.TH MPI_Intercomm_create 3 "11/12/2019" " " "MPI"

.SH NAME

MPI_Intercomm_create \-  Creates an intercommuncator from two intracommunicators 

.SH SYNOPSIS

.nf

int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,

MPI_Comm peer_comm, int remote_leader, int tag, MPI_Comm * newintercomm)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B local_comm 

- Local (intra)communicator

.PD 1

.PD 0

.TP

.B local_leader 

- Rank in local_comm of leader (often 0)

.PD 1

.PD 0

.TP

.B peer_comm 

- Communicator used to communicate between a

designated process in the other communicator.

Significant only at the process in 

.I local_comm

with

rank 

.I local_leader

\&.

.PD 1

.PD 0

.TP

.B remote_leader 

- 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 PARAMETERS

.PD 0

.TP

.B newintercomm 

- Created intercommunicator

.PD 1

.SH NOTES

.I peer_comm

is significant only for the process designated the

.I local_leader

in the 

.I 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 

.B reason

given for this in the standard is

.B not

the reason for this choice; rather, the 

.B other

operations on

intercommunicators (like 

.I MPI_Intercomm_merge

) do not make sense if the

groups are not disjoint.

.SH 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 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.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

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.  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 

.I MPI_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  The predefined error handler

.I MPI_ERRORS_RETURN

may be used to cause error values to be returned.

Note that MPI does 

.B not

guarentee that an MPI program can continue past

an error; however, MPI implementations will attempt to continue whenever

possible.

.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_INTERN 

- This error is returned when some part of the MPICH 

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