MPIDATATYPENULL
MPIDATATYPENULL is an internal sentinel value used within some MPI implementations to represent the absence of a valid MPI datatype at the MPID (Message Passing Interface Device) layer. It acts as a placeholder for situations where a datatype is not required, not yet created, or has been released, allowing the internal code to distinguish “no datatype” from a real, usable datatype.
In practice, MPIDATATYPENULL corresponds to the public MPI_Datatype value MPI_DATATYPE_NULL, but it is not a part
Usage and role: MPIDATATYPENULL is used within the MPID code paths to indicate that a datatype parameter
Scope and portability: MPIDATATYPENULL is not part of the MPI standard and is intended for internal use