NAME

       MPI_Allgatherv,  MPI_Iallgatherv  -  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(const void *sendbuf, int sendcount,
            MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
            const int displs[], MPI_Datatype recvtype, MPI_Comm comm)

       int MPI_Iallgatherv(const void *sendbuf, int sendcount,
            MPI_Datatype sendtype, void *recvbuf, const int recvcounts[],
            const int displs[], MPI_Datatype recvtype, MPI_Comm comm,
               MPI_Request *request)

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

       MPI_IALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF,
                 RECVCOUNT, DISPLS, RECVTYPE, COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT(*),
            INTEGER   DISPLS(*), RECVTYPE, COMM, REQUEST, 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).

       request   Request (handle, non-blocking only).

       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