MPI_GATHER, MPI_Gather

Purpose

Collects individual messages from each task in comm at the root task.

C synopsis

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

C++ synopsis

#include mpi.h
void MPI::Comm::Gather(const void* sendbuf, int sendcount,
                       const MPI::Datatype& sendtype, void* recvbuf,
                       int recvcount, const MPI::Datatype& recvtype,
                       int root) const;

FORTRAN synopsis

include 'mpif.h' or use mpi
MPI_GATHER(CHOICE SENDBUF,INTEGER SENDCOUNT,INTEGER SENDTYPE,
    CHOICE RECVBUF,INTEGER RECVCOUNT,INTEGER RECVTYPE,INTEGER ROOT,
    INTEGER COMM,INTEGER IERROR)

Description

This subroutine collects individual messages from each task in comm at the
root task and stores them in rank order.

The type signature of sendcount, sendtype on task i must be equal to the
type signature of recvcount, recvtype at the root. This means the amount of
data sent must be equal to the amount of data received, pairwise between
each task and the root. Distinct type maps between sender and receiver are
allowed.

The following is information regarding MPI_GATHER arguments and tasks:

   * On the task root, all arguments to the function are significant.
   * On other tasks, only the arguments sendbuf, sendcount, sendtype, root,
     and comm are significant.
   * The argument root must be the same on all tasks.

Note that the argument revcount at the root indicates the number of items
it receives from each task. It is not the total number of items received.

A call where the specification of counts and types causes any location on
the root to be written more than once is erroneous.

The "in place" option for intracommunicators is specified by passing
MPI_IN_PLACE as the value of sendbuf at the root. In such a case, sendcount
and sendtype are ignored, and the contribution of the root to the gathered
vector is assumed to be already in the correct place in the receive buffer.

If comm is an intercommunicator, the call involves all tasks in the
intercommunicator, but with one group (group A) defining the root task. All
tasks in the other group (group B) pass the same value in root, which is
the rank of the root in group A. The root passes the value MPI_ROOT in
root. All other tasks in group A pass the value MPI_PROC_NULL in root. Data
is gathered from all tasks in group B to the root. The send buffer
arguments of the tasks in group B must be consistent with the receive
buffer argument of the root.

MPI_IN_PLACE is not supported for intercommunicators.

When you use this subroutine in a threads application, make sure all
collective operations on a particular communicator occur in the same order
at each task. See IBM Parallel Environment for AIX: MPI Programming Guide
for more information on programming with MPI in a threads environment.

Parameters

sendbuf
     is the starting address of the send buffer (choice) (IN)
sendcount
     is the number of elements in the send buffer (integer) (IN)
sendtype
     is the datatype of the send buffer elements (handle) (IN)
recvbuf
     is the address of the receive buffer (choice, significant only at
     root) (OUT)
recvcount
     is the number of elements for any single receive (integer, significant
     only at root) (IN)
recvtype
     is the datatype of the receive buffer elements (handle, significant
     only at root) (IN)
root
     is the rank of the receiving task (integer) (IN)
comm
     is the communicator (handle) (IN)
IERROR
     is the FORTRAN return code. It is always the last argument.

Notes

In the 64-bit library, this function uses a shared memory optimization
among the tasks on a node. This optimization is discussed in the chapter
"Using shared memory" of IBM Parallel Environment for AIX: MPI Programming
Guide, and is enabled by default. This optimization is not available to
32-bit programs.

Errors

Fatal errors:

Invalid communicator

Invalid count(s)
     count < 0

Invalid datatype(s)

Type not committed

Invalid root

     For an intracommunicator: root < 0 or root >= groupsize

     For an intercommunicator: root < 0 and is neither MPI_ROOT nor
     MPI_PROC_NULL, or root >= groupsize of the remote group

Unequal message lengths

Invalid use of MPI_IN_PLACE

MPI not initialized

MPI already finalized

Develop mode error if:

Inconsistent root

Inconsistent message length

Related information

     MPE_IGATHER
     MPI_ALLGATHER
     MPI_GATHER
     MPI_SCATTER