MPI_Allgather(3)                   Open MPI                   MPI_Allgather(3)



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 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_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




1.4.1                            Jan 14, 2010                 MPI_Allgather(3)
