MPI_Graph_create(3OpenMPI)												MPI_Graph_create(3OpenMPI)

NAME

       MPI_Graph_create  - Makes a new communicator to which topology information has been attached.

SYNTAX

C Syntax
       #include <mpi.h>
       int MPI_Graph_create(MPI_Comm comm_old, int nnodes, int *index,
	    int *edges, int reorder, MPI_Comm *comm_graph)

Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_GRAPH_CREATE(COMM_OLD, NNODES, INDEX, EDGES, REORDER,
		 COMM_GRAPH, IERROR)
	    INTEGER   COMM_OLD, NNODES, INDEX(*), EDGES(*)
	    INTEGER   COMM_GRAPH, IERROR
	    LOGICAL   REORDER

C++ Syntax
       #include <mpi.h>
       Graphcomm Intracomm::Create_graph(int nnodes, const int index[],
	    const int edges[], bool reorder) const

INPUT PARAMETERS

       comm_old  Input communicator without topology (handle).

       nnodes	 Number of nodes in graph (integer).

       index	 Array of integers describing node degrees (see below).

       edges	 Array of integers describing graph edges (see below).

       reorder	 Ranking may be reordered (true) or not (false) (logical).

OUTPUT PARAMETERS

       comm_graph
		 Communicator with graph topology added (handle).

       IERROR	 Fortran only: Error status (integer).

DESCRIPTION

       MPI_Graph_create  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
       MPI_Cart_create and MPI_Comm_split. The call is erroneous if it specifies a graph that is larger than the group size of the input  communi-
       cator.

       The  three  parameters  nnodes, index, and edges define the graph structure. nnodes is the number of nodes of the graph. The nodes are num-
       bered 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 neigh-
       bors  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.

       The definitions of the arguments nnodes, index, and edges are illustrated with the following simple example.

       Example: Assume there are four processes 0, 1, 2, 3 with the following adjacency matrix:

	   Process    Neighbors
	      0 	 1, 3
	      1 	 0
	      2 	 3
	      3 	 0, 2

       Then, the input arguments are:
	   nnodes = 4
	   index  = 2, 3, 4, 6
	   edges  = 1, 3, 0, 3, 0, 2

       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.

       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).

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ func-
       tions  do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except  for
       I/O  function  errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be
       used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Graph_get
       MPI_Graphdims_get

Open MPI 1.2							  September 2006					MPI_Graph_create(3OpenMPI)