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