NAME

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

SYNTAX

C Syntax

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

       int MPI_Iallgather(const void *sendbuf, int  sendcount,
             MPI_Datatype sendtype, void *recvbuf, int recvcount,
             MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                 RECVTYPE, COMM, IERROR)
            <type>    SENDBUF (*), RECVBUF (*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
            INTEGER   IERROR

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

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                 comm, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
            TYPE(*), DIMENSION(..) :: recvbuf
            INTEGER, INTENT(IN) :: sendcount, recvcount
            TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

       MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                 comm, request, ierror)
            TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
            TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
            INTEGER, INTENT(IN) :: sendcount, recvcount
            TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
            TYPE(MPI_Comm), INTENT(IN) :: comm
            TYPE(MPI_Request), INTENT(OUT) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: 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).

       recvbuf   Starting address of recv buffer (choice).

       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).

       request   Request (handle, non-blocking only).

       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