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.TH "MPI_GRAPH_CREATE" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPI_Graph_create\fP — Makes a new communicator to which topology
information has been attached.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Graph_create(MPI_Comm comm_old, int nnodes, const int index[],
    const int edges[], int reorder, MPI_Comm *comm_graph)
.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_GRAPH_CREATE(COMM_OLD, NNODES, INDEX, EDGES, REORDER,
        COMM_GRAPH, IERROR)
    INTEGER COMM_OLD, NNODES, INDEX(*), EDGES(*)
    INTEGER COMM_GRAPH, IERROR
    LOGICAL   REORDER
.ft P
.fi
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.UNINDENT
.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE mpi_f08

MPI_Graph_create(comm_old, nnodes, index, edges, reorder, comm_graph,
        ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm_old
    INTEGER, INTENT(IN) :: nnodes, index(nnodes), edges(*)
    LOGICAL, INTENT(IN) :: reorder
    TYPE(MPI_Comm), INTENT(OUT) :: comm_graph
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBcomm_old\fP : Input communicator without topology (handle).
.IP \(bu 2
\fBnnodes\fP : Number of nodes in graph (integer).
.IP \(bu 2
\fBindex\fP : Array of integers describing node degrees (see below).
.IP \(bu 2
\fBedges\fP : Array of integers describing graph edges (see below).
.IP \(bu 2
\fBreorder\fP : Ranking may be reordered (true) or not (false) (logical).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBcomm_graph\fP : Communicator with graph topology added (handle).
.IP \(bu 2
\fBierror\fP : Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
\fI\%MPI_Graph_create\fP returns a handle to a new communicator to which the
graph topology information is attached. If reorder = false then the rank
of each process in the new group is identical to its rank in the old
group. Otherwise, the function may reorder the processes. If the size,
nnodes, of the graph is smaller than the size of the group of comm_old,
then some processes are returned MPI_COMM_NULL, in analogy to
\fI\%MPI_Cart_create\fP and \fI\%MPI_Comm_split\fP\&. The call is erroneous if it
specifies a graph that is larger than the group size of the input
communicator.
.sp
The three parameters nnodes, index, and edges define the graph
structure. nnodes is the number of nodes of the graph. The nodes are
numbered from 0 to nnodes\-1. The ith entry of array index stores the
total number of neighbors of the first i graph nodes. The lists of
neighbors of nodes 0, 1, …, nnodes\-1 are stored in consecutive
locations in array edges. The array edges is a flattened representation
of the edge lists. The total number of entries in index is nnodes and
the total number of entries in edges is equal to the number of graph
edges.
.sp
The definitions of the arguments nnodes, index, and edges are
illustrated with the following simple example.
.sp
Example: Assume there are four processes 0, 1, 2, 3 with the following
adjacency matrix:
.sp
——\- ———
Process Neighbors
——\- ———
0       1, 3
1       0
2       3
3       0, 2
——\- ———
.sp
Then, the input arguments are:
.INDENT 0.0
.IP \(bu 2
nodes = 4
.IP \(bu 2
index = 2, 3, 4, 6
.IP \(bu 2
edges = 1, 3, 0, 3, 0, 2
.UNINDENT
.sp
Thus, in C, index[0] is the degree of node zero, and index[i] \-
index[i\-1] is the degree of node i, i=1, … , nnodes\-1; the list of
neighbors of node zero is stored in edges[j], for 0 <= j <= index[0] \- 1
and the list of neighbors of node i, i > 0 , is stored in edges[j],
index[i\-1] <= j <= index[i] \- 1.
.sp
In Fortran, index(1) is the degree of node zero, and index(i+1) \-
index(i) is the degree of node i, i=1, … , nnodes\-1; the list of
neighbors of node zero is stored in edges(j), for 1 <= j <= index(1) and
the list of neighbors of node i, i > 0, is stored in edges(j), index(i)
+ 1 <= j <= index(i + 1).
.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:
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.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
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.INDENT 3.5
\fI\%MPI_Graph_get\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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