21.5. Handle Constants

Predefined handle constants are represented as integers in the range 1 to 4095. To make it easy for implementations and tools to decode handle constants, their values are derived from a Huffman code. The Huffman code currently uses the lower 10 bits of the 12 bits allocated above. In the following, we will describe constants in their binary representation, 0b***_***_***_***, where underscores are added for readability. We count bits from the right starting from zero; the rightmost three bits will be denoted 2:0.

Datatypes are the most common type of predefined handle and use the range 0b00_10_********. Bits 7:3 identify the category and the bits 2:0 identify the specific instances thereof. The first set of predefined datatypes are not fixed-sized. While long has a fixed size for a given platform ABI, it is not fixed across all platforms. In contrast, types like int32_t and REAL*8 (or its Fortran standard equivalents) have the same size on all platforms. Fixed-sized types allow the implementation to determine the size of the datatype from the predefined handle value itself, without a lookup table.

Table 30: Predefined MPI datatype categories and instance values, for types without a specified size. All unassigned values are reserved.

Table 37: Predefined MPI datatype categories and instance values, for types with a specified width. All unassigned values are reserved.

All other predefined handles use the range 0b00_01_********. In general, predefined null handles have all zero bits other than those required to detect the handle type.

Table 39: Predefined MPI_Op categories and instance values. All unassigned values are reserved.