Provided by: openmpi-doc_2.1.1-8_all 
NAME
MPI_Op_create - Creates a user-defined combination function handle.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Op_create(MPI_User_function *function, int commute,
MPI_Op *op)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_OP_CREATE(FUNCTION, COMMUTE, OP, IERROR)
EXTERNAL FUNCTION
LOGICAL COMMUTE
INTEGER OP, IERROR
C++ Syntax
#include <mpi.h>
void Op::Init(User function* function, bool commute)
INPUT PARAMETERS
function User-defined function (function).
commute True if commutative; false otherwise.
OUTPUT PARAMETERS
op Operation (handle).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Op_create binds a user-defined global operation to an op handle that can subsequently
be used in MPI_Reduce, MPI_Allreduce, MPI_Reduce_scatter, and MPI_Scan. The user-defined
operation is assumed to be associative. If commute = true, then the operation should be
both commutative and associative. If commute = false, then the order of operands is fixed
and is defined to be in ascending, process rank order, beginning with process zero. The
order of evaluation can be changed, taking advantage of the associativity of the
operation. If commute = true then the order of evaluation can be changed, taking advantage
of commutativity and associativity.
function is the user-defined function, which must have the following four arguments:
invec, inoutvec, len, and datatype.
The ANSI-C prototype for the function is the following:
typedef void MPI_User_function(void *invec, void *inoutvec,
int *len,
MPI_Datatype *datatype);
The Fortran declaration of the user-defined function appears below.
FUNCTION USER_FUNCTION( INVEC(*), INOUTVEC(*), LEN, TYPE)
<type> INVEC(LEN), INOUTVEC(LEN)
INTEGER LEN, TYPE
The datatype argument is a handle to the data type that was passed into the call to
MPI_Reduce. The user reduce function should be written such that the following holds: Let
u[0], ..., u[len-1] be the len elements in the communication buffer described by the
arguments invec, len, and datatype when the function is invoked; let v[0], ..., v[len-1]
be len elements in the communication buffer described by the arguments inoutvec, len, and
datatype when the function is invoked; let w[0], ..., w[len-1] be len elements in the
communication buffer described by the arguments inoutvec, len, and datatype when the
function returns; then w[i] = u[i] o v[i], for i=0 ,..., len-1, where o is the reduce
operation that the function computes.
Informally, we can think of invec and inoutvec as arrays of len elements that function is
combining. The result of the reduction over-writes values in inoutvec, hence the name.
Each invocation of the function results in the pointwise evaluation of the reduce operator
on len elements: i.e, the function returns in inoutvec[i] the value invec[i] o
inoutvec[i], for i = 0..., count-1, where o is the combining operation computed by the
function.
By internally comparing the value of the datatype argument to known, global handles, it is
possible to overload the use of a single user-defined function for several different data
types.
General datatypes may be passed to the user function. However, use of datatypes that are
not contiguous is likely to lead to inefficiencies.
No MPI communication function may be called inside the user function. MPI_Abort may be
called inside the function in case of an error.
NOTES
Suppose one defines a library of user-defined reduce functions that are overloaded: The
datatype argument is used to select the right execution path at each invocation, according
to the types of the operands. The user-defined reduce function cannot "decode" the
datatype argument that it is passed, and cannot identify, by itself, the correspondence
between the datatype handles and the datatype they represent. This correspondence was
established when the datatypes were created. Before the library is used, a library
initialization preamble must be executed. This preamble code will define the datatypes
that are used by the library and store handles to these datatypes in global, static
variables that are shared by the user code and the library code.
Example: Example of user-defined reduce:
Compute the product of an array of complex numbers, in C.
typedef struct {
double real,imag;
} Complex;
/* the user-defined function
*/
void myProd( Complex *in, Complex *inout, int *len,
MPI_Datatype *dptr )
{
int i;
Complex c;
for (i=0; i< *len; ++i) {
c.real = inout->real*in->real -
inout->imag*in->imag;
c.imag = inout->real*in->imag +
inout->imag*in->real;
*inout = c;
in++; inout++;
}
}
/* and, to call it...
*/
...
/* each process has an array of 100 Complexes
*/
Complex a[100], answer[100];
MPI_Op myOp;
MPI_Datatype ctype;
/* explain to MPI how type Complex is defined
*/
MPI_Type_contiguous( 2, MPI_DOUBLE, &ctype );
MPI_Type_commit( &ctype );
/* create the complex-product user-op
*/
MPI_Op_create( myProd, True, &myOp );
MPI_Reduce( a, answer, 100, ctype, myOp, root, comm );
/* At this point, the answer, which consists of 100 Complexes,
* resides on process root
*/
The Fortran version of MPI_Reduce will invoke a user-defined reduce function using the
Fortran calling conventions and will pass a Fortran-type datatype argument; the C version
will use C calling convention and the C representation of a datatype handle. Users who
plan to mix languages should define their reduction functions accordingly.
NOTES ON COLLECTIVE OPERATIONS
The reduction functions ( MPI_Op ) do not return an error value. As a result, if the
functions detect an error, all they can do is either call MPI_Abort or silently skip the
problem. Thus, if you change the error handler from MPI_ERRORS_ARE_FATAL to something
else, for example, MPI_ERRORS_RETURN , then no error may be indicated.
The reason for this is the performance problems in ensuring that all collective routines
return the same error value.
ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and
Fortran routines in the last argument. C++ functions do not return errors. If the default
error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
mechanism will be used to throw an MPI::Exception object.
Before the error value is returned, the current MPI error handler is called. By default,
this error handler aborts the MPI job, except for I/O function errors. The error handler
may be changed with MPI_Comm_set_errhandler; the predefined error handler
MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.
SEE ALSO
MPI_Reduce
MPI_Reduce_scatter
MPI_Allreduce
MPI_Scan
MPI_Op_free