MPI_GRAPH_MAP(3)

MPI_Graph_map 💅

okay bestie, welcome to the Open MPI manual, section 3, and today's main character is MPI_Graph_map — she maps processes to graph topology info. yes ma'am. let's get into it 🫶

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✨ NAME ✨

MPI_Graph_map — maps process to graph topology information. that's her whole job. iconic.

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🦋 SYNTAX 🦋

🌸 C Syntax

#include <mpi.h>

int MPI_Graph_map(MPI_Comm comm, int nnodes, const int index[],
    const int edges[], int *newrank)

🌸 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_GRAPH_MAP(COMM, NNODES, INDEX, EDGES, NEWRANK, IERROR)
    INTEGER COMM, NNODES, INDEX(*), EDGES(*), NEWRANK, IERROR

🌸 Fortran 2008 Syntax

USE mpi_f08

MPI_Graph_map(comm, nnodes, index, edges, newrank, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, INTENT(IN) :: nnodes, index(nnodes), edges(*)
    INTEGER, INTENT(OUT) :: newrank
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

three whole flavors, she's multilingual, we love that for her 💜

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🍓 INPUT PARAMETERS 🍓

• comm — input communicator (handle). the group chat you're operating in.
• nnodes — number of graph nodes (integer). like, how many girlies are in the graph.
• index — integer array specifying the graph structure, see MPI_Graph_create. (this is the one that tells everyone who's connected to who, iykyk)
• edges — integer array specifying the graph structure.

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💖 OUTPUT PARAMETERS 💖

• newrank — reordered rank of the calling process; MPI_UNDEFINED if the calling process does not belong to graph (integer). so if you're not invited to the graph, you get MPI_UNDEFINED, which is literally the group-chat equivalent of being left on read 😭
• ierror — Fortran only: error status (integer).

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🌈 DESCRIPTION 🌈

MPI_Cart_map and MPI_Graph_map can be used to implement all other topology functions. in general they will not be called by the user directly, unless he or she is creating additional virtual topology capability other than that provided by MPI.

translation: these two are the behind-the-scenes besties making the whole topology machine run, and normally you don't text them directly unless you're out here building your own custom topology capabilities beyond what MPI already gives you. periodt.

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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.

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. whole squad, 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. (selective chaos, we respect it)
• 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. so like... once it's over, it's over. no comeback tour. 🙈

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_Cart_map

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👑 Copyright 👑

2003-2026, The Open MPI Community

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and that's the whole page bestie, no cap, every last flag and caveat preserved 💅✨

sign-off: this absolute banger of a manpage comes from Open MPI, and the version stamp on the bottom is dated Sep 10, 2026. go forth and map those graphs, you're literally the main character 🦋🌸💖