Thanks. I get it now. <div class="gmail_quote">2012/7/29 Hans de Goede <<a href="mailto:hdegoede@redhat.com" target="_blank">hdegoede@redhat.com</a>> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Hi,<div class="im"> On 07/25/2012 03:36 PM, flooding Controlled wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Hi: I am puzzled by some functions calling in the function : "spice_usb_device_manager_initable_init" At this block: /* Do coldplug (detection of already connected devices) */ libusb_get_device_list(priv->context, &priv->coldplug_list); list = g_udev_client_query_by_subsystem(priv->udev, "usb"); ... ... libusb_free_device_list(priv->coldplug_list, 1); priv->coldplug_list = NULL; My questions are as follows: 1. Well, it seems that there are two routines to gather information of the already connected devices : "libusb_get_device_list" and " g_udev_client_query_by_subsystem " , which is the really for wok one ? "g_udev_client_query_by_subsystem" seems to be , am I right ? </blockquote> </div> g_udev gives us a list of USB devices present in the system, and not much more then that, where as we use libusb for low level device access, so we manage usb devices through a list of libusb_device references, the only reason we're also using gusb is because it gives us dynamic notification when device get plugged in / removed.<div class="im"> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> 2. So, the second question is, why we gather info to "priv->coldplug_list", as the codes above, it just be freed, and has never been used! </blockquote> </div> That is not true, the coldplug_list gets used in spice_usb_device_manager_add_dev to not have to call libusb_get_device_list repeatedly when enumerating devices already plugged in on spice-gtk startup. Regards, Hans </blockquote></div>