ioctl calls on fd passed from Java UsbManager

[Benedikt Tutzer]

Hi.
I am trying to communicate with a Webcam connected via USB. Since I do not have permissions on /dev/video4 (the device node for my webcam) I thought it would be best to ask for permission via the UsbManager class and then pass the file descriptor to C. This looks good, the fd is a read-write file descriptor and is open (both checked with fcntl), but whenever I try to perform any ioctl calls, I get a ENOTTY error.

This is how I pass the descriptor:

private 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){
                        Log.v(TAG, "connected to " + device);
                        UsbDeviceConnection con = manager.openDevice(device);
                        int fd = con.getFileDescriptor();
                        Log.e(TAG, "fd " +fd);
                        int ret = new V4L2Camera(fd, null, 640, 480, 30).initCamera();
                    }
                }
                else {
                    Log.d(TAG, "permission denied for device " + device);
                }
            }
        }
    }
};
And here is the calls to ioctl_loop

jint ...nInitCamera
  (JNIEnv *env, jobject obj){
   struct v4l2_capability capability;
   struct v4l2_cropcap cropcap;
   struct v4l2_crop crop;
   struct v4l2_format format;
   unsigned int min;

    jclass ClassV4L2Camera = env->GetObjectClass(obj);
    jfieldID fdID = env->GetFieldID(ClassV4L2Camera, "fd", "I");
    jfieldID widthID = env->GetFieldID(ClassV4L2Camera, "width", "I");
    jfieldID heightID = env->GetFieldID(ClassV4L2Camera, "height", "I");
    int fd = env->GetIntField(obj, fdID);

    int width = env->GetIntField(obj, widthID);
    int height = env->GetIntField(obj, heightID);

   if (ioctl_loop(fd, VIDIOC_QUERYCAP, &capability) == -1) {
      if (errno == EINVAL) {
         LOGE("Device does not have any V4L capabilities");
         return ERROR_NO_V4L_DEVICE;
      } else {
         if (errno == ENOTTY)
            LOGE("ENOTTY");
         return ERROR_CAPABILITIES_QUERRY;
      }
   }
....

This is the actual call to ioctl, I just retry until I do not get EINTR. In fact, I get ENOTTY.

int ioctl_loop(int fd, int request, void *arg)
{
   int r;

    do r = ioctl (fd, request, arg);
    while (-1 == r && EINTR == errno);

    return r;
}

Does anybody have an Idea why the ioctl calls do not work? I am sure the VIDIOC_QUERYCAP must work with any UVC camera, and all the cameras I have tried are UVC compliant.

I am stuck on this for quite some time, any input is very valuable to me. Thanks in advance.

[Benedikt Tutzer]

I just found out whats wrong. The UsbDeviceConnection filedescriptor points to the generic /dev/bus/usb/001/004, but to send the V4L ioctl's I need the pointer to /dev/video4, the device file created by the V4L kernel module. Can I somehow tell the device I want to use the V4L file?
I cannot open /dev/video4 directly because I do not have the necessary permissions (although the file has the same permissions).

My only idea would be to somehow build a userspace V4L library, I should then be able to open /dev/bus/usb/001/004, but I would prefer to use the build in V4L module.

[יהודה שר שלום‎]

you can use libuvc

https://github.com/saki4510t/UVCCamera/blob/master/README.md

בתאריך יום רביעי, 13 באפריל 2016 בשעה 21:34:18 UTC+3, מאת Benedikt Tutzer: