MPI_GRAPH_NEIGHBORS(3)

💅 MPI_GRAPH_NEIGHBORS(3) — bestie edition 🦋

okay so pack it up, we're reading the Open MPI manual together and honestly? this page is a bop. It's MPI_GRAPH_NEIGHBORS(3), and it's about asking a graph-topology communicator who are your neighbors. yes really, that's the whole gig. let's get into it 💖

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✨ 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

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🌸 INPUT PARAMETERS 🌸

• comm — the communicator that's got that graph topology on it, i.e. a handle. it's the group chat you're asking about 🫶
• rank — the rank of the process inside the group of comm (integer). literally just who you're asking about.
• maxneighbors — the size of the neighbors array (integer). how much room you're giving her to spill the tea.

🎀 OUTPUT PARAMETERS 🎀

• neighbors — the ranks of the processes that are neighbors to the process you specified (array of integers). your little list of besties, delivered fresh ✨
• ierror — Fortran only: error status (integer). classic.

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👑 DESCRIPTION 👑

Example time, bestie. 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. no cap, this is the actual move 💅

!  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)

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💀 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. yes, even this one. even your fave. 😭

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. like, everyone. gone. 💀
• 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. (so dramatic and for what)
• MPI_ERRORS_RETURN — Returns an error code to the application. the chill one. the reasonable one. 🫶

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. fr fr. don't yolo it.

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 🌈

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📜 Copyright 📜

2003-2026, The Open MPI Community

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and that's the whole page, bestie!! you ate that. go query your graph topology and find your neighbors, they're waiting for you 🧚‍♀️✨ this has been the MPI_GRAPH_NEIGHBORS(3) page from the Open MPI manual, dated Sep 10, 2026 — mwah, slay, periodt 💋🍓