Creates an intracommuncator from an intercommunicator


int MPI_Intercomm_merge(MPI_Comm intercomm, int high, MPI_Comm *newintracomm)

Input Parameters

inter-communicator (handle)
ordering of the local and remote groups in the new intra-communicator (logical)

Output Parameters

new intra-communicator (handle)


While all processes may provide the same value for the high parameter, this requires the MPI implementation to determine which group of processes should be ranked first.

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.

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.


  1. Allocate contexts
  2. Local and remote group leaders swap high values
  3. Determine the high value.
  4. Merge the two groups and make the intra-communicator


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 guarantee 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).
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Other error; use MPI_Error_string to get more information about this error code.

See Also

MPI_Intercomm_create, MPI_Comm_free