---
name: "MPI_Graph_neighbors"
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_Graph_neighbors"
---

# MPI\_Graph\_neighbors(3)

*MPI\_Graph\_neighbors* — Returns the neighbors of a node associated with a graph topology.

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors,
     int neighbors[])
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
     INTEGER COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Graph_neighbors(comm, rank, maxneighbors, neighbors, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(IN) :: rank, maxneighbors
     INTEGER, INTENT(OUT) :: neighbors(maxneighbors)
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **comm**: Communicator with graph topology (handle).

- •

  **rank**: Rank of process in group of comm (integer).

- •

  **maxneighbors**: Size of array neighbors (integer).

## OUTPUT PARAMETERS

- •

  **neighbors**: Ranks of processes that are neighbors to specified process (array of integers).

- •

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

## DESCRIPTION

**Example:** Suppose that comm is a communicator with a shuffle-exchange topology. The group has 2n members. Each process is labeled by a(1), …, a(n) with a(i) E{0,1}, and has three neighbors: exchange (a(1), …, a(n) = a(1), …, a(n-1), a(n) (a = 1 - a), shuffle (a(1), …, a(n)) = a(2), …, a(n), a(1), and unshuffle (a(1), …, a(n)) = a(n), a(1), …, a(n-1). The graph adjacency list is illustrated below for n=3.

```
                 exchange                shuffle         unshuffle
node             neighbors(1)    neighbors(2)    neighbors(3)
0(000)       1               0               0
1(001)       0               2               4
2(010)       3               4               1
3(011)       2               6               5
4(100)       5               1               2
5(101)       4               3               6
6(110)       7               5               3
7(111)       6               7               7
```

Suppose that the communicator comm has this topology associated with it. The following code fragment cycles through the three types of neighbors and performs an appropriate permutation for each.

```
!  assume: each process has stored a real number A.
!  extract neighborhood information
CALL MPI_COMM_RANK(comm, myrank, ierr)
CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr)
!  perform exchange permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0, &
                          neighbors(1), 0, comm, status, ierr)
!  perform shuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0, &
                          neighbors(3), 0, comm, status, ierr)
!  perform unshuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0, &
                          neighbors(2), 0, comm, status, ierr)
```

## 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\_Graph\_neighbors\_count*

## COPYRIGHT

2003-2026, The Open MPI Community