MPI_Graph_create(3)                Open MPI                MPI_Graph_create(3)



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
       communicator.

       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.

       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++
       functions 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




1.4.1                            Jan 14, 2010              MPI_Graph_create(3)
