20.3.9. Constants

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MPI constants have the same value in all languages, unless specified otherwise. This does not apply to constant handles ( MPI_INT, MPI_COMM_WORLD, MPI_ERRORS_RETURN, MPI_SUM, etc.) These handles need to be converted, as explained in Section Transfer of Handles. Constants that specify maximum lengths of strings (see Section Defined Constants for a listing) have a value one less in Fortran than C since in C the length includes the null terminating character. Thus, these constants represent the amount of space that must be allocated to hold the largest possible such string, rather than the maximum number of printable characters the string could contain.

Advice to users.

This definition means that it is safe in C to allocate a buffer to receive a string using a declaration like

Image file

( End of advice to users.)
Also constant ``addresses,'' i.e., special values for reference arguments that are not handles, such as MPI_BOTTOM or MPI_STATUS_IGNORE may have different values in different languages.


The current MPI standard specifies that MPI_BOTTOM can be used in initialization expressions in C, but not in Fortran. Since Fortran does not normally support call by value, then MPI_BOTTOM in Fortran must be the name of a predefined static variable, e.g., a variable in an MPI declared COMMON block. On the other hand, in C, it is natural to take MPI_BOTTOM = 0 (Caveat: Defining MPI_BOTTOM = 0 implies that NULL pointer cannot be distinguished from MPI_BOTTOM; it may be that MPI_BOTTOM = 1 is better. See the advice to implementors in the Datatypes subsection in Section MPI Opaque Objects) Requiring that the Fortran and C values be the same will complicate the initialization process. ( End of rationale.)

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