---
name: "MPI_Cart_sub"
section: "3"
platform: "linux"
package: "openmpi"
version: "5.0.11-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Cart_sub"
---

# MPI\_Cart\_sub(3)

*MPI\_Cart\_sub* — Partitions a communicator into subgroups, which form lower-dimensional Cartesian subgrids.

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Cart_sub(MPI_Comm comm, const int remain_dims[], MPI_Comm *comm_new)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_CART_SUB(COMM, REMAIN_DIMS, COMM_NEW, IERROR)
    INTEGER COMM, COMM_NEW, IERROR
    LOGICAL REMAIN_DIMS(*)
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Cart_sub(comm, remain_dims, newcomm, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    LOGICAL, INTENT(IN) :: remain_dims(*)
    TYPE(MPI_Comm), INTENT(OUT) :: newcomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **comm** : Communicator with Cartesian structure (handle).

- •

  - **remain\_dims**

    The ith entry of remain\_dims specifies whether the ith dimension is kept in the subgrid (true) or is dropped (false) (logical vector).

## OUTPUT PARAMETERS

- •

  - **comm\_new**

    Communicator containing the subgrid that includes the calling process (handle).

- •

  **ierror** : Fortran only: Error status (integer).

## DESCRIPTION

If a Cartesian topology has been created with *MPI\_Cart\_create*, the function *MPI\_Cart\_sub* can be used to partition the communicator group into subgroups that form lower-dimensional Cartesian subgrids, and to build for each subgroup a communicator with the associated subgrid Cartesian topology. (This function is closely related to *MPI\_Comm\_split*.)

Example: Assume that **MPI\_Cart\_create( ..., comm)** has defined a (2 x 3 x 4) grid. Let remain\_dims = (true, false, true). Then a call to

```
MPI_Cart_sub(comm, remain_dims, comm_new);
```

will create three communicators, each with eight processes in a 2 x 4 Cartesian topology. If remain\_dims = (false, false, true) then the call to MPI\_Cart\_sub(comm, remain\_dims, comm\_new) will create six nonoverlapping communicators, each with four processes, in a one-dimensional Cartesian topology.

## 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.

- •

  **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.

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.

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\_create*

## COPYRIGHT

2003-2026, The Open MPI Community