MPI_ALLGATHER, MPI_Allgather

Purpose

Gathers individual messages from each task in comm and distributes
the resulting message to each task.

C synopsis

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

C++ synopsis

#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;

FORTRAN synopsis

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

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) (OUT)

recvcount
     is the number of elements received from any task (integer) (IN)

recvtype
     is the datatype of the receive buffer elements (handle) (IN)

comm
     is the communicator (handle) (IN)

IERROR
     is the FORTRAN return code. It is always the last argument.

Description

MPI_ALLGATHER is similar to MPI_GATHER except that all tasks receive the
result instead of just the root.

The block of data sent from task j is received by every task and placed in
the jth block of the buffer recvbuf.

The type signature associated with sendcount, sendtype at a task must be
equal to the type signature associated with recvcount, recvtype at any
other task.

The "in place" option for intracommunicators is specified by passing the
value MPI_IN_PLACE to sendbuf at all tasks. The sendcount and sendtype
arguments are ignored. The input data of each task is assumed to be in the
area where that task would receive its own contribution to the receive
buffer. Specifically, the outcome of a call to MPI_ALLGATHER in the "in
place" case is as if all tasks executed n calls to:

MPI_GATHER(MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf, recvcount, recvtype,
           root, comm)

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

If comm is an intercommunicator, each task in group A contributes a data
item. These items are concatenated and the result is stored at each task in
group B. Conversely, the concatenation of the contributions of the tasks in
group B is stored at each task in group A. The send buffer arguments in
group A must be consistent with the receive buffer arguments in group B,
and vice versa.

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.

Errors

Fatal errors:

Invalid communicator

Invalid count(s)
     count < 0

Invalid datatype(s)

Type not committed

Unequal message lengths

Invalid use of MPI_IN_PLACE

MPI not initialized

MPI already finalized

Develop mode error if:

Inconsistent message length

Related information

     MPE_IALLGATHER
     MPI_ALLGATHER
     MPI_GATHER