Normally, an invalid handle to an MPI object is not a valid argument for a call that expects an object. There is one exception to this rule: communication-complete calls can be passed request handles with value MPI_REQUEST_NULL. MPI_REQUEST_NULL A communication complete call with such an argument is a ``no-op'': the null handles are ignored. The same rule applies to persistent handles that are not associated with an active communication operation. request, null handlenull request handle request, inactive vs active active request handle inactive request handle status, empty
We shall use the following terminology. A null request handle is a handle with value MPI_REQUEST_NULL. A handle to a MPI_REQUEST_NULL persistent request is inactive if the request is not currently associated with an ongoing communication. A handle is active, if it is neither null nor inactive. An empty status is a status that is set to tag = MPI_ANY_TAG, source = MPI_ANY_SOURCE, and is also internally configured so that calls to MPI_ANY_TAG MPI_ANY_SOURCE MPI_GET_COUNT and MPI_GET_ELEMENT return count = 0. We set a status variable to empty in cases when the value returned is not significant. Status is set this way to prevent errors due to access of stale information.
A call to MPI_WAIT with a null or inactive request argument returns immediately with an empty status.
A call to MPI_TEST with a null or inactive request argument returns immediately with flag = true and an empty status.
The list of requests passed to MPI_WAITANY may contain null or inactive requests. If some of the requests are active, then the call returns when an active request has completed. If all the requests in the list are null or inactive then the call returns immediately, with index = MPI_UNDEFINED and an empty status.
The list of requests passed to MPI_TESTANY may contain null or inactive requests. The call returns flag = false if there are active requests in the list, and none have completed. It returns flag = true if an active request has completed, or if all the requests in the list are null or inactive. In the later case, it returns index = MPI_UNDEFINED and an empty status.
The list of requests passed to MPI_WAITALL may contain null or inactive requests. The call returns as soon as all active requests have completed. The call sets to empty each status associated with a null or inactive request.
The list of requests passed to MPI_TESTALL may contain null or inactive requests. The call returns flag = true if all active requests have completed. In this case, the call sets to empty each status associated with a null or inactive request. Otherwise, the call returns flag = false.
The list of requests passed to MPI_WAITSOME may contain null or inactive requests. If the list contains active requests, then the call returns when some of the active requests have completed. If all requests were null or inactive, then the call returns immediately, with outcount = MPI_UNDEFINED. MPI_UNDEFINED
The list of requests passed to MPI_TESTSOME may contain null or inactive requests. If the list contains active requests and some have completed, then the call returns in outcount the number of completed request. If it contains active requests, and none have completed, then it returns outcount = 0. If the list contains no active requests, then it returns outcount = MPI_UNDEFINED.
In all these cases, null or inactive request handles are not modified by the call.
