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

.SH NAME

MPI_Comm_split \-  Creates new communicators based on colors and keys 

.SH SYNOPSIS

.nf

int MPI_Comm_split(MPI_Comm comm, int color, int key, MPI_Comm * newcomm)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B comm 

- communicator (handle)

.PD 1

.PD 0

.TP

.B color 

- control of subset assignment (nonnegative integer).  Processes

with the same color are in the same new communicator

.PD 1

.PD 0

.TP

.B key 

- control of rank assignment (integer)

.PD 1

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B newcomm 

- new communicator (handle)

.PD 1

.SH NOTES

The 

.I color

must be non-negative or 

.I MPI_UNDEFINED

\&.

.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 ALGORITHM

.nf

1. Use MPI_Allgather to get the color and key from each process

2. Count the number of processes with the same color; create a

communicator with that many processes.  If this process has

.I MPI_UNDEFINED

as the color, create a process with a single member.

3. Use key to order the ranks

.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_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_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