Re: com1 access denied - win xp

[Brian Ford <ford@vss.fsi.com>]

On Thu, 20 May 2004, Michael Wood wrote:

> I get an "Access is denied" error message when attempting to create a
> serial port connection through Cygwin, running on Win XP.
>
> In cygwin, when I execute:
> 	ls -l COM1
>
> I get the following:
> 	-rw-r--r--    1 mwood    mkgroup-        0 Jan  1  1970 COM1

Not related, but note the mkgroup- in the group field.  That means your
group is not part of /etc/group.  Please see "man mkgroup" to fix this.

I get the following with the latest snapshot:

-rwxrwxrwx    1 Administ SYSTEM          0 Dec 31  1969 COM1*

which may or may not make more sense to you.

> However, when I execute:
> 	chmod a+rw COM1
>
> the command exits normally (no error message), but the permissions on
> COM1 stay the same.

AFAIK, you can't change the POSIX derived permissions of DOS devices.

> Furthermore, I do not particularly understand why if I am the owner of
> COM1 (as illustrated by the 'ls -l'), why I would get a permission
> error.

That may depend on the version of Cygwin you are using.  Please see:

http://cygwin.com/problems.html

for the required information in a problem report.

> I have successfully created and used a connection on the serial port on
> the same machine through a VMWare session running a Linux Red Hat image.
> I used a very similar procedure above, in that I simply changed the
> permissions on "/dev/ttyS0" to grant all users read and write
> permissions to the serial port.

Does this help?

http://cygwin.com/cygwin-ug-net/using-specialnames.html#AEN806

Don't use a DOS device if you want POSIX behavior.

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained pilot...