MPI_CART_CREATE(3)

💅 MPI_Cart_create 💅

okay so bestie, you've opened the Open MPI manual page for MPI_Cart_create, and it's giving "let's make a grid communicator" main character energy. 🫶 No cap, every parameter and every caveat is below.

🌸 NAME 🌸

MPI_Cart_create — makes a new communicator to which Cartesian topology information has been attached. Literally a brand-new communicator, grid info included. ✨

✨ 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 — input communicator (handle). This is the group you're starting from, bestie. 💖
• ndims — number of dimensions of Cartesian grid (integer). How many directions are we gridding in? 🎀
• dims — integer array of size ndims specifying the number of processes in each dimension. It's the shape of the grid, periodt.
• periods — logical array of size ndims specifying whether the grid is periodic (true) or not (false) in each dimension. Wrap-around vibes per axis. 🌀
• reorder — ranking may be reordered (true) or not (false) (logical). Yes, that's literally what it says. 👑

🎀 OUTPUT PARAMETERS 🎀

• comm_cart — communicator with new Cartesian topology (handle). The shiny new grid communicator. ✨
• ierror — Fortran only: error status (integer). For the Fortran girlies, the error lives here. 🫶

👑 DESCRIPTION 👑

MPI_Cart_create returns a handle to a new communicator to which the Cartesian topology information is attached. Okay so if reorder = false, then the rank of each process in the new group is identical to its rank in the old group. Literally no shuffling. Otherwise, the function may reorder the processes — possibly so as to choose a good embedding of the virtual topology onto the physical machine. It's giving "let me place y'all nicely." 💅

If the total size of the Cartesian grid is smaller than the size of the group of comm, then some processes are returned MPI_COMM_NULL, in analogy to MPI_Comm_split. And heads up: the call is erroneous if it specifies a grid that is larger than the group size. 🚨 Like, don't do that.

🚨 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. Dramatic? Yes. But that's the assignment. 💀
• 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. Very "here's your receipt" of it all. 🧾

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. That's not a vibe, that's a warning.

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.

📜 COPYRIGHT 📜

2003-2026, The Open MPI Community

xoxo bestie 💋 — source/version: Open MPI, date: Sep 11, 2026, manual tag: MPI_CART_CREATE(3).