opaqoverview.tex

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\subsection{MPI Opaque Objects}

  MPI Opaque objects such as \texttt{MPI_Comm} or \texttt{MPI_Datatype} are specified by 
  integers (in the MPICH2 implementation); the MPI standard calls these
  handles.  
  Out of range values are invalid; the value 0 is reserved.
  For most (with the possible exception of 
  \texttt{MPI_Request} for performance reasons) MPI Opaque objects, the integer
  encodes both the kind of object (allowing runtime tests to detect a datatype
  passed where a communicator is expected) and important properties of the 
  object.  Even the \texttt{MPI_xxx_NULL} values should be encoded so that 
  different null handles can be distinguished.  The details of the encoding
  of the handles is covered in more detail in the MPICH2 Design Document.
  For the most part, the ADI uses pointers to the underlying structures
  rather than the handles themselves.  However, each structure contains an 
  \texttt{handle} field that is the corresponding integer handle for the MPI object.

  MPID objects (objects used within the implementation of MPI) are not opaque.