# 18.2.79. MPI_Comm_split

MPI_Comm_split — Creates new communicators based on colors and keys.

## 18.2.79.1. SYNTAX

### 18.2.79.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_split(MPI_Comm comm, int color, int key,
         MPI_Comm *newcomm)

### 18.2.79.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_COMM_SPLIT(COMM, COLOR, KEY, NEWCOMM, IERROR)
         INTEGER COMM, COLOR, KEY, NEWCOMM, IERROR

### 18.2.79.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Comm_split(comm, color, key, newcomm, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         INTEGER, INTENT(IN) :: color, key
         TYPE(MPI_Comm), INTENT(OUT) :: newcomm
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.79.2. INPUT PARAMETERS

- `comm`: Communicator (handle).

- `color`: Control of subset assignment (nonnegative integer).

- `key`: Control of rank assignment (integer).

## 18.2.79.3. OUTPUT PARAMETERS

- `newcomm`: New communicator (handle).

- `ierror`: Fortran only: Error status (integer).

## 18.2.79.4. DESCRIPTION

This function partitions the group associated with comm into disjoint subgroups, one for each value of color. Each subgroup contains all processes of the same color. Within each subgroup, the processes are ranked in the order defined by the value of the argument key, with ties broken according to their rank in the old group. A new communicator is created for each subgroup and returned in newcomm. A process may supply the color value MPI_UNDEFINED, in which case newcomm returns MPI_COMM_NULL. This is a collective call, but each process is permitted to provide different values for color and key.

When you call MPI_Comm_split on an inter-communicator, the processes on the left with the same color as those on the right combine to create a new inter-communicator. The key argument describes the relative rank of processes on each side of the inter-communicator. The function returns MPI_COMM_NULL for those colors that are specified on only one side of the inter-communicator, or for those that specify MPI_UNDEFINED as the color.

A call to MPI_Comm_create(comm, group, newcomm) is equivalent to a call to MPI_Comm_split(comm, color,key, newcomm), where all members of group provide color = 0 and key = rank in group, and all processes that are not members of group provide color = MPI_UNDEFINED. The function MPI_Comm_split allows more general partitioning of a group into one or more subgroups with optional reordering.

The value of color must be nonnegative or MPI_UNDEFINED.

## 18.2.79.5. NOTES

This is an extremely powerful mechanism for dividing a single communicating group of processes into k subgroups, with k chosen implicitly by the user (by the number of colors asserted over all the processes). Each resulting communicator will be nonoverlapping. Such a division could be useful for defining a hierarchy of computations, such as for multigrid or linear algebra.

Multiple calls to MPI_Comm_split can be used to overcome the requirement that any call have no overlap of the resulting communicators (each process is of only one color per call). In this way, multiple overlapping communication structures can be created. Creative use of the color and key in such splitting operations is encouraged.

Note that, for a fixed color, the keys need not be unique. It is MPI_Comm_split’s responsibility to sort processes in ascending order according to this key, and to break ties in a consistent way. If all the keys are specified in the same way, then all the processes in a given color will have the relative rank order as they did in their parent group. (In general, they will have different ranks.)

Essentially, making the key value zero for all processes of a given color means that one needn’t really pay attention to the rank-order of the processes in the new communicator.

## 18.2.79.6. 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_Comm_create

- MPI_Intercomm_create

- MPI_Comm_dup

- MPI_Comm_free