Strange NPE when opening USB device

[MariusVolkhart]

Hi,

I was hoping you could join me in exploring a strange NPE coming from inside the library.

My program keeps encountering the following error on MacOS.

java.lang.NullPointerException: Cannot invoke "jdk.internal.foreign.AbstractMemorySegmentImpl.isAlignedForElement(long, java.lang.foreign.MemoryLayout)" because "segment" is null
    at java.base/jdk.internal.foreign.LayoutPath.checkAlign(LayoutPath.java:290)
    at net.codecrete.usb.macos.IoKitHelper.getVtable(IoKitHelper.java:96)
    at net.codecrete.usb.macos.IoKitUsb.USBDeviceOpenSeize(IoKitUsb.java:54)
    at net.codecrete.usb.macos.MacosUsbDevice.open(MacosUsbDevice.java:115)

My application code is:

    private fun UsbDevice.openIfNeeded() {
        LOGGER.atDebug().log("USB device is open: {}.", isOpened)
        if (!isOpened) {
            try {
                open()
                try {
                    claimInterface(USB_INTERFACE_NUMBER)
                } catch (e: Exception) {
                    LOGGER.atWarn().withThrowable(e).log("Failed to claim interface")
                }
            } catch (e: Exception) {
                LOGGER.atWarn().withThrowable(e).log("Failed to open")
            }
        }
    }

I've bee experience this issue since 0.7.0 (first version I used). It happens after several hours of the application running. No USB devices added/removed. Restarting the application immediately makes it work. At no point does my application close the USB device. I added this openIfNeeded function because I found the USB device was releasing endpoints itself. As in, without me explicitly releasing them or closing the device, after several hours, I'd find the device to be unable to transferOut.

I don't know if I'm using the underlying USB library incorrectly (as in, should I not be holding the device open for hours on end without sending traffic), or if there's something else, but from a library perspective, I'd expect the open() call to not fail with an NPE :)

Please let me know if there's more information you'd like or something you'd like me to try out. Thanks for the fun FFM library!

Version: 1.0.0 MacOS Apple Silicon Java 22

[manuel-ti8m]

Thank you for reporting it. I will investigate it. It doesn't sound as if you are using the library incorrectly.

During the hours when the USB device is not used:

• Does the computer go to sleep / suspend?
• Does the application keep the device open and has interfaces that are still claimed?
• If so, why is isOpen set the false in your application? Does it detect the device is not responding and closes and reopens it?

[manuelbl]

The same exception can be trigger when trying to open a device that was disconnected (the application kept it in a variable). The behavior has now been changed. It now triggers a UsbException that clearly indicates that the device has been disconnected.

I suspect that your case could be similar or the same, i.e. the USB device has been disconnected, if not physically, then by rebooting the USB device, USB hub or a similar action. At least macOS thinks it has been disconnected (and possibly reconnected).

The fixed code is on the main branch. It hasn't been published as a release it.

[MariusVolkhart]

Thanks for the analysis. Indeed, I made some code changes and open the USB on-demand now, and that resolves my issue. I haven't yet tried with the changes you made. I'm OK closing this ticket though. Again, thank you for responding and investigating.