Question

Hello I am working on an embedded linux device with a usb port that uses the g_ether driver for usb networking.

When the usb plug is connected the dmesg output is:

g_ether gadget: full speed config #2: RNDIS

When the usb cable is unplugged no message is written to dmesg.

Using C how can I listen for the connect/disconnect events?

The embedded linux OS does not have any extras. There is no dbus daemon or hotplug helper script. I am not even sure if these would of been helpful.

Answer 1

If you want everything in your single process, you’ll have to use libudev to either get events from udevd or directly from the kernel.

Seeing that it might be a problem to use libudev in your application (lack of documentation?), an alternative is to use the udevadm program, which can:

• report device events after being processed by udevd (udevadm monitor --udev --property),
• report devive events directly from the kernel (udevadm monitor --kernel --property), and
• dump udevd’s database of current devices (but not the kernel’s!) (udevadm info --query all --export-db)

udevadm is part of the udev package, but shouldn’t need udevd if you only use it to report kernel events. You can use it by having your process spawn it and parse its standard output (but you’ll have to launch it via stdbuf -o L ).

Either way, it’ll probably be a lot of work. I’ve already implemented a lot of this in my NCD programming language, including monitoring of USB devices. You might want to take a look at NCD; it’s useful for a lot of configuration tasks, and handles hotplugging well. For example, this NCD program will print USB device events to standard output:

process main {
    sys.watch_usb() watcher;
    println(watcher.event_type, " ", watcher.devname, " ", watcher.vendor_id, ":", watcher.model_id);
    watcher->nextevent();
}

This will make NCD print something like that (with an initial added event for any USB device that was already plugged in):

added /dev/bus/usb/002/045 0409:0059
added /dev/bus/usb/002/046 046d:c313
added /dev/bus/usb/002/047 046d:c03e
added /dev/bus/usb/002/048 0557:2008
removed /dev/bus/usb/002/048 0557:2008

You can also use NCD just for this, and parse this standard output – which is much easier to work with than messing with udevadm directly.

Note that NCD itself uses udevadm, and it does require udevd to be running; but why is that a problem anyway? (with some work this dependency could be removed)