MPIREQUESTNULL

MPI_REQUEST_NULL is a predefined constant in the MPI (Message Passing Interface) standard that represents a null or empty nonblocking request handle. It serves as a sentinel to indicate that no nonblocking operation is associated with a given request variable, or that an operation was not started.

In practice, a nonblocking operation such as MPI_Isend or MPI_Irecv returns a real request handle when the operation is initiated. MPI_REQUEST_NULL is commonly used to initialize a request variable, or to pass a placeholder to routines that expect a request, signaling that there is no outstanding operation in that position. It is not a real communication operation itself.

When used with completion or test routines, MPI_REQUEST_NULL behaves as already completed. For example, MPI_Wait(MPI_REQUEST_NULL, ...) returns MPI_SUCCESS immediately, and MPI_Test(MPI_REQUEST_NULL, ...) sets the test flag to true without blocking. In routines that operate on arrays of requests, such as MPI_Waitall or MPI_Testall, MPI_REQUEST_NULL entries are ignored, and statuses corresponding to non-null requests are filled as usual; MPI_STATUS_IGNORE may be used for the entries that are MPI_REQUEST_NULL.

Regarding resource management, MPI_Request_free releases resources associated with a real request; if called with MPI_REQUEST_NULL, the call is a no-op and returns MPI_SUCCESS. MPI_REQUEST_NULL is not a valid handle for performing communication operations, and it should not be treated as an active request.

Portability across C and Fortran interfaces is ensured by this sentinel value, which provides consistent semantics for indicating “no operation” or “no request” in MPI programs.