2009 18:00 0 beta

   C:\test> _

Here type nul is just a command that doesnt produce any output. Any command
that doesnt produce any output, e.g. cmd /c exit, would do. Its the
redirection operator > that creates  or empties  the file.

If you want to create a file with just a few lines of text then that can also be
done in about the same way, namely by typeing the con device, which for input is
just your keyboard, and redirecting the text to a file  the idiomatic poor
mans editor:

   C:\test> type con >gamma
   Programmers do the most amazing things via commands.
   Instead of mousing they use their hands.

   C:\test> dir | find "gamma"
   05.07.2009 18:34 96 gamma

   C:\test> type gamma
   Programmers do the most amazing things via commands.
   Instead of mousing they use their hands.

   C:\test> _

OK, its a poor mans poem, too! J I meant to write Instead of mousing they use
the keyboard, but that didnt rhyme. Oh well&

Anyway, with 2 empty files and one little two-line text file at hand I can
demonstrate the final special naming convention that you may  or may not 
encounter. Namely, that for filesystems that support it, and the default Windows
NTFS filesystem does support this, a Windows file may not just have the normal
main content thats reported by the size shown by the dir command and in
Windows Explorer, but may additionally have hidden alternate streams of data.
For example, thats where Windows Explorer stores additional file property

For example, the [alfa] file that we made above has absolutely no main content.
But you can add a stream named sillypoem and copy the contents of [gamma]
there. The full name of that stream is then [alfa:sillypoem] , and even though
it contains 96 bytes of data  the poem text  it doesnt show up in the
reported file size of [alfa]:

   C:\test> type gamma >alfa:poemtext

   C:\test> del gamma

   C:\test> dir
    Volume in drive C is maindisk
    Volume Serial Number is C469-4FA2

    Directory of C:\test

   05.07.2009 18:45 <DIR> .
   05.07.2009 18:45 <DIR> ..
   05.07.2009 18:45 0 alfa
   05.07.2009 18:00 0 beta
                  2 File(s) 0 bytes
                  2 Dir(s) 15 201 169 408 bytes free

   C:\test> more <alfa:poemtext
   Programmers do the most amazing things via commands.
   Instead of mousing they use their hands.

   C:\test> _

The ordinary commands dont accept the : notation for specifying stream
names, which is why I had to use the redirection operators (and hence, theres
no way to delete an alternate stream by using standard commands). So at the
command interpreter level the alternate streams are not very practically useful,
except perhaps as a way to hide stuff  then for binary data using e.g. a
*nix-like cat command. But in programming you will sooner or later need to know
about this functionality, if for nothing else then as common knowledge and to
avoid using the colon!

F Suggested exercises:
1. Find out which characters are not allowed in ordinary file- and directory-names.
2. Create empty files via commands (as opposed to using e.g. an editor).
3. On the theme of hacking, which the limited abilities of the Windows command
interpreter sometimes force people to do, one old trick for checking whether a
path is a directory path (as opposed to being a file path) is to check whether
the file nul exists in the assumed-to-be directory. Test this using if and
echo. For basic information about if, see that commands quickhelp.

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