NAME

       MPI_Allgather - Gathers data from all processes and distributes it to all processes

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Allgather(void *sendbuf, int  sendcount,
            MPI_Datatype sendtype, void *recvbuf, int recvcount,
             MPI_Datatype recvtype, MPI_Comm comm)

Fortran Syntax

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

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Allgather(const void* sendbuf, int sendcount, const
            MPI::Datatype& sendtype, void* recvbuf, int recvcount,
            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 Number of elements received from any process (integer).

       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_Allgather  is  similar  to  MPI_Gather,  except that all processes receive the result,
       instead of just the root. In other words, all processes contribute to the result, and  all
       processes receive the result.

       The  type  signature associated with sendcount, sendtype at a process must be equal to the
       type signature associated with recvcount, recvtype at any other process.

       The outcome of a call to MPI_Allgather(...) is as if all processes executed n calls to

         MPI_Gather(sendbuf,sendcount,sendtype,recvbuf,recvcount,
                    recvtype,root,comm),

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

       Example:  The all-gather version of Example 1 in MPI_Gather. Using  MPI_Allgather, we will
       gather 100 ints from every process in the group to every process.

       MPI_Comm comm;
           int gsize,sendarray[100];
           int *rbuf;
           ...
           MPI_Comm_size( comm, &gsize);
           rbuf = (int *)malloc(gsize*100*sizeof(int));
           MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);

       After the call, every process has the group-wide concatenation of the sets of data.

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_GATHER ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
          recvcount, 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 and received by all the
       members of the second group.  Then the data is gathered from all the members of the second
       group  and  received by all the members of the first.  The operation, however, need not be
       symmetric.  The number of items sent by the processes in first group need not be equal  to
       the  number  of items sent by the the processes in the second group.  You can move data in
       only one direction by giving sendcount a value of  0  for  communication  in  the  reverse
       direction.

       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_Allgatherv
       MPI_Gather