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.TH "MPI_CART_SUB" "3" "Jul 22, 2024" "" "Open MPI"
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\fI\%MPI_Cart_sub\fP — Partitions a communicator into subgroups, which form
lower\-dimensional Cartesian subgrids.
.SH SYNTAX
.SS C Syntax
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.ft C
#include <mpi.h>

int MPI_Cart_sub(MPI_Comm comm, const int remain_dims[], MPI_Comm *comm_new)
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.SS Fortran Syntax
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.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq

MPI_CART_SUB(COMM, REMAIN_DIMS, COMM_NEW, IERROR)
    INTEGER COMM, COMM_NEW, IERROR
    LOGICAL REMAIN_DIMS(*)
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.SS Fortran 2008 Syntax
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.ft C
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
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBcomm\fP : Communicator with Cartesian structure (handle).
.IP \(bu 2
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.B \fBremain_dims\fP
The ith entry of remain_dims specifies whether the ith
dimension is kept in the subgrid (true) or is dropped (false)
(logical vector).
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.SH OUTPUT PARAMETERS
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.IP \(bu 2
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.B \fBcomm_new\fP
Communicator containing the subgrid that includes the
calling process (handle).
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.IP \(bu 2
\fBierror\fP : Fortran only: Error status (integer).
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.SH DESCRIPTION
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If a Cartesian topology has been created with \fI\%MPI_Cart_create\fP, the
function \fI\%MPI_Cart_sub\fP 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
\fI\%MPI_Comm_split\fP\&.)
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Example: Assume that \fBMPI_Cart_create( ..., comm)\fP has defined a (2 x 3 x
4) grid. Let remain_dims = (true, false, true). Then a call to
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MPI_Cart_sub(comm, remain_dims, comm_new);
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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.
.SH ERRORS
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Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
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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.
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Open MPI includes three predefined error handlers that can be used:
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\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.
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MPI applications can also implement their own error handlers by calling:
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.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
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Note that MPI does not guarantee that an MPI program can continue past
an error.
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See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
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See the Error Handling section of the MPI\-3.1 standard for
more information.
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\fBSEE ALSO:\fP
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.IP \(bu 2
\fI\%MPI_Cart_create\fP
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.SH COPYRIGHT
2003-2024, The Open MPI Community
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