# 18.2.86. MPI_Dist_graph_neighbors

MPI_Dist_graph_neighbors — Returns the neighbors of the calling process in a distributed graph topology.

## 18.2.86.1. SYNTAX

### 18.2.86.1.1. C Syntax

    #include <mpi.h>

    int MPI_Dist_graph_neighbors(MPI_Comm comm, int maxindegree, int sources[], int sourceweights[],
                                 int maxoutdegree, int destinations[], int destweights[])

### 18.2.86.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_DIST_GRAPH_NEIGHBORS(COMM, MAXINDEGREE, SOURCES, SOURCEWEIGHTS,
            MAXOUTDEGREE, DESTINATIONS, DESTWEIGHTS, IERROR)
            INTEGER COMM, MAXINDEGREE, SOURCES(*), SOURCEWEIGHTS(*), MAXOUTDEGREE,
                    DESTINATIONS(*), DESTWEIGHTS(*), IERROR

### 18.2.86.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Dist_Graph_neighbors(comm, maxindegree, sources, sourceweights,
                 maxoutdegree, destinations, destweights, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         INTEGER, INTENT(IN) :: maxindegree, maxoutdegree
         INTEGER, INTENT(OUT) :: sources(maxindegree), destinations(maxoutdegree)
         INTEGER :: sourceweights(*), destweights(*)
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.86.2. INPUT PARAMETERS

- `comm`: Communicator with distributed graph topology (handle).

- `maxindegree`: Size of sources and sourceweights arrays (non-negative integer).

- `maxoutdegree`: Size of destinations and destweights arrays (non-negative integer).

## 18.2.86.3. OUTPUT PARAMETERS

- `sources`: Processes for which the calling process is a destination (array of non-negative integers).

- `sourceweights`: Weights of the edges into the calling process (array of non-negative integers).

- `destinations`: Processes for which the calling process is a source (array of non-negative integers).

- `destweights`: Weights of the edges out of the calling process (array of non-negative integers).

- `ierror`: Fortran only: Error status (integer).

## 18.2.86.4. DESCRIPTION

MPI_Dist_graph_neighbors returns the source and destination ranks in a distributed graph topology for the calling process. This call will return up to maxindegree source ranks in the sources array and up to maxoutdegree destination ranks in the destinations array. If weights were specified at the time of the communicator’s creation then the associated weights are returned in the sourceweights and destweights arrays. If the communicator was created with MPI_Dist_graph_create_adjacent then the order of the values in sources and destinations is identical to the input that was used by the process with the same rank in comm_old in the creation call.

## 18.2.86.5. 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_Dist_graph_neighbors_count