Thread: USB Plugin Detection in C/C++

Hardware is a "kernel space" realm; the things you refer to are "user space" applications that rely on the kernel 100% to inform them.

So, you could write a simple kernel module and that will be a sure fire, custom method if you want notification of USB events. But then you have to learn some kernel programming.

On the other hand, I have not looked at the API for libusb. "kudbd" or whatever is a distro specific thing, maybe you could use that. There are more universal high level methods you could use such as putting a hook into the kernel log, or into /proc, both of which record USB events.

Just keep googling and find a method that works for what you want to do...but on a low level, using the kernel C API is probably the only way.

Well, so write something that monitors /var/log/messages. You could make that generic, since it may have other uses, and then use a socket or signals for communication to your other program.

Or you could right something specific that deals with /proc, you will have to investigate how the "usb subsystem" is referred to there. Another possibility, if the device has a corresponding device file in /dev and all you need to know is whether and when it is plugged in, would be to poll that file somehow.

Originally Posted by Kennedy

If you are just looking to do something user-space, then just interject a script into the hotplug scripts.

I think you're getting to the root of the issue here. How does a "hotplug script" work? It seems like the correct approach isn't to modify those scripts, but to figure out how they are triggered and base the solution on that same method.

(And if it comes down to monitoring messages in a log file I think I'll vomit)

Nah, it doesn't. It really doesn't even need to be a script (though that is the "normal" way of doing things). Depending on what the hotplug system is (the old style was really freaky), one needs to simply modify the configuration of it (I think this is under /etc/hotplug/) to ensure to trap the notification of the device matching type X [ and vendor Y]. If the hotplug system is udevd, I know that the whole "real" configuration of this beast is under /dev/.udevd but I have yet to work with it (I know how to interface this sucker from kernel space, but not from user space). I recall that it will allow each device attached to run a specific script for user-space configuration, but I have never done this. I've only worked with hotplug (so far).

[edit]In the past, the easiest thing that I did was to watch the device (in this case /dev/sda) for insertion by doing a call to dd attempting to read one byte from the device. If the device is not there, dd posts an error (echo $? == "1"). If, however, the device shows up, then I would kick out of my 10 second sleep loop and do something real. When I was done, I'd hit the top of my sleep loop again. Yes, this is a serious hack, but it was lots easier than attempting to keep hotplug happy.[/edit]

Originally Posted by brewbuck

(And if it comes down to monitoring messages in a log file I think I'll vomit)

Oh come on it would work. It's also easy, and reliable.

If you are actually going to put some days into this, you might as well try and tackle the kernel API and write a little module -- I'm sure it would not be more than 50 lines, and you could communicate with it in user space. Unfortunately the online documentation for this is pretty skimpy tho. Or maybe not...I just found this, which looks like a good, short intro:

http://www.freesoftwaremagazine.com/.../drivers_linux

Here's the API:
http://www.gnugeneration.com/books/l...20/kernel-api/

And if you google around, there are now at least TWO free books online on kernel programming/linux device drivers.

Originally Posted by RockyMarrone

From the discussion above i m tryin to get all the messages from udev and there is a utility udevmonitor which tells always when ever the device is plugged in or out....

Of course, if you can find a little system command and use that, you're set. Also check out lsusb:

Code:

[root~] lsusb
Bus 005 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 005: ID 045e:00dd Microsoft Corp. 
Bus 002 Device 004: ID 045e:00b9 Microsoft Corp. Wireless Optical Mouse 3.0
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

The microsoft stuff is a USB keyboard and mouse, which I stole from some dumbass windows user and is the only USB things connected to my system right now.

You could also try "apropos usb" which will give you a list of the manual pages for all commands that have "usb" in their short description.