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How to properly use the asynchronous libusb?

  • I worked on the synchronous libusb in my Qt project with good results and now I need the asynchronous features of this library. I understood reading here, here and here that, after I've registered my callback function using the libusb_fill_control_transfer and submitted a transfer with libusb_submit_transfer , I need to "keep live" the libusb_handle_events_completed inside a while loop to get the transfer related events since the libusb doesn't have its own thread. For example you can read a code like this

    libusb_fill_control_transfer(transfer, dev, buffer, cb, &completed, 1000);
    while (!completed) {
        libusb_handle_events_completed(ctx, &completed);

    Now if I want read a packet that I don't know when it occurs, I think that goes against the asynchronous nature submit a read and wait in the while with libusb_handle_events_completed until the event is triggered.

    Then, do I need to create a separate thread within the libusb_handle_events_completed in an infinite while loop? Like

    // I create the thread when the application starts
    QFuture<void> usbEventHandlerPoller;
    usbEventHandlerPoller = QtConcurrent::run( usbConnection, &USBConnection::PollUSBEventHandler, &threadAbortFlag );
    // Here the cuncurrent thread with the infinite loop
    void USBConnection::PollUSBEventHandler( volatile bool* pThreadAbortFlag )
        int completed = 0;
        while ( 1 )
            libusb_handle_events_completed( NULL, &completed );
            if ( *pThreadAbortFlag == true )

    Can anyone, with experience in the asynchronous features of libusb library, give some suggestions on the right approach to handle the transfer events?

  • Lifetime Qt Champion


    I haven't used that feature but from the looks of it you should properly encapsulate your libusb code within a class. In this case likely a QThread subclass so you can implement the loop handling in the run reimplementation.

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