QT: Not getting XIDeviceEvent data in X11EventFilter

Question

I have a Qt Application that works fine on Ubuntu 14.04 and Ubuntu 12.04.I need to redirect Touch and button press XEvents to my application. The piece of code that does this is as follows-

XIEventMask eventmask;
eventmask.deviceid = XIAllMasterDevices;
eventmask.mask_len = XIMaskLen(XI_LASTEVENT);
eventmask.mask = (unsigned char*) calloc(eventmask.mask_len, sizeof(char));
XISetMask(eventmask.mask, XI_TouchBegin);
XISetMask(eventmask.mask, XI_TouchUpdate);
XISetMask(eventmask.mask, XI_TouchEnd);
XISetMask(eventmask.mask, XI_ButtonPress);
XISetMask(eventmask.mask, XI_ButtonRelease);
XISetMask(eventmask.mask, XI_Motion);
printf("Return value from XISelectevents: %d\n",XISelectEvents(QX11Info::display(), viewer->winId(), &eventmask, 1));

The XISelectEvents() function returns 0 which I assume is the return value for success. Then I have a oveeride function bool MainApplication::x11EventFilter(XEvent *event) to process the events.

XEvent ev = *event;
if (ev.xcookie.type == GenericEvent)
{
    //printf("event: %d\n", ev.xcookie.evtype);
    XIDeviceEvent* evData = (XIDeviceEvent*)(ev.xcookie.data);

    int id = 0;
    if(evData != 0)
        id = evData->detail;
    else{
        printf("Device Event data not coming\n");
       // return false;
    }
...

The above code works fine on Ubuntu but on Debian 8 the value of (XIDeviceEvent*)(ev.xcookie.data) is 0. Is there any reason why this shouldn't work on Debian 8?

Answer 1

I needed to call XGetEventData(QX11Info::display(),ev.xcookie) in order to get the event data. This is what I found out after doing some digging. The cookie is simply a unique ID for every event. The XGenericEventCookie data structure is as follows-

typedef struct
{
    int            type;         /* of event. Always GenericEvent */
    unsigned long  serial;       /* # of last request processed */
    Bool           send_event;   /* true if from SendEvent request */
    Display        *display;     /* Display the event was read from */
    int            extension;    /* major opcode of extension that caused the event */
    int            evtype;       /* actual event type. */
    unsigned int   cookie;       /* unique event cookie */
    void           *data;        /* actual event data */
} XGenericEventCookie;

Turns out simply retrieving the event retrieves the event cookie with the data pointer NULL. XGetEventData has to be called in order to actually claim the cookie.