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       MPI_Allgatherv -  Gathers data from all tasks and deliver the combined data to all tasks


       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)


              - starting address of send buffer (choice)
              - number of elements in send buffer (integer)
              - data type of send buffer elements (handle)
              -  integer  array (of length group size) containing the number of elements that are
              to be 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

              - data type of receive buffer elements (handle)
       comm   - communicator (handle)


              - address of receive buffer (choice)


       The MPI standard (1.0 and 1.1) says that

       The  jth  block  of data sent from each process is received by every process and placed in
       the jth block of the buffer recvbuf .

       This is misleading; a better description is

       The block of data sent from the jth process is received by every process and placed in the
       jth block of the buffer recvbuf .

       This  text  was suggested by Rajeev Thakur, and has been adopted as a clarification to the
       MPI standard by the MPI-Forum.


       This routine is thread-safe.  This means that this routine may be safely used by  multiple
       threads  without the need for any user-provided thread locks.  However, the routine is not
       interrupt safe.  Typically, this is due to the use of memory allocation routines  such  as
       malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.


       All  MPI  routines  in  Fortran  (except  for MPI_WTIME and MPI_WTICK ) have an additional
       argument ierr at the end of the argument list.  ierr  is  an  integer  and  has  the  same
       meaning  as  the  return  value  of  the  routine  in  C.   In  Fortran,  MPI routines are
       subroutines, and are invoked with the call statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in Fortran.


       All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error value;  C  routines  as
       the  value of the function and Fortran routines in the last argument.  Before the value is
       returned, the current MPI error handler is called.  By default, this error handler  aborts
       the  MPI  job.   The  error  handler  may  be  changed  with  MPI_Comm_set_errhandler (for
       communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler  (for  RMA
       windows).   The  MPI-1  routine  MPI_Errhandler_set may be used but its use is deprecated.
       The predefined error handler MPI_ERRORS_RETURN may be used to cause  error  values  to  be
       returned.   Note  that  MPI  does  not  guarentee that an MPI program can continue past an
       error; however, MPI implementations will attempt to continue whenever possible.

              - Invalid buffer pointer.  Usually a null buffer where one is not valid.
              - Invalid count argument.  Count arguments must be non-negative; a count of zero is
              often valid.
              -  Invalid datatype argument.  Additionally, this error can occur if an uncommitted
              MPI_Datatype (see MPI_Type_commit ) is used in a communication call.

                                            11/21/2018                          MPI_Allgatherv(3)