# 17.2.84. MPI_Dist_graph_create

MPI_Dist_graph_create — Makes a new communicator to which topology information has been attached.

## 17.2.84.1. SYNTAX

### 17.2.84.1.1. C Syntax

    #include <mpi.h>

    int MPI_Dist_graph_create(MPI_Comm comm_old, int n, const int sources[],
         const int degrees[], const int destinations[], const int weights[],
            MPI_Info info, int reorder, MPI_Comm *comm_dist_graph)

### 17.2.84.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_DIST_GRAPH_CREATE(COMM_OLD, N, SOURCES, DEGREES, DESTINATIONS, WEIGHTS,
                    INFO, REORDER, COMM_DIST_GRAPH, IERROR)
         INTEGER COMM_OLD, N, SOURCES(*), DEGRES(*), WEIGHTS(*), INFO
         INTEGER COMM_DIST_GRAPH, IERROR
         LOGICAL   REORDER

### 17.2.84.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Dist_Graph_create(comm_old, n, sources, degrees, destinations, weights,
                 info, reorder, comm_dist_graph, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm_old
         INTEGER, INTENT(IN) :: n, sources(n), degrees(n), destinations(*)
         INTEGER, INTENT(IN) :: weights(*)
         TYPE(MPI_Info), INTENT(IN) :: info
         LOGICAL, INTENT(IN) :: reorder
         TYPE(MPI_Comm), INTENT(OUT) :: comm_dist_graph
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.84.2. INPUT PARAMETERS

- `comm_old`: Input communicator without topology (handle).

- `n`: Number of source nodes for which this process specifies edges (non-negative integer).

- `sources`: Array containing the n source nodes for which this process specifies edges (array of non-negative integers).

- `degrees`: Array specifying the number of destinations for each source node in the source node array (array of non-negative integers).

- `destinations`: Destination nodes for the source nodes in the source node array (array of non-negative integers).

- `weights`: Weights for source to destination edges (array of non-negative integers).

- `info`: Hints on optimization and interpretation of weights (handle).

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

## 17.2.84.3. OUTPUT PARAMETERS

- `comm_dist_graph`: Communicator with distributed graph topology added (handle).

- `ierror`: Fortran only: Error status (integer).

## 17.2.84.4. DESCRIPTION

MPI_Dist_graph_create creates a new communicator comm_dist_graph with distributed graph topology and returns a handle to the new communicator. The number of processes in comm_dist_graph is identical to the number of processes in comm_old. Concretely, each process calls the constructor with a set of directed (source,destination) communication edges as described below. Every process passes an array of n source nodes in the sources array. For each source node, a non-negative number of destination nodes is specified in the degrees array. The destination nodes are stored in the corresponding consecutive segment of the destinations array. More precisely, if the i-th node in sources is s, this specifies degrees[i] edges (s,d) with d of the j-th such edge stored in destinations[degrees[0]+…+degrees[i-1]+j]. The weight of this edge is stored in weights[degrees[0]+…+degrees[i-1]+j]. Both the sources and the destinations arrays may contain the same node more than once, and the order in which nodes are listed as destinations or sources is not signicant. Similarly, different processes may specify edges with the same source and destination nodes. Source and destination nodes must be process ranks of comm_old. Different processes may specify different numbers of source and destination nodes, as well as different source to destination edges. This allows a fully distributed specification of the communication graph. Isolated processes (i.e., processes with no outgoing or incoming edges, that is, processes that do not occur as source or destination node in the graph specification) are allowed. The call to MPI_Dist_graph_create is collective.

If reorder = false, all processes will have the same rank in comm_dist_graph as in comm_old. If reorder = true then the MPI library is free to remap to other processes (of comm_old) in order to improve communication on the edges of the communication graph. The weight associated with each edge is a hint to the MPI library about the amount or intensity of communication on that edge, and may be used to compute a

## 17.2.84.5. WEIGHTS

Weights are specified as non-negative integers and can be used to influence the process remapping strategy and other internal MPI optimizations. For instance, approximate count arguments of later communication calls along specific edges could be used as their edge weights. Multiplicity of edges can likewise indicate more intense communication between pairs of processes. However, the exact meaning of edge weights is not specified by the MPI standard and is left to the implementation. An application can supply the special value MPI_UNWEIGHTED for the weight array to indicate that all edges have the same (effectively no) weight. It is erroneous to supply MPI_UNWEIGHTED for some but not all processes of comm_old. If the graph is weighted but n = 0, then MPI_WEIGHTS_EMPTY or any arbitrary array may be passed to weights. Note that MPI_UNWEIGHTED and MPI_WEIGHTS_EMPTY are not special weight values; rather they are special values for the total array argument. In Fortran, MPI_UNWEIGHTED and MPI_WEIGHTS_EMPTY are objects like MPI_BOTTOM (not usable for initialization or assignment). See MPI-3 section 2.5.4.

## 17.2.84.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

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 MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. 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.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort 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.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Dist_graph_create_adjacent

- MPI_Dist_graph_neighbors

- MPI_Dist_graph_neighbors_count