int VT_countsetcallback( VT_CountCallback_t callback, void *custom, int ncounters )
address of the callback function or
opaque data that is passed to the callback function by the Intel® Trace Collector; can be used by the callback function to identify the thread or enabled counters
upper limit for the number of counters returned by the callback. It is not a problem to specify a number that is larger than the one that will be actually used, because the extra memory provided to the callback function will be reused efficiently.