MPI_COMM_TEST_INTER(3)				     Open MPI				    MPI_COMM_TEST_INTER(3)

MPI_Comm_test_inter — Tests to see if a comm is an intercommunicator.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Comm_test_inter(MPI_Comm comm, int *flag)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_COMM_TEST_INTER(COMM, FLAG, IERROR)
	       INTEGER COMM, IERROR
	       LOGICAL FLAG

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Comm_test_inter(comm, flag, ierror)
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       LOGICAL, INTENT(OUT) :: flag
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETER
       • comm: Communicator (handle).

OUTPUT PARAMETERS
       • flag (Logical.):

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       This  local routine allows the calling process to determine the type of a communicator. It returns true for
       an intercommunicator, false for an intracommunicator.

       The type of communicator also affects the value returned by three other functions.  When	 dealing  with	an
       intracommunicator  (enables  communication  within  a  single group), the functions listed below return the
       expected values, group size, group, and rank. When dealing with an inter-communicator, however, they return
       the following values:

       MPI_Comm_size <#mpi-comm-size>
	      Returns the size of the local group.

       MPI_Comm_group <#mpi-comm-group>
	      Returns the local group.

       MPI_Comm_rank <#mpi-comm-rank>
	      Returns the rank in the local group.

       To return the remote group and remote group size of an inter-communicator, use the MPI_Comm_remote_group <#
       mpi-comm-remote-group> and MPI_Comm_remote_size <#mpi-comm-remote-size> functions.

       The operation MPI_Comm_compare <#mpi-comm-compare> is valid for intercommunicators. Both communicators must
       be either intra- or intercommunicators, or else MPI_UNEQUAL results. Both corresponding	local  and  remote
       groups  must  compare  correctly	 to  get  the  results MPI_CONGRUENT and MPI_SIMILAR. In particular, it is
       possible for MPI_SIMILAR to result because  either  the	local  or  remote  groups  were	 similar  but  not
       identical.

       The   following	accessors  provide  consistent	access	to  the	 remote	 group	of  an	intercommunicator:
       MPI_Comm_remote_size <#mpi-comm-remote-size>, MPI_Comm_remote_group <#mpi-comm-remote-group>.

       The  intercommunicator  accessors   (MPI_Comm_test_inter,   MPI_Comm_remote_size	  <#mpi-comm-remote-size>,
       MPI_Comm_remote_group) are all local operations.

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_Comm_remote_group <#mpi-comm-remote-group>

	  • MPI_Comm_remote_size <#mpi-comm-remote-size>

	  • MPI_Intercomm_create <#mpi-intercomm-create>

	  • MPI_Intercomm_merge <#mpi-intercomm-merge>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				    MPI_COMM_TEST_INTER(3)
