The Great and Terrible implementation of MPI-2

function index


Compares two communicators
int MPI_Comm_compare(
  MPI_Comm comm1,
  MPI_Comm comm2,
  int *result


[in] comm1 (handle)
[in] comm2 (handle)
[out] 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


MPI_IDENT results if and only if comm1 and comm2 are handles for the same object (identical groups and same contexts). MPI_CONGRUENT results if the underlying groups are identical in constituents and rank order; these communicators differ only by context. MPI_SIMILAR results if the group members of both communicators are the same but the rank order differs. MPI_UNEQUAL results 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.


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.

No error; MPI routine completed successfully.
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Comm_compare.

#include "mpi.h"
#include <stdio.h>

int main( int argc, char *argv[] )
    int errs = 0;
int size, dims[2], periods[2], remain[2];
int result;
    MPI_Comm comm, newcomm;

    MPI_Init( &argc, &argv );

    /* First, create a 1-dim cartesian communicator */
periods[0] = 0;
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    dims[0] = size;
    MPI_Cart_create( MPI_COMM_WORLD, 1, dims, periods, 0, &comm );

/* Now, extract a communicator with no dimensions */
remain[0] = 0;
    MPI_Cart_sub( comm, remain, &newcomm );

/* This should be congruent to MPI_COMM_SELF */
    MPI_Comm_compare( MPI_COMM_SELF, newcomm, &result );
    if (result != MPI_CONGRUENT) {
        printf( "cart sub to size 0 did not give self\n" );fflush(stdout);

/* Free the new communicator */
MPI_Comm_free( &newcomm );
    MPI_Comm_free( &comm );

return 0;