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 of the MPI standard itself. MPI_DATATYPE_NULL is the constant documented for application programmers, while MPIDATATYPENULL is an implementation detail used inside the library to manage datatype handles, initialize data structures, and perform error checking without inadvertently dereferencing a invalid datatype.

Usage and role: MPIDATATYPENULL is used within the MPID code paths to indicate that a datatype parameter is not defined or to mark slots in internal tables that do not hold a valid datatype. It helps the MPI library cleanly handle edge cases in datatype creation, packing/unpacking, and communication operations, by providing a clear sentinel value separate from any real datatype handle.

Scope and portability: MPIDATATYPENULL is not part of the MPI standard and is intended for internal use only. Application code should rely on MPI_DATATYPE_NULL for indicating a null datatype. Because it is an implementation detail, the exact representation and name can vary between MPI implementations and versions.