EventSelectForWindow(WindowPtr pWin, ClientPtr client, Mask mask)
{
    Mask check;
    OtherClients *others;
    DeviceIntPtr dev;
    int rc;

    if (mask & ~AllEventMasks) {
        client->errorValue = mask;
        return BadValue;
    }
    check = (mask & ManagerMask);
    if (check) {
        rc = XaceHook(XACE_RESOURCE_ACCESS, client, pWin->drawable.id,
                      RT_WINDOW, pWin, RT_NONE, NULL, DixManageAccess);
        if (rc != Success)
            return rc;
    }
    check = (mask & AtMostOneClient);
    if (check & (pWin->eventMask | wOtherEventMasks(pWin))) {
        /* It is illegal for two different clients to select on any of the
           events for AtMostOneClient. However, it is OK, for some client to
           continue selecting on one of those events.  */
        if ((wClient(pWin) != client) && (check & pWin->eventMask))
            return BadAccess;
        for (others = wOtherClients(pWin); others; others = others->next) {
            if (!SameClient(others, client) && (check & others->mask))
                return BadAccess;
        }
    }
    if (wClient(pWin) == client) {
        check = pWin->eventMask;
        pWin->eventMask = mask;
    }
    else {
        for (others = wOtherClients(pWin); others; others = others->next) {
            if (SameClient(others, client)) {
                check = others->mask;
                if (mask == 0) {
                    FreeResource(others->resource, RT_NONE);
                    return Success;
                }
                else
                    others->mask = mask;
                goto maskSet;
            }
        }
        check = 0;
        if (!pWin->optional && !MakeWindowOptional(pWin))
            return BadAlloc;
        others = malloc(sizeof(OtherClients));
        if (!others)
            return BadAlloc;
        others->mask = mask;
        others->resource = FakeClientID(client->index);
        others->next = pWin->optional->otherClients;
        pWin->optional->otherClients = others;
        if (!AddResource(others->resource, RT_OTHERCLIENT, (void *) pWin))
            return BadAlloc;
    }
 maskSet:
    if ((mask & PointerMotionHintMask) && !(check & PointerMotionHintMask)) {
        for (dev = inputInfo.devices; dev; dev = dev->next) {
            if (dev->valuator && dev->valuator->motionHintWindow == pWin)
                dev->valuator->motionHintWindow = NullWindow;
        }
    }
    RecalculateDeliverableEvents(pWin);
    return Success;
}

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.