MPI_Allgatherv(3)                  Open MPI                  MPI_Allgatherv(3)



NAME
       MPI_Allgatherv - Gathers data from all processes and delivers it to
       all. Each process may contribute a different amount of data.


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Allgatherv(void *sendbuf, int sendcount,
            MPI_Datatype sendtype, void *recvbuf, int *recvcount,
            int *displs, MPI_Datatype recvtype, MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF,
                 RECVCOUNT, DISPLS, RECVTYPE, COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT(*),
            INTEGER   DISPLS(*), RECVTYPE, COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Allgatherv(const void* sendbuf, int sendcount,
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int displs[],
            const MPI::Datatype& recvtype) const = 0


INPUT PARAMETERS
       sendbuf   Starting address of send buffer (choice).

       sendcount Number of elements in send buffer (integer).

       sendtype  Datatype of send buffer elements (handle).

       recvcount Integer array (of length group size) containing the number of
                 elements that are received from each process.

       displs    Integer array (of length group size). Entry i specifies the
                 displacement (relative to recvbuf) at which to place the
                 incoming data from process i.

       recvtype  Datatype of receive buffer elements (handle).

       comm      Communicator (handle).


OUTPUT PARAMETERS
       recvbuf   Address of receive buffer (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPI_Allgatherv is similar to MPI_Allgather in that all processes gather
       data from all other processes, except that each process can send a
       different amount of data. The block of data sent from the jth process
       is received by every process and placed in the jth block of the buffer
       recvbuf.

       The type signature associated with sendcount, sendtype, at process j
       must be equal to the type signature associated with recvcounts[j],
       recvtype at any other process.

       The outcome is as if all processes executed calls to
       MPI_Gatherv(sendbuf,sendcount,sendtype,recvbuf,recvcount,
                   displs,recvtype,root,comm)

       for root = 0 , ..., n-1. The rules for correct usage of MPI_Allgatherv
       are easily found from the corresponding rules for MPI_Gatherv.


USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform an all-
       gather operation in-place (the output buffer is used as the input
       buffer).  Use the variable MPI_IN_PLACE as the value of both sendbuf
       and recvbuf.  In this case, sendcount and sendtype are ignored.  The
       input data of each process is assumed to be in the area where that
       process would receive its own contribution to the receive buffer.
       Specifically, the outcome of a call to MPI_Allgather that used the in-
       place option is identical to the case in which all processes executed n
       calls to

          MPI_GATHERV ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
          recvcounts, displs, recvtype, root, comm )

       for root =0, ... , n-1.

       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.


WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When the communicator is an inter-communicator, the gather operation
       occurs in two phases.  The data is gathered from all the members of the
       first group, concatenated, and received by all the members of the
       second group.  Then the data is gathered from all the members of the
       second group, concatenated, and received by all the members of the
       first.  The send buffer arguments in the one group must be consistent
       with the receive buffer arguments in the other group, and vice versa.
       The operation must exhibit symmetric, full-duplex behavior.

       The first group defines the root process.  The root process uses
       MPI_ROOT as the value of root.  All other processes in the first group
       use MPI_PROC_NULL as the value of root.  All processes in the second
       group use the rank of the root process in the first group as the value
       of root.

       When the communicator is an intra-communicator, these groups are the
       same, and the operation occurs in a single phase.



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_Gatherv
       MPI_Allgather



1.4.1                            Jan 14, 2010                MPI_Allgatherv(3)
