# MPI_Intercomm_merge

Creates an intracommuncator from an intercommunicator

## Synopsis

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

## Input Parameters

intercomm
  Intercommunicator (handle)
high
  Used to order the groups within comm (logical) when creating the new communicator. This is a boolean value; the group that sets high true has its processes ordered after the group that sets this value to false. If all processes in the intercommunicator provide the same value, the choice of which group is ordered first is arbitrary.

## Output Parameters

newintracomm
  Created intracommunicator (handle)

## Notes

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.

## Algorithm

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

## 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_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_INTERN
  This error is returned when some part of the MPICH implementation is unable to acquire memory.

See Also MPI_Intercomm_create, MPI_Comm_free