My reading of section 2.5.4 (Named Constants) is that predefined MPI datatypes need not be implemented as compile-time constants/macros, so one cannot do #if defined(MPI_INT64_T) or use in a switch/case statement. Perhaps this makes sense if the Forum some day decides to standardize the ABI, yet all implementations I know of use macros for these types. Is there some other language in the standard that effectively requires them to be macros already? Since some vendors have been less than precise about setting MPI_VERSION and MPI_SUBVERSION, configure tests are the only reliable way to determine availability of such types.