24. Named Constants

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MPI procedures sometimes assign a special meaning to a special value of a basic type argument; e.g., tag is an integer-valued argument of point-to-point communication operations, with a special wild-card value, MPI_ANY_TAG. Such arguments will have a range of regular values, which is a proper subrange of the range of values of the corresponding basic type; special values (such as MPI_ANY_TAG) will be outside the regular range. The range of regular values, such as tag, can be queried using environmental inquiry functions (Chapter 7 of the MPI-1 document). The range of other values, such as source, depends on values given by other MPI routines (in the case of source it is the communicator size).

MPI also provides predefined named constant handles, such as MPI_COMM_WORLD.

All named constants, with the exceptions noted below for Fortran, can be used in initialization expressions or assignments, but not necessarily in array declarations or as labels in C/C++ switch or Fortran select/case statements. This implies named constants to be link-time but not necessarily compile-time constants. The named constants listed below are required to be compile-time constants in both C/C++ and Fortran. These constants do not change values during execution. Opaque objects accessed by constant handles are defined and do not change value between MPI initialization ( MPI_INIT) and MPI completion ( MPI_FINALIZE). The handles themselves are constants and can be also used in initialization expressions or assignments.

The constants that are required to be compile-time constants (and can thus be used for array length declarations and labels in C/C++ switch and Fortran case/select statements) are:

and their C++ counterparts where appropriate.

The constants that cannot be used in initialization expressions or assignments in Fortran are:

Advice to implementors.

In Fortran the implementation of these special constants may require the use of language constructs that are outside the Fortran standard. Using special values for the constants (e.g., by defining them through PARAMETER statements) is not possible because an implementation cannot distinguish these values from legal data. Typically, these constants are implemented as predefined static variables (e.g., a variable in an MPI-declared COMMON block), relying on the fact that the target compiler passes data by address. Inside the subroutine, this address can be extracted by some mechanism outside the Fortran standard (e.g., by Fortran extensions or by implementing the function in C). ( End of advice to implementors.)

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