# MPI_Intercomm_create

Creates an intercommuncator from two intracommunicators

## Synopsis

    int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,
                           MPI_Comm peer_comm, int remote_leader, int tag,
                           MPI_Comm *newintercomm)

## Input Parameters

local_comm
  Local (intra)communicator
local_leader
  Rank in local_comm of leader (often 0)
peer_comm
  Communicator used to communicate between a designated process in the other communicator. Significant only at the process in `local_comm` with rank `local_leader`.
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

`peer_comm` is significant only for the process designated the `local_leader` in the `local_comm`.

The MPI 1.1 Standard contains two mutually exclusive comments on the input intercommunicators. 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.

## 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.

## 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_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:intercomm_create.c