# 18.2.81. MPI_Comm_test_inter

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

## 18.2.81.1. SYNTAX

### 18.2.81.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_test_inter(MPI_Comm comm, int *flag)

### 18.2.81.1.2. Fortran Syntax

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

### 18.2.81.1.3. 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

## 18.2.81.2. INPUT PARAMETER

- `comm`: Communicator (handle).

## 18.2.81.3. OUTPUT PARAMETERS

- `flag (Logical.)`:

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

## 18.2.81.4. 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

  Returns the size of the local group.

MPI_Comm_group

  Returns the local group.

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 and MPI_Comm_remote_size functions.

The operation 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_group.

The intercommunicator accessors (MPI_Comm_test_inter, MPI_Comm_remote_size, MPI_Comm_remote_group) are all local operations.

## 18.2.81.5. 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_Comm_remote_group

- MPI_Comm_remote_size

- MPI_Intercomm_create

- MPI_Intercomm_merge