MPI_COMM_SPLIT(3)				     Open MPI					 MPI_COMM_SPLIT(3)

MPI_Comm_split — Creates new communicators based on colors and keys.

SYNTAX
   C Syntax
	  #include <mpi.h>

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

   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

   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

INPUT PARAMETERS
       • comm: Communicator (handle).

       • color: Control of subset assignment (nonnegative integer).

       • key: Control of rank assignment (integer).

OUTPUT PARAMETERS
       • newcomm: New communicator (handle).

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

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.

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.

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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Comm_create <#mpi-comm-create>

	  • MPI_Intercomm_create <#mpi-intercomm-create>

	  • MPI_Comm_dup <#mpi-comm-dup>

	  • MPI_Comm_free <#mpi-comm-free>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					 MPI_COMM_SPLIT(3)
