Android 用 libusb 操作 USB 设备，无须 root

- 日理万妓 2022-09-24 15:24 357阅读 0赞

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 CSDN 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