# MPI_Comm_compare

Compares two communicators

## Synopsis

    int MPI_Comm_compare(MPI_Comm comm1, MPI_Comm comm2, int *result)

## Input Parameters

comm1
  comm1 (handle)
comm2
  comm2 (handle)

## Output Parameters

result
  integer which is `MPI_IDENT` if the contexts and groups are the same, `MPI_CONGRUENT` if different contexts but identical groups, `MPI_SIMILAR` if different contexts but similar groups, and `MPI_UNEQUAL` otherwise

## Using 'MPI_COMM_NULL' with 'MPI_Comm_compare'

It is an error to use `MPI_COMM_NULL` as one of the arguments to `MPI_Comm_compare`. The relevant sections of the MPI standard are

.(2.4.1 Opaque Objects) A null handle argument is an erroneous `IN` argument in MPI calls, unless an exception is explicitly stated in the text that defines the function.

.(5.4.1. Communicator Accessors) where there is no text in `MPI_COMM_COMPARE` allowing a null handle.

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe. (To perform the communicator comparisions, this routine may need to allocate some memory. Memory allocation is not interrupt-safe, and hence this routine is only thread-safe.)

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. The predefined error handler `MPI_ERRORS_RETURN` may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).