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A derived datatype must be committed before it can be used in a communication. A committed datatype can continue to be used as an input argument in datatype constructors (so that other datatypes can be derived from the committed datatype). There is no need to commit primitive datatypes.

   INOUT    datatype               datatype that is to be committed
MPI_Type_commit(MPI_Datatype *datatype)


MPI_TYPE_COMMIT commits the datatype. Commit should be thought of as a possible ``flattening'' or ``compilation'' of the formal description of a type map into an efficient representation. Commit does not imply that the datatype is bound to the current content of a communication buffer. After a datatype has been committed, it can be repeatedly reused to communicate different data.

 Advice to implementors. The system may "compile" at commit time an
internal representation for the datatype that facilitates communication. End of 
advice to implementors.)

Jack Dongarra
Fri Sep 1 06:16:55 EDT 1995