Unload kext which still has instances

Question

I'm updating the driver of one of my devices and would like to kextunload the old driver and kextload the new one so a restart isn't required anymore.

After the installation I try:

sudo kextunload /System/Library/Extensions/Driver.kext

The error which happens looks like this:

(kernel) Can't unload kext com.driver.Driver; classes have instances: (kernel) Kext com.driver.Driver class com_driver_Driver_USBAudioDevice has 1 instance. Failed to unload com.driver.Driver - (libkern/kext) kext is in use or retained (cannot unload).

The device is not present in ioreg. How can I find out what this instance is which prevents me from unloading the driver?

Answer 1

If you call the command kextstat, it will display a list of all the loaded kernel extensions with info like this one:-

115 3 0xffffff7f814f4000 0x32000 0x32000 com.apple.iokit.IOAudioFamily (1.8.9fc11) <114 5 4 3 1>

The first number (115) is the id of the kernel extension.

At the end, the set of numbers <114 5 4 3 1> are the other kernel extension ids that are being referenced by this particular kext.

If you match the id of your kext to any of those listed in another, then that is a reference that kextunload is referring to.

Also note that when writing any code in the kernel, globals variables are global across the whole kernel, so if you've declared any that may not have a unique name and is used by another kext, I have found this to cause a similar issue. The solution here is to prepend any globals with a reverse company URI (e.g. com_apple_globalVarName).