.\" Man page generated from reStructuredText.
.
.TH "MPI_COMM_SPLIT" "3" "May 30, 2025" "" "Open MPI"
.
.nr rst2man-indent-level 0
.
.de1 rstReportMargin
\\$1 \\n[an-margin]
level \\n[rst2man-indent-level]
level margin: \\n[rst2man-indent\\n[rst2man-indent-level]]
-
\\n[rst2man-indent0]
\\n[rst2man-indent1]
\\n[rst2man-indent2]
..
.de1 INDENT
.\" .rstReportMargin pre:
. RS \\$1
. nr rst2man-indent\\n[rst2man-indent-level] \\n[an-margin]
. nr rst2man-indent-level +1
.\" .rstReportMargin post:
..
.de UNINDENT
. RE
.\" indent \\n[an-margin]
.\" old: \\n[rst2man-indent\\n[rst2man-indent-level]]
.nr rst2man-indent-level -1
.\" new: \\n[rst2man-indent\\n[rst2man-indent-level]]
.in \\n[rst2man-indent\\n[rst2man-indent-level]]u
..
.sp
\fI\%MPI_Comm_split\fP — Creates new communicators based on colors and keys.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Comm_split(MPI_Comm comm, int color, int key,
     MPI_Comm *newcomm)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_COMM_SPLIT(COMM, COLOR, KEY, NEWCOMM, IERROR)
     INTEGER COMM, COLOR, KEY, NEWCOMM, IERROR
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBcomm\fP: Communicator (handle).
.IP \(bu 2
\fBcolor\fP: Control of subset assignment (nonnegative integer).
.IP \(bu 2
\fBkey\fP: Control of rank assignment (integer).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBnewcomm\fP: New communicator (handle).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
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.
.sp
When you call \fI\%MPI_Comm_split\fP 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.
.sp
A call to MPI_Comm_create(comm, \fIgroup\fP, \fInewcomm\fP) is equivalent to a
call to MPI_Comm_split(comm, \fIcolor\fP,\fIkey\fP, \fInewcomm\fP), where all
members of \fIgroup\fP provide \fIcolor\fP = 0 and \fIkey\fP = rank in group, and
all processes that are not members of \fIgroup\fP provide \fIcolor\fP =
MPI_UNDEFINED. The function \fI\%MPI_Comm_split\fP allows more general
partitioning of a group into one or more subgroups with optional
reordering.
.sp
The value of \fIcolor\fP must be nonnegative or MPI_UNDEFINED.
.SH NOTES
.sp
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.
.sp
Multiple calls to \fI\%MPI_Comm_split\fP 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.
.sp
Note that, for a fixed color, the keys need not be unique. It is
\fI\%MPI_Comm_split\fP’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.)
.sp
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.
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
.INDENT 0.0
.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
.UNINDENT
.sp
MPI applications can also implement their own error handlers by calling:
.INDENT 0.0
.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
.UNINDENT
.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
\fBSEE ALSO:\fP
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.IP \(bu 2
\fI\%MPI_Comm_create\fP
.IP \(bu 2
\fI\%MPI_Intercomm_create\fP
.IP \(bu 2
\fI\%MPI_Comm_dup\fP
.IP \(bu 2
\fI\%MPI_Comm_free\fP
.UNINDENT
.UNINDENT
.UNINDENT
.SH COPYRIGHT
2003-2025, The Open MPI Community
.\" Generated by docutils manpage writer.
.