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<TITLE>MPI_Intercomm_create</TITLE>
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MPI_Intercomm_create
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Creates an intercommuncator from two intracommunicators
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Synopsis
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int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,
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MPI_Comm peer_comm, int remote_leader, int tag,
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MPI_Comm *newintercomm)
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Input Parameters
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local_comm Local (intra)communicator
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local_leader Rank in local_comm of leader (often 0)
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peer_comm Communicator used to communicate between a
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designated process in the other communicator.
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Significant only at the process in <tt>local_comm</tt> with
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rank <tt>local_leader</tt>.
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remote_leader Rank in peer_comm of remote leader (often 0)
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tag Message tag to use in constructing intercommunicator; if multiple
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<tt>MPI_Intercomm_creates</tt> are being made, they should use different tags (more
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precisely, ensure that the local and remote leaders are using different
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tags for each <tt>MPI_intercomm_create</tt>).
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Output Parameters
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- newintercomm
- Created intercommunicator
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Notes
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<tt>peer_comm</tt> is significant only for the process designated the
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<tt>local_leader</tt> in the <tt>local_comm</tt>.
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The MPI 1.1 Standard contains two mutually exclusive comments on the
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input intercommunicators. One says that their repective groups must be
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disjoint; the other that the leaders can be the same process. After
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some discussion by the MPI Forum, it has been decided that the groups must
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be disjoint. Note that the reason given for this in the standard is
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not the reason for this choice; rather, the other operations on
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intercommunicators (like <tt>MPI_Intercomm_merge</tt>) do not make sense if the
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groups are not disjoint.
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Thread and Interrupt Safety
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This routine is thread-safe. This means that this routine may be
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safely used by multiple threads without the need for any user-provided
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thread locks. However, the routine is not interrupt safe. Typically,
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this is due to the use of memory allocation routines such as <tt>malloc
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</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
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Notes for Fortran
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All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
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an additional argument <tt>ierr</tt> at the end of the argument list. <tt>ierr
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</tt>is an integer and has the same meaning as the return value of the routine
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in C. In Fortran, MPI routines are subroutines, and are invoked with the
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<tt>call</tt> statement.
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All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
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</tt>in Fortran.
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Errors
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All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) return an error value;
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C routines as the value of the function and Fortran routines in the last
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argument. Before the value is returned, the current MPI error handler is
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called. By default, this error handler aborts the MPI job. The error handler
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may be changed with <tt>MPI_Comm_set_errhandler</tt> (for communicators),
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<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
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RMA windows). The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
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its use is deprecated. The predefined error handler
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<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
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Note that MPI does not guarentee that an MPI program can continue past
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an error; however, MPI implementations will attempt to continue whenever
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possible.
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- MPI_SUCCESS
- No error; MPI routine completed successfully.
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- MPI_ERR_COMM
- Invalid communicator. A common error is to use a null
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communicator in a call (not even allowed in <tt>MPI_Comm_rank</tt>).
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- MPI_ERR_TAG
- Invalid tag argument. Tags must be non-negative; tags
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in a receive (<tt>MPI_Recv</tt>, <tt>MPI_Irecv</tt>, <tt>MPI_Sendrecv</tt>, etc.) may
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also be <tt>MPI_ANY_TAG</tt>. The largest tag value is available through the
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the attribute <tt>MPI_TAG_UB</tt>.
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- MPI_ERR_INTERN
- This error is returned when some part of the MPICH
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implementation is unable to acquire memory.
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- MPI_ERR_RANK
- Invalid source or destination rank. Ranks must be between
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zero and the size of the communicator minus one; ranks in a receive
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(<tt>MPI_Recv</tt>, <tt>MPI_Irecv</tt>, <tt>MPI_Sendrecv</tt>, etc.) may also be <tt>MPI_ANY_SOURCE</tt>.
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<H0>See Also</H0>
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MPI_Intercomm_merge, MPI_Comm_free, MPI_Comm_remote_group,
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MPI_Comm_remote_size
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