MPIScatterv(3OpenMPI) MPIScatterv(3OpenMPI)
NAME
MMPPIISSccaatttteerrvv - Scatters a buffer in parts to all tasks in a group.
SSYYNNTTAAXX
CC SSyynnttaaxx
#include
int MPIScatterv(void *sendbuf, int *sendcounts, int *displs,
MPIDatatype sendtype, void *recvbuf, int recvcount,
MPIDatatype recvtype, int root, MPIComm comm)
FFoorrttrraann SSyynnttaaxx
INCLUDE 'mpif.h'
MPISCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
SENDBUF(*), RECVBUF(*)
INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR
CC++++ SSyynnttaaxx
#include
void MPI::Comm::Scatterv(const void* sendbuf, const int sendcounts[],
const int displs[], const MPI::Datatype& sendtype,
void* recvbuf, int recvcount, const MPI::Datatype&
recvtype, int root) const
IINNPPUUTT PPAARRAAMMEETTEERRSS
sendbuf Address of send buffer (choice, significant only at root).
sendcounts
Integer array (of length group size) specifying the number of
elements to send to each processor.
displs Integer array (of length group size). Entry i specifies the
displacement (relative to sendbuf) from which to take the
outgoing data to process i.
sendtype Datatype of send buffer elements (handle).
recvcount Number of elements in receive buffer (integer).
recvtype Datatype of receive buffer elements (handle).
root Rank of sending process (integer).
comm Communicator (handle).
OOUUTTPPUUTT PPAARRAAMMEETTEERRSS
recvbuf Address of receive buffer (choice).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPIScatterv is the inverse operation to MPIGatherv.
MPIScatterv extends the functionality of MPIScatter by allowing a
varying count of data to be sent to each process, since sendcounts is
now an array. It also allows more flexibility as to where the data is
taken from on the root, by providing the new argument, displs.
The outcome is as if the root executed n send operations,
MPISend(sendbuf + displs[i] * extent(sendtype), \
sendcounts[i], sendtype, i, ...)
and each process executed a receive,
MPIRecv(recvbuf, recvcount, recvtype, root, ...)
The send buffer is ignored for all nonroot processes.
The type signature implied by sendcount[i], sendtype at the root must
be equal to the type signature implied by recvcount, recvtype at
process i (however, the type maps may be different). This implies that
the amount of data sent must be equal to the amount of data received,
pairwise between each process and the root. Distinct type maps between
sender and receiver are still allowed.
All arguments to the function are significant on process root, while on
other processes, only arguments recvbuf, recvcount, recvtype, root,
comm are significant. The arguments root and comm must have identical
values on all processes.
The specification of counts, types, and displacements should not cause
any location on the root to be read more than once.
EExxaammppllee 11:: The reverse of Example 5 in the MPIGatherv manpage. We have
a varying stride between blocks at sending (root) side, at the receiv-
ing side we receive 100 - i elements into the ith column of a 100 x 150
C array at process i.
MPIComm comm;
int gsize,recvarray[100][150],*rptr;
int root, *sendbuf, myrank, bufsize, *stride;
MPIDatatype rtype;
int i, *displs, *scounts, offset;
...
MPICommsize( comm, &gsize);
MPICommrank( comm, &myrank );
stride = (int *)malloc(gsize*sizeof(int));
...
/* stride[i] for i = 0 to gsize-1 is set somehow
* sendbuf comes from elsewhere
*/
...
displs = (int *)malloc(gsize*sizeof(int));
scounts = (int *)malloc(gsize*sizeof(int));
offset = 0;
for (i=0; i scounts[i] = 100 - i;
}
/* Create datatype for the column we are receiving
*/
MPITypevector( 100-myrank, 1, 150, MPIINT, &rtype);
MPITypecommit( &rtype );
rptr = &recvarray[0][myrank];
MPIScatterv(sendbuf, scounts, displs, MPIINT,
rptr, 1, rtype, root, comm);
EExxaammppllee 22:: The reverse of Example 1 in the MPIGather manpage. The root
process scatters sets of 100 ints to the other processes, but the sets
of 100 are stride ints apart in the sending buffer. Requires use of
MPIScatterv, where stride >= 100.
MPIComm comm;
int gsize,*sendbuf;
int root, rbuf[100], i, *displs, *scounts;
...
MPICommsize(comm, &gsize);
sendbuf = (int *)malloc(gsize*stride*sizeof(int));
...
displs = (int *)malloc(gsize*sizeof(int));
scounts = (int *)malloc(gsize*sizeof(int));
for (i=0; iUUSSEE OOFF IINN-PPLLAACCEE OOPPTTIIOONN
When the communicator is an intracommunicator, you can perform a gather
operation in-place (the output buffer is used as the input buffer).
Use the variable MPIINPLACE as the value of the root process recvbuf.
In this case, recvcount and recvtype are ignored, and the root process
sends no data to itself.
Note that MPIINPLACE is a special kind of value; it has the same
restrictions on its use as MPIBOTTOM.
Because the in-place option converts the receive buffer into a send-
and-receive buffer, a Fortran binding that includes INTENT must mark
these as INOUT, not OUT.
WWHHEENN CCOOMMMMUUNNIICCAATTOORR IISS AANN IINNTTEERR-CCOOMMMMUUNNIICCAATTOORR
When the communicator is an inter-communicator, the root process in the
first group sends data to all processes in the second group. The first
group defines the root process. That process uses MPIROOT as the
value of its root argument. The remaining processes use MPIPROCNULL
as the value of their root argument. All processes in the second group
use the rank of that root process in the first group as the value of
their root argument. The receive buffer argument of the root process
in the first group must be consistent with the receive buffer argument
of the processes in the second group.
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 ALSO
MPIGather
MPIGatherv
MPIScatter
Open MPI 1.2 September 2006 MPIScatterv(3OpenMPI)