MPI_GRAPH_CREATE(3)				     Open MPI				       MPI_GRAPH_CREATE(3)

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, const int index[],
	      const int edges[], int reorder, MPI_Comm *comm_graph)

   Fortran Syntax
	  USE MPI
	  ! or the older form: 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

   Fortran 2008 Syntax
	  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

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 <#mpi-cart-create> and MPI_Comm_split  <#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:

       • nodes = 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 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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:
	  MPI_Graph_get <#mpi-graph-get>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				       MPI_GRAPH_CREATE(3)
