The usual type matching rules apply to size-specific datatypes: a
value sent with datatype
MPI_TYPE
n can be received with this same datatype on
another process.
Most modern computers use 2's complement for integers and
IEEE format for floating point. Thus, communication using these
size-specific datatypes will not entail loss of precision or
truncation errors.
real(selected_real_kind(5)) x(100) call MPI_SIZEOF(x, size, ierror) call MPI_TYPE_MATCH_SIZE(MPI_TYPECLASS_REAL, size, xtype, ierror) if (myrank .eq. 0) then ... initialize x ... call MPI_SEND(x, xtype, 100, 1, ...) else if (myrank .eq. 1) then call MPI_RECV(x, xtype, 100, 0, ...) endifThis may not work in a heterogeneous environment if the value of size is not the same on process 1 and process 0. There should be no problem in a homogeneous environment. To communicate in a heterogeneous environment, there are at least four options. The first is to declare variables of default type and use the MPI-/ datatypes for these types, e.g., declare a variable of type REAL and use MPI_REAL. The second is to use selected_real_kind or selected_int_kind and with the functions of the previous section. The third is to declare a variable that is known to be the same size on all architectures (e.g., selected_real_kind(12) on almost all compilers will result in an 8-byte representation). The fourth is to carefully check representation size before communication. This may require explicit conversion to a variable of size that can be communicated and handshaking between sender and receiver to agree on a size.
Note finally that using the ``external32'' representation for I/O requires explicit attention to the representation sizes. Consider the following code:
real(selected_real_kind(5)) x(100) call MPI_SIZEOF(x, size, ierror) call MPI_TYPE_MATCH_SIZE(MPI_TYPECLASS_REAL, size, xtype, ierror) if (myrank .eq. 0) then call MPI_FILE_OPEN(MPI_COMM_SELF, 'foo', & MPI_MODE_CREATE+MPI_MODE_WRONLY, & MPI_INFO_NULL, fh, ierror) call MPI_FILE_SET_VIEW(fh, 0, xtype, xtype, 'external32', & MPI_INFO_NULL, ierror) call MPI_FILE_WRITE(fh, x, 100, xtype, status, ierror) call MPI_FILE_CLOSE(fh, ierror) endif call MPI_BARRIER(MPI_COMM_WORLD, ierror) if (myrank .eq. 1) then call MPI_FILE_OPEN(MPI_COMM_SELF, 'foo', MPI_MODE_RDONLY, & MPI_INFO_NULL, fh, ierror) call MPI_FILE_SET_VIEW(fh, 0, xtype, xtype, 'external32', & MPI_INFO_NULL, ierror) call MPI_FILE_WRITE(fh, x, 100, xtype, status, ierror) call MPI_FILE_CLOSE(fh, ierror) endifIf processes 0 and 1 are on different machines, this code may not work as expected if the size is different on the two machines.(End of advice to users.)
MPI-Standard for MARMOT