17.2.313. MPI_Scatterv

MPI_Scatterv, MPI_Iscatterv, MPI_Scatterv_init - Scatters a buffer in parts to all tasks in a group.

17.2.313.1. SYNTAX

17.2.313.1.1. C Syntax

#include <mpi.h>

int MPI_Scatterv(const void *sendbuf, const int sendcounts[], const int displs[],
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, int root, MPI_Comm comm)

int MPI_Iscatterv(const void *sendbuf, const int sendcounts[], const int displs[],
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Request *request)

int MPI_Scatterv_init(const void *sendbuf, const int sendcounts[], const int displs[],
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Info info, MPI_Request *request)

17.2.313.1.2. Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_SCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
             RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
     INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR

MPI_ISCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
             RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
     INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR

MPI_SCATTERV_INIT(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
             RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
     INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR

17.2.313.1.3. Fortran 2008 Syntax

USE mpi_f08
MPI_Scatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
             recvtype, root, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcounts(*), displs(*), recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iscatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
             recvtype, root, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
     INTEGER, INTENT(IN) :: recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_scatterv_init(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
             recvtype, root, comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
     INTEGER, INTENT(IN) :: recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

17.2.313.2. INPUT PARAMETERS

• 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).

• info: Info (handle, persistent only).

17.2.313.3. OUTPUT PARAMETERS

• recvbuf: Address of receive buffer (choice).

• request: Request (handle, non-blocking only).

• ierror: Fortran only: Error status (integer).

17.2.313.4. DESCRIPTION

MPI_Scatterv is the inverse operation to MPI_Gatherv.

MPI_Scatterv extends the functionality of MPI_Scatter 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,

MPI_Send(sendbuf + displs[i] * extent(sendtype),
         sendcounts[i], sendtype, i, ...);

// and each process executed a receive,

MPI_Recv(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.

Example 1: The reverse of Example 5 in the MPI_Gatherv manpage. We have a varying stride between blocks at sending (root) side, at the receiving side we receive 100 - i elements into the ith column of a 100 x 150 C array at process i.

MPI_Comm comm;
int gsize,recvarray[100][150],*rptr;
int root, *sendbuf, myrank, bufsize, *stride;
MPI_Datatype rtype;
int i, *displs, *scounts, offset;
...
MPI_Comm_size( comm, &gsize);
MPI_Comm_rank( 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<gsize; ++i) {
    displs[i] = offset;
    offset += stride[i];
    scounts[i] = 100 - i;
}
/* Create datatype for the column we are receiving
 */
MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &rtype);
MPI_Type_commit( &rtype );
rptr = &recvarray[0][myrank];
MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
             rptr, 1, rtype, root, comm);

Example 2: The reverse of Example 1 in the MPI_Gather 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 MPI_Scatterv, where stride >= 100.

MPI_Comm comm;
int gsize,*sendbuf;
int root, rbuf[100], i, *displs, *scounts;

...

MPI_Comm_size(comm, &gsize);
sendbuf = (int *)malloc(gsize*stride*sizeof(int));
...
displs = (int *)malloc(gsize*sizeof(int));
scounts = (int *)malloc(gsize*sizeof(int));
for (i=0; i<gsize; ++i) {
   displs[i] = i*stride;
   scounts[i] = 100;
}
MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
             rbuf, 100, MPI_INT, root, comm);

17.2.313.5. USE OF IN-PLACE OPTION

When the communicator is an intracommunicator, you can perform a scatter operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE 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 MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

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.

17.2.313.6. WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

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 MPI_ROOT as the value of its root argument. The remaining processes use MPI_PROC_NULL 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.

17.2.313.7. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

• MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

• MPI_ERRORS_RETURN Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

• MPI_File_create_errhandler then MPI_File_set_errhandler

• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

• MPI_Win_create_errhandler then MPI_Win_set_errhandler

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 the Error Handling section of the MPI-3.1 standard for more information.

See also

• MPI_Gather

• MPI_Gatherv

• MPI_Scatter