Android 用 libusb 操作 USB 设备，无须 root

http://www.freehum.com/2014/06/operate-usb-device-under-android-by-libusb-without-root.html

对原作者做了一定的改进

之前有段时间在 Android 下调用我们自己开发的 USB 摄像头，在 优快云 上讨论了一番，发现感兴趣的人蛮多了，不断网友发邮件来问，干脆放到博客上吧

基本思路：

利用 Android （3.1版本以上）的 USB HOST API 获得 USB 设备的 FileDescriptor，然后libusb 使用 FileDescriptor 打开 USB 设备，当然 libusb 需要做少量修改，后面有代码。

总体的效果就是，用户插入USB 设备，或者启动 Android 系统，你的 App 会根据事先设定的 device filter 自动启动。如果是第一次启动，会询问用户是否授权使用该USB设备。

关键代码（如何使用libusb，请参考官方文档）：

获取 FileDescriptor

// get FileDescriptor by Android USB Host API
UsbManager manager = (UsbManager) mContext.getSystemService(Context.USB_SERVICE);
final String ACTION_USB_PERMISSION = "com.android.example.USB_PERMISSION";
HashMap<String, UsbDevice> deviceList = manager.getDeviceList();
Iterator<UsbDevice> deviceIterator = deviceList.values().iterator();
 
PendingIntent mPermissionIntent = PendingIntent.getBroadcast(mContext, 0, 
                                                             new Intent(ACTION_USB_PERMISSION), 0);
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
mContext.registerReceiver(mUsbReceiver, filter);
 
int fd = -1;
while(deviceIterator.hasNext()){
    UsbDevice device = deviceIterator.next();
    Log.i(TAG, device.getDeviceName() + " " + Integer.toHexString(device.getVendorId()) + 
               " " + Integer.toHexString(device.getProductId()));
 
    manager.requestPermission(device, mPermissionIntent);
    UsbDeviceConnection connection = manager.openDevice(device);
    if(connection != null){
       fd = connection.getFileDescriptor();
    } else
       Log.e(TAG, "UsbManager openDevice failed");
    break;
}

 

接着修改 libusb，我用的是libusbx-1.0.17，主要工作是增加一个 libusb_open_fd 函数，替代原来的 libusb_open，将上一步的 fd 传入即可。

libusbx\libusb\core.c 中在 libusb_open 后面增加 libusb_open_fd 函数

#ifdef ANDROID
int LIBUSB_CALL libusb_open_fd(libusb_device *dev, libusb_device_handle **handle, int fd)
{
struct libusb_context *ctx = DEVICE_CTX(dev);
struct libusb_device_handle *_handle;
size_t priv_size = usbi_backend->device_handle_priv_size;
int r;
usbi_dbg("open %d.%d", dev->bus_number, dev->device_address);
 
_handle = malloc(sizeof(*_handle) + priv_size);
if (!_handle)
return LIBUSB_ERROR_NO_MEM;
 
r = usbi_mutex_init(&_handle->lock, NULL);
if (r) {
free(_handle);
return LIBUSB_ERROR_OTHER;
}
 
_handle->dev = libusb_ref_device(dev);
_handle->claimed_interfaces = 0;
memset(&_handle->os_priv, 0, priv_size);
 
r = usbi_backend->open_fd(_handle, fd);
if (r < 0) {
usbi_dbg("open %d.%d returns %d", dev->bus_number, dev->device_address, r);
libusb_unref_device(dev);
usbi_mutex_destroy(&_handle->lock);
free(_handle);
return r;
}
 
usbi_mutex_lock(&ctx->open_devs_lock);
list_add(&_handle->list, &ctx->open_devs);
usbi_mutex_unlock(&ctx->open_devs_lock);
*handle = _handle;
 
/* At this point, we want to interrupt any existing event handlers so
 * that they realise the addition of the new device's poll fd. One
 * example when this is desirable is if the user is running a separate
 * dedicated libusb events handling thread, which is running with a long
 * or infinite timeout. We want to interrupt that iteration of the loop,
 * so that it picks up the new fd, and then continues. */
usbi_fd_notification(ctx);
 
return 0;
}
#endif

在libusbx\libusb\libusb.h中找到libusb_open函数的声明，在其下面添加libusb_open_fd声明：
 

int LIBUSB_CALL libusb_open_fd(libusb_device *dev, libusb_device_handle **handle, int fd);

在libusbx\libusb\libusbi.h中找到结构体usbi_os_backend，在其内部成员open下面添加
 

int (*open_fd)(struct libusb_device_handle *handle, int fd);

libusbx\libusb\os\linux_usbfs.c 中在 op_open 后面增加 op_open_fd 函数
 

#ifdef ANDROID//可以去掉，否则要到config.h中去定义才能是同
static int op_open_fd(struct libusb_device_handle *handle, int fd) {
struct linux_device_handle_priv *hpriv = _device_handle_priv(handle);
 
hpriv->fd = fd;
 
return usbi_add_pollfd(HANDLE_CTX(handle), hpriv->fd, POLLOUT);
}
#endif

libusbx\libusb\os\linux_usbfs.c 中在 结构体 linux_usbfs_backend 中 .open = op_open 一行后面增加一行
 

#ifdef ANDROID//可以去掉，否则要到config.h中去定义才能是同
    .open_fd = op_open_fd,
#endif