File: MPI_Comm_split.3

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.TH MPI_Comm_split 3 "1/6/1997" " " "MPI"
.SH NAME
MPI_Comm_split \-  Creates new communicators based on colors and keys 
.SH SYNOPSIS
.nf
#include "mpi.h"
int MPI_Comm_split ( comm, color, key, comm_out )
MPI_Comm  comm;
int       color, key;
MPI_Comm *comm_out;
.fi
.SH INPUT PARAMETERS
.PD 0
.TP
.B comm 
- communicator (handle) 
.PD 1
.PD 0
.TP
.B color 
- control of subset assignment (nonnegative integer) 
.PD 1
.PD 0
.TP
.B key 
- control of rank assigment (integer) 
.PD 1

.SH OUTPUT PARAMETER
.PD 0
.TP
.B newcomm 
- new communicator (handle) 
.PD 1

.SH NOTES
The 
.I color
must be non-negative or 
.I MPI_UNDEFINED
.

.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 ALGORITHM

The current algorithm used has quite a few (read: a lot of) inefficiencies
that can be removed.  Here is what we do for now

.nf
1) A table is built of colors, and keys (has a next field also).
2) The tables of all processes are merged using 
.I MPI_Allreduce
.
3) Two contexts are allocated for all the comms to be created.  These
same two contexts can be used for all created communicators since
the communicators will not overlap.
4) If the local process has a color of 
.I MPI_UNDEFINED
, it can return
a 
.I NULL
comm. 
5) The table entries that match the local process color are sorted 
by key/rank. 
6) A group is created from the sorted list and a communicator is created
with this group and the previously allocated contexts.
.fi

.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_Errhandler_set
; 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.

.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_INTERN 
- This error is returned when some part of the MPICH 
implementation is unable to acquire memory.  
.PD 1

.SH SEE ALSO
MPI_Comm_free
.br
.SH LOCATION
comm_split.c