# MPI_Intercomm_create

Creates an intercommuncator from two intracommunicators

## Synopsis

    #include "mpi.h"
    int MPI_Intercomm_create ( MPI_Comm local_comm, int local_leader,
                             MPI_Comm peer_comm, int remote_leader, int tag,
                             MPI_Comm *comm_out )

## Input Paramters

| local_comm | Local (intra)communicator
| local_leader | Rank in local_comm of leader (often 0)
| peer_comm | Remote communicator
| remote_leader | Rank in peer_comm of remote leader (often 0)
| tag | Message tag to use in constructing intercommunicator; if multiple `MPI_Intercomm_creates` are being made, they should use different tags (more precisely, ensure that the local and remote leaders are using different tags for each `MPI_intercomm_create`).

## Output Parameter

comm_out
  Created intercommunicator

## Notes

The MPI 1.1 Standard contains two mutually exclusive comments on the input intracommunicators. One says that their repective groups must be disjoint; the other that the leaders can be the same process. After some discussion by the MPI Forum, it has been decided that the groups must be disjoint. Note that the reason given for this in the standard is not the reason for this choice; rather, the other operations on intercommunicators (like `MPI_Intercomm_merge`) do not make sense if the groups are not disjoint.

## 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 a send context, an inter | coll context, and an intra-coll context
| 2) Send "send_context" and lrank_to_grank list from local comm group | if I'm the local_leader.
| 3) If I'm the local leader, then wait on the posted sends and receives | to complete. Post the receive for the remote group information and wait for it to complete.
| 4) Broadcast information received from the remote leader. | . 5) Create the inter_communicator from the information we now have.
| An inter | communicator ends up with three levels of communicators. The inter-communicator returned to the user, a "collective" inter-communicator that can be used for safe communications between local & remote groups, and a collective intra-communicator that can be used to allocate new contexts during the merge and dup operations.

For the resulting inter-communicator, `comm_out`

           comm_out                       = inter-communicator
           comm_out->comm_coll            = "collective" inter-communicator
           comm_out->comm_coll->comm_coll = safe collective 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_Errhandler_set`; 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.

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_TAG
  Invalid tag argument. Tags must be non-negative; tags in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_TAG`. The largest tag value is available through the the attribute `MPI_TAG_UB`.

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

MPI_ERR_RANK
  Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_SOURCE`.

## See Also

MPI_Intercomm_merge, MPI_Comm_free, MPI_Comm_remote_group,
MPI_Comm_remote_size

Location:ic_create.c