Parallels C API Reference Guide
PRL_EVENT_HANDLER_PTR is a function prototype for implementing callback functions for use in asynchronous calls.
Note: You must always free the handle before exiting from the function regardless of whether you actually used it or not. Failure to do so will result in a memory leak.
A pointer to a callback function can be passed to another function that accepts it as a parameter. The operation initiated by that function will be performed in the background thread, which will call the callback function (possibly more than once) passing to it intermediate results or the final result of the operation.