File I/O on a mounted USB storage device in USB Host mode (Android 3.1 and up)

Question

Ok, so I have a Android 3.1 tablet (Acer Iconia Tab, which is great by the way) which I can use with Android USB API to connect with a USB Mass Storage Device (a simple USB memory stick).

I use USB Host mode, find the device, get permission to connect to it (using BroadcastReceiver). All works great. The problem is that I don't know exactly what to do in order to copy a file from the External Storage Directory to the USB memory stick.

This is what I have so far:

final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (ACTION_USB_PERMISSION.equals(action)) {
            synchronized (this) {
            UsbDevice device = (UsbDevice) intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);

            if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                if (device != null) {

                    // Got to a point where I should set up connection
                    // I'm setting up a very simple connection, just want some file transfer

                    UsbInterface intf = device.getInterface(0);
                    UsbEndpoint endpoint = intf.getEndpoint(0);
                    UsbDeviceConnection connection = UsbManager.openDevice(device);

                    if (connection.claimInterface(intf, true)) {
                        UtilsAndDialogs.print(getApplicationContext(), "Connected to device");

                        // Copy file to USB...

                    } else
                        UtilsAndDialogs.print(getApplicationContext(), "Could not connect!");
                }
            } else {
                UtilsAndDialogs.print(getApplicationContext(), "Permission denied");
                Log.d(UtilsAndDialogs.LOG_TAG, "Permission denied for device " + device);
            }
        }
    } 
};

I read the documentation on the Android Dev Usb Host but it's not very explicit and I found a pretty good tutorial Android Usb Host Tutorial - AdbTest but it uses asynchronous communication.

I just want to know how should I set up the connection and use the endpoint (I didn't get the endpoint part, why they are needed) to just be able to create a new file on the USB storage device and copy contents of a different file in there, probably using bulkTransfer() method.

Any hints or pointers to more explicit documentation would be greatly appreciated.

Thank you

Answer 1

The Android USB host APIs provide only raw USB access. To access files on a memory device, your app must itself implement USB Mass Storage Mode on top of the USB Apis, and then the code of a filesystem on top of that.

A few vendor-customized versions of Android will mount a USB mass storage device with a recognized file system at operation system level, but that is not currently part of standard android. It is also possible that if you have a rooted device you may be able to use that to convince the kernel to mount such a filesystem.