MPIScan(3OpenMPI) MPIScan(3OpenMPI)

NAME
       MMPPIISSccaann - Computes an inclusive scan (partial reduction)


SSYYNNTTAAXX
CC SSyynnttaaxx
       #include 
       int MPIScan(void *sendbuf, void *recvbuf, int count,
            MPIDatatype datatype, MPIOp op, MPIComm comm)


FFoorrttrraann SSyynnttaaxx
       INCLUDE 'mpif.h'
       MPISCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
                SENDBUF(*), RECVBUF(*)
            INTEGER   COUNT, DATATYPE, OP, COMM, IERROR


CC++++ SSyynnttaaxx
       #include 
       void MPI::Intracomm::Scan(const void* sendbuf, void* recvbuf,
            int count, const MPI::Datatype& datatype,
            const MPI::Op& op) const


IINNPPUUTT PPAARRAAMMEETTEERRSS
       sendbuf   Send buffer (choice).

       count     Number of elements in input buffer (integer).

       datatype  Data type of elements of input buffer (handle).

       op        Operation (handle).

       comm      Communicator (handle).


OOUUTTPPUUTT PPAARRAAMMEETTEERRSS
       recvbuf   Receive buffer (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPIScan  is used to perform an inclusive prefix reduction on data dis-
       tributed across the calling processes. The operation  returns,  in  the
       recvbuf of the process with rank i, the reduction (calculated according
       to the function op) of the values in the  sendbufs  of  processes  with
       ranks  0,  ...,  i (inclusive). The type of operations supported, their
       semantics, and the constraints on send and receive buffers are  as  for
       MPIReduce.


EEXXAAMMPPLLEE
       This example uses a user-defined operation to produce a segmented scan.
       A segmented scan takes, as input, a set of values and a  set  of  logi-
       cals,  where  the  logicals delineate the various segments of the scan.
       For example,

       values     v1      v2      v3      v4      v5      v6      v7      v8
       logicals   0       0       1       1       1       0       0       1
       result     v1    v1+v2     v3    v3+v4  v3+v4+v5   v6    v6+v7     v8

       The result for rank j is thus the sum v(i) + ... + v(j), where i is the
       lowest  rank such that for all ranks n, i <= n <= j, logical(n) = logi-
       cal(j). The operator that produces this effect is

             [ u ]     [ v ]     [ w ]
             [   ]  o  [   ]  =  [   ]
             [ i ]     [ j ]     [ j ]

       where

                   ( u + v if i  = j
             w  =  (
                   ( v     if i != j

       Note that this is a noncommutative operator. C code that implements  it
       is given below.

            typedef struct {
                 double val;
                 int log;
            } SegScanPair;

            /*
             * the user-defined function
             */
            void segScan(SegScanPair *in, SegScanPair *inout, int *len,
                 MPIDatatype *dptr)
            {
                 int i;
                 SegScanPair c;

                 for (i = 0; i < *len; ++i) {
                      if (in->log == inout->log)
                           c.val = in->val + inout->val;
                      else
                           c.val = inout->val;

                      c.log = inout->log;
                      *inout = c;
                      in++;
                      inout++;
                 }
            }

       Note  that  the inout argument to the user-defined function corresponds
       to the right-hand operand of the operator. When using this operator, we
       must be careful to specify that it is noncommutative, as in the follow-
       ing:

            int            i, base;
            SeqScanPair    a, answer;
            MPIOp         myOp;
            MPIDatatype   type[2] = {MPIDOUBLE, MPIINT};
            MPIAint       disp[2];
            int            blocklen[2] = {1, 1};
            MPIDatatype   sspair;

            /*
             * explain to MPI how type SegScanPair is defined
             */
            MPIGetaddress(a, disp);
            MPIGetaddress(a.log, disp + 1);
            base = disp[0];
            for (i = 0; i < 2; ++i)
                 disp[i] -= base;
            MPITypestruct(2, blocklen, disp, type, &sspair);
            MPITypecommit(&sspair);

            /*
             * create the segmented-scan user-op
             * noncommutative - set commute (arg 2) to 0
             */
            MPIOpcreate((MPIUserfunction *)segScan, 0, &myOp);
            ...
            MPIScan(a, answer, 1, sspair, myOp, comm);


UUSSEE OOFF IINN-PPLLAACCEE OOPPTTIIOONN
       WHen the communicator is an intracommunicator, you can perform a  scan-
       ning  operation  in  place  (the  output  buffer  is  used as the input
       buffer).  Use the variable MPIINPLACE as the  value  of  the  sendbuf
       argument.  The input data is taken from the receive buffer and replaced
       by the output data.


NNOOTTEESS OONN CCOOLLLLEECCTTIIVVEE OOPPEERRAATTIIOONNSS
       The reduction functions of type MPIOp do not return  an  error  value.
       As  a  result,  if  the  functions  detect an error, all they can do is
       either call MPIAbort or silently skip the problem. Thus, if the  error
       handler  is  changed from MPIERRORSAREFATAL to something else (e.g.,
       MPIERRORSRETURN), 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.


EERRRROORRSS
       Almost  all MPI routines return an error value; C routines as the value
       of the function and Fortran routines in the last  argument.  C++  func-
       tions  do  not  return  errors.  If the default error handler is set to
       MPI::ERRORSTHROWEXCEPTIONS, 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
       MPICommseterrhandler; the predefined error handler MPIERRORSRETURN
       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 the MPI man page for a full list of MPI error codes.


SEE ALSO
       MPIExscan
       MPIOpcreate
       MPIReduce




Open MPI 1.2                    September 2006              MPIScan(3OpenMPI)