I Can't get the number of remaining bytes that can be read on an Android Accesory Mode connection

Question

I'm trying to get how many bytes are available for reading on a AOA connection but when I call the available () method of FileInputStream I get an IOException with the message: 'ioctl failed: EINVAL (Invalid argument)'

Note 1: I'm calling Java methods from C++ Using JNI

Note 2: If, instead of calling available() method I try to read, it work's perfectly

What am I doing wrong? Is there another way to ask for available bytes before reading from the socket on an AOA connection?

This is the Java Code:

public int testConnection () throws Exception {
    UsbManager usbManager = (UsbManager) this.context.getSystemService(Context.USB_SERVICE);
    final UsbAccessory[] accessoryList = usbManager.getAccessoryList();
    
    if (accessoryList == null || accessoryList.length == 0) {
        return -1;
    
    } else {
        UsbAccessory usb = accessoryList[0];
    
        try {
            ParcelFileDescriptor fileDescriptor = usbManager.openAccessory(usb);
            
            if (fileDescriptor != null) {
                FileDescriptor fd = fileDescriptor.getFileDescriptor();
                FileInputStream inStream = new FileInputStream(fd);
                
                inStream.available ();
            
            } else {
                return -2;
            }
            
        } catch (IOException e) {  
            return -3;
        
        } catch (Exception e) {
            return -4;
        }
    }

    return 0;
}

Answer 1

If you do a 'Ctrl' + 'Clic' on the available method, you will see that the method isn't available for Android targets. The implement is changed from bare native and always returns an Exception.