How to make /dev/ttyACM0 (and friends) exclusive?

[Jeffrey Walton]

On Sun, Mar 3, 2019 at 12:55 AM <valdis.kletnieks at vt.edu> wrote:
>
> On Sat, 02 Mar 2019 14:36:12 -0500, Jeffrey Walton said:
>
> > I feel like I am missing something... Does Linux consider the modem a
> > shared resource instead of an exclusive resource? What use cases
> > support two different programs sending commands to the modem at the
> > same time?
>
> The Linux kernel has exactly zero clue what a "modem" is.  It's talking to a
> serial port, and doesn't care where the other end of the serial cable is. If
> you have a onboard modem, that cable may be all of 2 mm long and consist of a
> bunch of traces between two chips on a PCB, or even internal connections
> between two sides of a chip, but it's still there.
>
> So the correct question is "what use cases have two programs talking to the
> same serial port"?

I agree about the general case of serial lines and /dev/ttySn. However...

/dev/ttyACMn are modems, not serial lines. For whatever reason the
kernel made a special case for the devices. The kernel knows exactly
what they are.

Jeff