MPI_CART_CREATE(3)

📦
version 5.0.10 · 7 versions
, in ubuntu stonking

💖 MPI_Cart_create 💖

okay bestie, welcome to the MPI_Cart_create page from the Open MPI manual (that's the MPI_CART_CREATE(3) one, in case you were wondering which page you wandered onto 🫶).

so like — this function makes a brand new communicator and slaps Cartesian topology info all over it. iconic behavior honestly.

✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

int MPI_Cart_create(MPI_Comm comm_old, int ndims, const int dims[],

    const int periods[], int reorder, MPI_Comm *comm_cart)

🎀 Fortran Syntax

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

MPI_CART_CREATE(COMM_OLD, NDIMS, DIMS, PERIODS, REORDER,
        COMM_CART, IERROR)
    INTEGER COMM_OLD, NDIMS, DIMS(*), COMM_CART, IERROR
    LOGICAL PERIODS(*), REORDER

🎀 Fortran 2008 Syntax

USE mpi_f08

MPI_Cart_create(comm_old, ndims, dims, periods, reorder, comm_cart, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm_old
    INTEGER, INTENT(IN) :: ndims, dims(ndims)
    LOGICAL, INTENT(IN) :: periods(ndims), reorder
    TYPE(MPI_Comm), INTENT(OUT) :: comm_cart
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🌸 INPUT PARAMETERS 🌸

  • comm_old — the input communicator (a handle). your starting point, bestie 💅
  • ndims — how many dimensions your Cartesian grid has (integer).
  • dims — integer array of size ndims telling you the number of processes in each dimension.
  • periods — logical array of size ndims saying whether the grid is periodic (true) or not (false) in each dimension.
  • reorder — ranking may be reordered (true) or not (false) (logical).

💜 OUTPUT PARAMETERS 💜

  • comm_cart — the communicator with the new Cartesian topology (handle). she's new, she's fresh, she's got topology ✨
  • ierror — Fortran only: error status (integer).

🦋 DESCRIPTION 🦋

MPI_Cart_create hands you back a handle to a new communicator with all the Cartesian topology info attached to it. no cap.

so here's the tea:

  • if reorder = false, then the rank of each process in the new group is identical to its rank in the old group. same energy, same position, no shuffling 🙈
  • otherwise (i.e. reorder is true), the function may reorder the processes — possibly so as to choose a good embedding of the virtual topology onto the physical machine. she's optimizing, let her cook 🔥
  • if the total size of the Cartesian grid is smaller than the size of the group of comm, then some processes get returned MPI_COMM_NULL, in analogy to MPI_Comm_split. not everyone makes the cut, bestie 😭
  • and fyi — the call is erroneous if it specifies a grid that is larger than the group size. that one's a hard no 💀

⭐ ERRORS ⭐

almost all MPI routines return an error value, btw: C routines as the return result of the function, and Fortran routines in the last argument.

before that error value comes back, the current MPI error handler associated with the communication object (like a communicator, window, or file) gets called. and 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. that 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
  • an info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple.

and 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 you can use, periodt:

  • MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic, but make it fatal 💀
  • 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, it 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, it aborts only the local process.
  • MPI_ERRORS_RETURN — returns an error code to the application. she just hands it back 🫴

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, don't assume you can just keep vibing after one, bestie 😭

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 📚

2003-2026, The Open MPI Community

and that's the whole page, my love 🌈 this was the MPI_CART_CREATE(3) page from Open MPI, dated Sep 10, 2026 — go forth and build your grids, they're gonna eat 💅✨🎀

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