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 sendbuf.  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