I’m grateful for having you as a friend! Sent from my iPhone
On Oct 7, 2020, at 4:41 AM, Daniel Santos <daniel.santos@pobox.com> wrote:
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
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