---
name: "MPI_Neighbor_alltoall_init.openmpi"
section: "3"
platform: "linux"
package: "openmpi-doc"
version: "5.0.7-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Neighbor_alltoall_init.openmpi"
---

# MPI\_Neighbor\_alltoall\_init.openmpi(3)

*MPI\_Neighbor\_alltoall*, *MPI\_Ineighbor\_alltoall*, *MPI\_Neighbor\_alltoall* - All processes send data to neighboring processes in a virtual topology communicator

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Neighbor_alltoall(const void *sendbuf, int sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm)
int MPI_Ineighbor_alltoall(const void *sendbuf, int sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)
int MPI_Neighbor_alltoall_init(const void *sendbuf, int sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)
```

### Fortran Syntax

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

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Neighbor_alltoall(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_Ineighbor_alltoall(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
MPI_Neighbor_alltoall_init(sendbuf, sendcount, sendtype, recvbuf, recvcount,
             recvtype, comm, info, 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_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **sendbuf**: Starting address of send buffer (choice).

- •

  **sendcount**: Number of elements to send to each process (integer).

- •

  **sendtype**: Datatype of send buffer elements (handle).

- •

  **recvcount**: Number of elements to receive from each process (integer).

- •

  **recvtype**: Datatype of receive buffer elements (handle).

- •

  **comm**: Communicator over which data is to be exchanged (handle).

- •

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

## OUTPUT PARAMETERS

- •

  **recvbuf**: Starting address of receive buffer (choice).

- •

  **request**: Request (handle, non-blocking only).

- •

  **ierror**: Fortran only: Error status (integer).

## DESCRIPTION

*MPI\_Neighbor\_alltoall* is a collective operation in which all processes send and receive the same amount of data to each neighbor. The operation of this routine can be represented as follows, where each process performs 2n (n being the number of neighbors in communicator *comm*) independent point-to-point communications. The neighbors and buffer layout are determined by the topology of *comm*.

Example of *MPI\_Neighbor\_alltoall* semantics for cartesian topologies:

```
MPI_Cart_get(comm, maxdims, dims, periods, coords);
for (dim = 0, i = 0 ; dim < dims ; ++dim) {
    MPI_Cart_shift(comm, dim, 1, &r0, &r1);
    MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
              sendcount, sendtype, r0, ..., comm, ...);
    MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
              recvcount, recvtype, r0, ..., comm, ...);
    ++i;
    MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
              sendcount, sendtype, r1, ..., comm, &req[i]);
    MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
              recvcount, recvtype, r1, ..., comm, ...);
    ++i;
}
MPI_Waitall (...);
```

Each process breaks up its local *sendbuf* into n blocks - each containing *sendcount* elements of type *sendtype* - and divides its *recvbuf* similarly according to *recvcount* and *recvtype*. Process j sends the k-th block of its local *sendbuf* to neighbor k, which places the data in the j-th block of its local *recvbuf*. The amount of data sent must be equal to the amount of data received, pairwise, between every pair of processes.

## NEIGHBOR ORDERING

For a distributed graph topology, created with *MPI\_Dist\_graph\_create*, the sequence of neighbors in the send and receive buffers at each process is defined as the sequence returned by *MPI\_Dist\_graph\_neighbors* for destinations and sources, respectively. For a general graph topology, created with *MPI\_Graph\_create*, the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by *MPI\_Graph\_neighbors*. Note that general graph topologies should generally be replaced by the distributed graph topologies.

For a Cartesian topology, created with *MPI\_Cart\_create*, the sequence of neighbors in the send and receive buffers at each process is defined by order of the dimensions, first the neighbor in the negative direction and then in the positive direction with displacement 1. The numbers of sources and destinations in the communication routines are 2\*ndims with ndims defined in *MPI\_Cart\_create*. If a neighbor does not exist, i.e., at the border of a Cartesian topology in the case of a non-periodic virtual grid dimension (i.e., periods\[…\]==false), then this neighbor is defined to be **MPI\_PROC\_NULL**.

If a neighbor in any of the functions is **MPI\_PROC\_NULL**, then the neighborhood collective communication behaves like a point-to-point communication with **MPI\_PROC\_NULL** in this direction. That is, the buffer is still part of the sequence of neighbors but it is neither communicated nor updated.

## NOTES

The MPI\_IN\_PLACE option for *sendbuf* is not meaningful for this function.

All arguments on all processes are significant. The *comm* argument, in particular, must describe the same communicator on all processes. *comm* must be either a cartesian, graph, or dist graph communicator.

There are two MPI library functions that are more general than *MPI\_Neighbor\_alltoall*. *MPI\_Neighbor\_alltoallv* allows all-to-all communication to and from buffers that need not be contiguous; different processes may send and receive different amounts of data. *MPI\_Neighbor\_alltoallw* expands *MPI\_Neighbor\_alltoallv*’s functionality to allow the exchange of data with different datatypes.

## ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI\_COMM\_SELF and will call the associated MPI error handler. When MPI\_COMM\_SELF is not initialized (i.e., before *MPI\_Init*/*MPI\_Init\_thread*, after *MPI\_Finalize*, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling *MPI\_Comm\_set\_errhandler* on MPI\_COMM\_SELF when using the World model, or the mpi\_initial\_errhandler CLI argument to mpiexec or info key to *MPI\_Comm\_spawn*/*MPI\_Comm\_spawn\_multiple*. If no other appropriate error handler has been set, then the MPI\_ERRORS\_RETURN error handler is called for MPI I/O functions and the MPI\_ERRORS\_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- •

  **MPI\_ERRORS\_ARE\_FATAL** Causes the program to abort all connected MPI processes.

- •

  **MPI\_ERRORS\_ABORT** An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if *MPI\_Abort* was called on that communicator. If called on a window or file, acts as if *MPI\_Abort* was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- •

  **MPI\_ERRORS\_RETURN** Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- •

  *MPI\_Comm\_create\_errhandler* then *MPI\_Comm\_set\_errhandler*

- •

  *MPI\_File\_create\_errhandler* then *MPI\_File\_set\_errhandler*

- •

  *MPI\_Session\_create\_errhandler* then *MPI\_Session\_set\_errhandler* or at *MPI\_Session\_init*

- •

  *MPI\_Win\_create\_errhandler* then *MPI\_Win\_set\_errhandler*

Note that MPI does not guarantee that an MPI program can continue past an error.

See the *MPI man page* for a full list of *MPI error codes*.

See the Error Handling section of the MPI-3.1 standard for more information.

**SEE ALSO:**

- •

  *MPI\_Neighbor\_alltoallv*

- •

  *MPI\_Neighbor\_alltoallw*

- •

  *MPI\_Cart\_create*

- •

  *MPI\_Graph\_create*

- •

  *MPI\_Dist\_graph\_create*

- •

  *MPI\_Dist\_graph\_create\_adjacent*

## COPYRIGHT

2003-2025, The Open MPI Community