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

[Daniel Santos]

Hello!

I'm not subscribed to this list, but I thought I would resurrect this
old thread, as I ran across it on the web.

> On Mon, 04 Mar 2019 16:38:44 +0100, Yann Droneaud said:
>
> > O_EXCL is intended to be used to prevent opening an existing file. Said
> > differently, it's used to ensure a new file is created, useful to
> > prevent race condition, where multiple processes compete to create a
> > file. For example think of temporary file created with random name.
>
> Which means that if you're trying to regulate exclusive use of a character
> device or similar, you use O_EXCL|O_CREAT to create a lock file, and
make sure
> that all the programs competing for the character device use the same
pathname
> to create the lock file - which has been a tried-and-true way of handling
> exclusive locking for serial ports for 4 decades now.
>
> The fact that 'strings /usr/sbin/ModemManager | grep /' proves pretty
clearly
> that ModemManager is trusting DBus to keep track of things rather than
using
> lock files on the modems is almost certainly part of the reason it's a
steaming
> pile of dingo's kidneys rather than a helpful piece of software.

My solution to this particular dingo terd is this:

cat << EOF >> /var/udev/rules.d/81-ttyUSB_ACM_no_mm.rules
KERNEL=="ttyUSB[0-9]*", ENV{ID_MM_DEVICE_IGNORE}="1"
KERNEL=="ttyACM[0-9]*", ENV{ID_MM_DEVICE_IGNORE}="1"
EOF

Very evil indeed.  I have a lot of strange devices attached to serial
ports or emulating USB CDC ACM devices, and this cost me a lot of time. 
The funny thing is that I went CDC ACM (on one device) instead of raw
URBs because I thought it would save me time, but I spent much longer
trying to figure this out than I saved.

Daniel