Technical Q&A QA1038

HID Manager Event Data Underruns

Q: I am receiving kIOReturnUnderrun errors when calling the HID Manager's getNextEvent function with a timoutMS of zero (the non-blocking mode). What does this mean and what should I do about it?

A: This is a normal condition and signifies no events pending in the queue and thus no data to return. The event parameter should be ignored in this case and will not contain valid data. Your event handling should take this into consideration as a normal mode of operation. See listing 1 for an example.

 AbsoluteTime zeroTime = {0,0};
result = (*queue)->getNextEvent (queue, &event, zeroTime, 0);
if (result == kIOReturnUnderrun)
{
    // do nothing queue empty, event invalid
}
else if (result != kIOReturnSuccess)
{
    // error
}
else
{
    // retrieved valid event
}

[Jun 13 2001]