.TH MPI_Intercomm_create 3 "6/24/2006" "LAM/MPI 7.1.4" "LAM/MPI"
.SH NAME
MPI_Intercomm_create \-  Creates an intercommuncator from two intracommunicators 
.SH SYNOPSIS
.nf
#include <mpi.h>
int MPI_Intercomm_create(MPI_Comm lcomm, int lleader, 
                       MPI_Comm pcomm, int pleader, int tag, 
                       MPI_Comm *newcomm)
.fi
.SH INPUT PARAMTERS
.PD 0
.TP
.B lcomm 
- Local (intra)communicator
.PD 1
.PD 0
.TP
.B lleader 
- Rank in local_comm of leader (often 0)
.PD 1
.PD 0
.TP
.B pcomm 
- Remote communicator
.PD 1
.PD 0
.TP
.B pleader 
- Rank in peer_comm of remote leader (often 0)
.PD 1
.PD 0
.TP
.B tag 
- Message tag to use in constructing intercommunicator; if
multiple 
.I 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 
.I MPI_intercomm_create
).
.PD 1

.SH OUTPUT PARAMETER
.PD 0
.TP
.B newcomm 
- Created intercommunicator
.PD 1

.SH 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 
.I reason
given for this in the
standard is 
.I not
the reason for this choice; rather, the 
.I other
operations on intercommunicators (like 
.I MPI_Intercomm_merge
) do not
make sense if the groups are not disjoint.

.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 
.I MPI_WTIME
and 
.I MPI_WTICK
)
have an additional argument 
.I ierr
at the end of the argument list.
.I 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 
.I call
statement.

All MPI objects (e.g., 
.I MPI_Datatype
, 
.I MPI_Comm
) are of type
.I INTEGER
in Fortran.

.SH ERRORS

If an error occurs in an MPI function, the current MPI error handler
is called to handle it.  By default, this error handler aborts the
MPI job.  The error handler may be changed with 
.I MPI_Errhandler_set
;
the predefined error handler 
.I MPI_ERRORS_RETURN
may be used to cause
error values to be returned (in C and Fortran; this error handler is
less useful in with the C++ MPI bindings.  The predefined error
handler 
.I MPI::ERRORS_THROW_EXCEPTIONS
should be used in C++ if the
error value needs to be recovered).  Note that MPI does 
.I not
guarantee that an MPI program can continue past an error.

All MPI routines (except 
.I MPI_Wtime
and 
.I MPI_Wtick
) return an error
value; C routines as the value of the function and Fortran routines
in the last argument.  The C++ bindings for MPI do not return error
values; instead, error values are communicated by throwing exceptions
of type 
.I MPI::Exception
(but not by default).  Exceptions are only
thrown if the error value is not 
.I MPI::SUCCESS
\&.


Note that if the 
.I MPI::ERRORS_RETURN
handler is set in C++, while
MPI functions will return upon an error, there will be no way to
recover what the actual error value was.
.PD 0
.TP
.B MPI_SUCCESS 
- No error; MPI routine completed successfully.
.PD 1
.PD 0
.TP
.B MPI_ERR_COMM 
- Invalid communicator.  A common error is to use a
null communicator in a call (not even allowed in 
.I MPI_Comm_rank
).
.PD 1
.PD 0
.TP
.B MPI_ERR_TAG 
- Invalid tag argument.  Tags must be non-negative;
tags in a receive (
.I MPI_Recv
, 
.I MPI_Irecv
, 
.I MPI_Sendrecv
, etc.)
may also be 
.I MPI_ANY_TAG
\&.
The largest tag value is available
through the the attribute 
.I MPI_TAG_UB
\&.

.PD 1
.PD 0
.TP
.B MPI_ERR_ARG 
- Invalid argument.  Some argument is invalid and is not
identified by a specific error class.  This is typically a NULL
pointer or other such error.
.PD 1
.PD 0
.TP
.B MPI_ERR_OTHER 
- This error is returned when some part of the
LAM/MPI implementation is unable to acquire memory.
.PD 1
.PD 0
.TP
.B 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 (
.I MPI_Recv
, 
.I MPI_Irecv
, 
.I MPI_Sendrecv
, etc.) may also be
.I MPI_ANY_SOURCE
\&.

.PD 1

.SH SEE ALSO
MPI_Intercomm_merge, MPI_Comm_free, MPI_Comm_remote_group, 
.br
MPI_Comm_remote_size

.SH MORE INFORMATION

For more information, please see the official MPI Forum web site,
which contains the text of both the MPI-1 and MPI-2 standards.  These
documents contain detailed information about each MPI function (most
of which is not duplicated in these man pages).

.I http://www.mpi-forum.org/


.SH ACKNOWLEDGEMENTS

The LAM Team would like the thank the MPICH Team for the handy program
to generate man pages ("doctext" from
.I ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz
), the initial
formatting, and some initial text for most of the MPI-1 man pages.
.SH LOCATION
iccreate.c