The Great and Terrible implementation of MPI-2

function index


Return the parent communicator for this process
int MPI_Comm_get_parent(
  MPI_Comm *parent


[out] the parent communicator (handle)


If a process was started with MPI_Comm_spawn or MPI_Comm_spawn_multiple, MPI_Comm_get_parent returns the parent intercommunicator of the current process. This parent intercommunicator is created implicitly inside of MPI_Init and is the same intercommunicator returned by MPI_Comm_spawn in the parents.

If the process was not spawned, MPI_Comm_get_parent returns MPI_COMM_NULL.

After the parent communicator is freed or disconnected, MPI_Comm_get_parent returns MPI_COMM_NULL.

Advice to users.

MPI_COMM_GET_PARENT returns a handle to a single intercommunicator. Calling MPI_COMM_GET_PARENT a second time returns a handle to the same intercommunicator. Freeing the handle with MPI_COMM_DISCONNECT or MPI_COMM_FREE will cause other references to the intercommunicator to become invalid (dangling). Note that calling MPI_COMM_FREE on the parent communicator is not useful.


The desire of the Forum was to create a constant MPI_COMM_PARENT similar to MPI_COMM_WORLD. Unfortunately such a constant cannot be used (syntactically) as an argument to MPI_COMM_DISCONNECT, which is explicitly allowed.

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

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_get_parent.

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

#define NUM_SPAWNS 2

int main( int argc, char *argv[] )
int np = NUM_SPAWNS;
int errcodes[NUM_SPAWNS];
    MPI_Comm parentcomm, intercomm;

    MPI_Init( &argc, &argv );
    MPI_Comm_get_parent( &parentcomm );
    if (parentcomm == MPI_COMM_NULL)
/* Create 2 more processes - this example must be called spawn_example.exe for this to work. */
        MPI_Comm_spawn( "spawn_example", MPI_ARGV_NULL, np, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &intercomm, errcodes );
        printf("I'm the parent.\n");
        printf("I'm the spawned.\n");
    return 0;