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

[valdis.kletnieks at vt.edu]

On Fri, 01 Mar 2019 23:29:53 -0500, Jeffrey Walton said:

> I cannot seem to open the device in exclusive mode. The current open
> is (I also tried with O_EXCL):
>
>     int modem = open(device_path, O_RDWR | O_NOCTTY | O_SYNC);

So what happens?  Does the open fail? If so, what does perror() say about it?
Or does the open work but the flag is ignored?

Have you tried making the *other* program open it with O_EXCL so your
caller-id program can't mess with its state?