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