MPI Programming Guide

Examples

Examples of user-defined reduction functions for integer vector addition follow.

C example

void int_sum (int *in, int *inout,
     int *len, MPI_Datatype *type);
 
{
   int i
   for (i=0; i<*len; i++)  {
      inout[i] + = in[i];
   }
}

FORTRAN example

      SUBROUTINE INT_SUM(IN,INOUT,LEN,TYPE)
 
      INTEGER IN(*),INOUT(*),LEN,TYPE,I
 
      DO I = 1,LEN
         INOUT(I) = IN(I) + INOUT(I)
      ENDDO
      END

User-supplied reduction operations have four arguments:

• The first argument, in, is an array or scalar variable. The length, in elements, is specified by the third argument, len.

  This argument is an input array to be reduced.

• The second argument, inout, is an array or scalar variable. The length, in elements, is specified by the third argument, len.

  This argument is an input array to be reduced and the result of the reduction will be placed here.

• The third argument, len is the number of elements in in and inout to be reduced.
• The fourth argument type is the datatype of the elements to be reduced.

Users may code their own reduction operations, with the restriction that the operations must be associative. Also, C programmers should note that the values of len and type will be passed as pointers. No communication calls are allowed in user-defined reduction operations. See Limitations in setting the thread stack size for thread stack size considerations when using the MPI threads library.