MPI_GRAPH_NEIGHBORS(3)				     Open MPI				    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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Graph_neighbors_count <#mpi-graph-neighbors-count>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				    MPI_GRAPH_NEIGHBORS(3)
