MPI_CART_MAP(3)

💖 MPI_CART_MAP 💖

okay bestie, welcome to the Open MPI manual page for MPI_Cart_map and honestly? it's giving "topology girlie who does the behind-the-scenes work." let's get into it 🎀

✨ NAME ✨

MPI_Cart_map — Maps process to Cartesian topology information. no cap, that's literally the whole job 💅

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🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_Cart_map(MPI_Comm comm, int ndims, const int dims[],
    const int periods[], int *newrank)

Fortran Syntax

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

MPI_CART_MAP(COMM, NDIMS, DIMS, PERIODS, NEWRANK, IERROR)
    INTEGER COMM, NDIMS, DIMS(*), NEWRANK, IERROR
    LOGICAL PERIODS(*)

Fortran 2008 Syntax

USE mpi_f08

MPI_Cart_map(comm, ndims, dims, periods, newrank, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, INTENT(IN) :: ndims, dims(ndims)
    LOGICAL, INTENT(IN) :: periods(ndims)
    INTEGER, INTENT(OUT) :: newrank
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

three whole syntaxes, we love a multilingual queen 🌈

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🫶 INPUT PARAMETERS 🫶

• comm — Input communicator (handle). the group chat you're working with, basically.
• ndims — Number of dimensions of Cartesian structure (integer). how many directions this grid vibe has.
• dims — Integer array of size ndims specifying the number of processes in each coordinate direction. so like, how many besties per axis 🦋
• periods — Logical array of size ndims specifying the periodicity specification in each coordinate direction. does it wrap around? yes or no, per axis.

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👑 OUTPUT PARAMETERS 👑

• newrank — Reordered rank of the calling process; MPI_UNDEFINED if calling process does not belong to grid (integer). so if you're not in the group chat, you get MPI_UNDEFINED, and honestly? felt 💀
• 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. they're the blueprint, the blueprint-iest of blueprints. periodt.

ngl, in general they will not be called by the user directly, unless you're out here creating additional virtual topology capability other than that provided by MPI. that's the "iykyk" tier of MPI, bestie — main character energy for library authors only.

so what does it actually do? MPI_Cart_map computes an “optimal” placement for the calling process on the physical machine. ✨ optimal ✨ in quotes, because a possible implementation of this function is to always return the rank of the calling process — that is, not to perform any reordering. iconic behaviour honestly, sometimes the best map is just... yourself. 🎀

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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. noted. 📝

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. she does NOT play around 💀
• 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. the polite 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

bestie, important: note that MPI does not guarantee that an MPI program can continue past an error. that's a real one, no cap. 🚨

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_map

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🎀 Copyright 🎀

2003-2026, The Open MPI Community

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and that's the whole page, bestie!! this was MPI_CART_MAP(3) from the Open MPI crew — page date Sep 10, 2026 — brought to you by the Open MPI community, 2003-2026, all rights reserved energy. go forth and map those processes optimally (or don't, apparently, that's a valid implementation too 💅✨🔮).