MPI_CART_RANK(3)

💅 MPI_CART_RANK(3) — like, the Open MPI manual page, bestie 🎀

okay so real talk: this is MPI_Cart_rank from Open MPI, and it's literally the glow-up translator between "cartesian coordinates" brain and "process rank" brain. no cap, it's iconic. let's get into it ✨

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🌸 SYNTAX 🌸

✨ C Syntax

#include <mpi.h>

int MPI_Cart_rank(MPI_Comm comm, int coords[], int *rank)

💜 Fortran Syntax

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

MPI_CART_RANK(COMM, COORDS, RANK, IERROR)
    INTEGER COMM, COORDS(*), RANK, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08

MPI_Cart_rank(comm, coords, rank, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, INTENT(IN) :: coords(*)
    INTEGER, INTENT(OUT) :: rank
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🫶 INPUT PARAMETERS 🫶

• comm — the communicator that's got that Cartesian structure going on (handle). it's giving grid energy 👑
• coords — an integer array (size ndims, which was defined by the MPI_Cart_create call, iykyk) that says which Cartesian coordinates a process is sitting at. like a little address for your process 🏠

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⭐ OUTPUT PARAMETER ⭐

• rank — the rank of the specified process (integer). that's the main character right there 💅
• ierror — Fortran only: error status (integer). classic Fortran behavior, we love her 🙈

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💖 DESCRIPTION 💖

so for a process group with Cartesian structure, MPI_Cart_rank translates the logical process coordinates into process ranks — like, the exact way the point-to-point routines use them. it's a whole translator arc, lowkey ✨

and here's the tea for dimension i where periods(i) = true: if the coordinate coords(i) is out of range — meaning coords(i) < 0 or coords(i) >= dims(i) — it gets shifted back into the interval 0 =< coords(i) < dims(i) automatically. wrapping around like it's nothing, periodt 🌀

but bestie, listen: out-of-range coordinates are erroneous for nonperiodic dimensions. no wrap-around for those, it's just wrong. don't do it 💀

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😭 ERRORS 😭

almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines stick it in the last argument. consistency queen 🎀

before that error value comes back to you, the current MPI error handler associated with the communication object (e.g., communicator, window, file) gets called. and if there's no communication object associated with the MPI call? then the call is considered attached to MPI_COMM_SELF, and it'll call the associated MPI error handler. when MPI_COMM_SELF isn't 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. and that initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or by the mpi_initial_errhandler CLI argument to mpiexec, or the 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. so many layers, honestly 🧚

Open MPI ships three predefined error handlers you can use:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic? yes. effective? also yes 💀
• 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. versatile icon 🌈
• MPI_ERRORS_RETURN — returns an error code to the application. simple, clean, hands you the receipts 🧾

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

heads up: note that MPI does not guarantee that an MPI program can continue past an error. like, don't count on it surviving, fr fr 😭

see the MPI man page for a full list of MPI error codes. and see the Error Handling section of the MPI-3.1 standard for more information 📚

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👀 SEE ALSO 👀

• MPI_Cart_create — the one that made your grid in the first place, the origin story 💫

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🍓 COPYRIGHT 🍓

2003-2026, The Open MPI Community — legends, every single one 🌟

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okay that's the whole page bestie, go forth and map those coordinates 🦋 and yeah — this was Open MPI, page MPI_CART_RANK(3), dated Sep 11, 2026. mwah 💋 slay responsibly ✨