.. _mpi_allgather:


MPI_Allgather
=============

.. include_body

:ref:`MPI_Allgather`, :ref:`MPI_Iallgather`, :ref:`MPI_Allgather_init` - Gathers data
from all processes and distributes it to all processes

.. The following directive tells the man page generation script to
   generate multiple bindings for this file.
.. mpi-bindings: MPI_Allgather, MPI_Iallgather, MPI_Allgather_init

.. The following file was automatically generated
.. include:: ./bindings/mpi_allgather.rst

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).
* ``info``: Info (handle, persistent only).

OUTPUT PARAMETERS
-----------------
* ``recvbuf``: Address of receive buffer (choice).
* ``request``: Request (handle, non-blocking and persistent only).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

:ref:`MPI_Allgather` is similar to :ref:`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 :ref:`MPI_Allgather` is as if all processes
executed n calls to

.. code-block:: c

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

     // for root = 0, ..., n-1.

The rules for correct usage of :ref:`MPI_Allgather`
are easily found from the corresponding rules for :ref:`MPI_Gather`.

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

.. code-block:: c

       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 :ref:`MPI_Allgather` that used the in-place option is
identical to the case in which all processes executed *n* calls to

.. code-block:: c

      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.


ERRORS
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Allgatherv`
   * :ref:`MPI_Gather`