---
name: "MPI_Neighbor_alltoallw"
section: "3"
platform: "linux"
package: "openmpi"
version: "5.0.11-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Neighbor_alltoallw"
---

# MPI\_Neighbor\_alltoallw(3)

*MPI\_Neighbor\_alltoallw*, *MPI\_Ineighbor\_alltoallw*, *MPI\_Neighbor\_alltoallw\_init* — All processes send data of different types to, and receive data of different types from, all processes

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Neighbor_alltoallw(const void *sendbuf, const int sendcounts[],
     const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
     void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
     const MPI_Datatype recvtypes[], MPI_Comm comm)
int MPI_Ineighbor_alltoallw(const void *sendbuf, const int sendcounts[],
     const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
     void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
     const MPI_Datatype recvtypes[], MPI_Comm comm, MPI_Request *request)
int MPI_Neighbor_alltoallw_init(const void *sendbuf, const int sendcounts[],
     const MPI_Aint sdispls[], const MPI_Datatype sendtypes[],
     void *recvbuf, const int recvcounts[], const MPI_Aint rdispls[],
     const MPI_Datatype recvtypes[], MPI_Comm comm, MPI_Info info, MPI_Request *request)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_NEIGHBOR_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SENDTYPES(*)
     INTEGER RECVCOUNTS(*), RECVTYPES(*)
     INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
     INTEGER COMM, IERROR
MPI_INEIGHBOR_ALLTOALLW(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SENDTYPES(*)
     INTEGER RECVCOUNTS(*), RECVTYPES(*)
     INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
     INTEGER COMM, REQUEST, IERROR
MPI_NEIGHBOR_ALLTOALLW_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SENDTYPES(*)
     INTEGER RECVCOUNTS(*), RECVTYPES(*)
     INTEGER(KIND=MPI_ADDRESS_KIND) SDISPLS(*), RDISPLS(*)
     INTEGER COMM, INFO, REQUEST, IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Neighbor_alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
             recvcounts, rdispls, recvtypes, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcounts(*), recvcounts(*)
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: sdispls(*), rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtypes(*), recvtypes(*)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ineighbor_alltoallw(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
             recvcounts, rdispls, recvtypes, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), recvcounts(*)
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN), ASYNCHRONOUS ::
     sdispls(*), rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
     recvtypes(*)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Neighbor_alltoallw_init(sendbuf, sendcounts, sdispls, sendtypes, recvbuf,
             recvcounts, rdispls, recvtypes, comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), recvcounts(*)
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN), ASYNCHRONOUS ::
     sdispls(*), rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN), ASYNCHRONOUS :: sendtypes(*),
     recvtypes(*)
     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.

- •

  **sendcounts**: Integer array, where entry i specifies the number of elements to send to neighbor i.

- •

  **sdispls**: Integer array, where entry i specifies the displacement (in bytes, offset from *sendbuf*) from which to send data to neighbor i.

- •

  **sendtypes**: Datatype array, where entry i specifies the datatype to use when sending data to neighbor i.

- •

  **recvcounts**: Integer array, where entry j specifies the number of elements to receive from neighbor j.

- •

  **rdispls**: Integer array, where entry j specifies the displacement (in bytes, offset from *recvbuf*) to which data from neighbor j should be written.

- •

  **recvtypes**: Datatype array, where entry j specifies the datatype to use when receiving data from neighbor j.

- •

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

- •

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

## OUTPUT PARAMETERS

- •

  **recvbuf**: Address of receive buffer.

- •

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

- •

  **ierror**: Fortran only: Error status.

## DESCRIPTION

*MPI\_Neighbor\_alltoallw* is a generalized collective operation in which all processes send data to and receive data from all neighbors. It adds flexibility to *MPI\_Neighbor\_alltoallv* by allowing the user to specify the datatype of individual data blocks (in addition to displacement and element count). Its operation can be thought of in the following way, where each process performs 2n (n being the number of neighbors in the topology of communicator *comm*) independent point-to-point communications. The neighbors and buffer layout are determined by the topology of *comm*.

```
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 + sdispls[i]  * extent(sendtype),
              sendcount, sendtypes[i], r0, ..., comm, ...);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtypes[i], r0, ..., comm, ...);
    ++i;
    MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
              sendcount, sendtypes[i], r1, ..., comm, &req[i]);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtypes[i], r1, ..., comm, ...);
    ++i;
}
MPI_Wait_all (...);
MPI_Comm_size(comm, &n);
for (i = 0, i < n; i++)
  MPI_Send(sendbuf + sdispls[i], sendcounts[i],
       sendtypes[i], i, ..., comm);
for (i = 0, i < n; i++)
  MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
       recvtypes[i], i, ..., comm);
```

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*.

When a pair of processes exchanges data, each may pass different element count and datatype arguments so long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive.

Note that process i may send a different amount of data to process j than it receives from process j. Also, a process may send entirely different amounts and types of data to different processes in the communicator.

## NOTES

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

The specification of counts, types, and displacements should not cause any location to be written more than once.

All arguments on all processes are significant. The *comm* argument, in particular, must describe the same communicator on all processes.

The offsets of *sdispls* and *rdispls* are measured in bytes. Compare this to *MPI\_Neighbor\_alltoallv*, where these offsets are measured in units of *sendtype* and *recvtype*, respectively.

## 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\_alltoall*

- •

  *MPI\_Neighbor\_alltoallv*

- •

  *MPI\_Cart\_create*

- •

  *MPI\_Graph\_create*

- •

  *MPI\_Dist\_graph\_create*

## COPYRIGHT

2003-2026, The Open MPI Community