NAME

       MPI_Comm_split -  Creates new communicators based on colors and keys

SYNOPSIS

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

INPUT PARAMETERS

       comm   - communicator (handle)
       color  -  control  of  subset assignment (nonnegative integer).  Processes with the same color are in the
              same new communicator
       key    - control of rank assignment (integer)

OUTPUT PARAMETERS

       newcomm
              - new communicator (handle)

NOTES

       The color must be non-negative or MPI_UNDEFINED .

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

NOTES FOR FORTRAN

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

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

ALGORITHM

       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
       MPI_UNDEFINED
       as the color, create a process with a single member.
       3. Use key to order the ranks

ERRORS

       All MPI routines (except MPI_Wtime and 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    MPI_Comm_set_errhandler   (for   communicators),   MPI_File_set_errhandler   (for   files),   and
       MPI_Win_set_errhandler (for RMA windows).  The MPI-1 routine MPI_Errhandler_set may be used but  its  use
       is  deprecated.   The  predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be
       returned.  Note that MPI does not guarentee that an MPI program can continue past an error; however,  MPI
       implementations will attempt to continue whenever possible.

       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_COMM
              -  Invalid communicator.  A common error is to use a null communicator in a call (not even allowed
              in MPI_Comm_rank ).
       MPI_ERR_INTERN
              - This error is returned when some part of the MPICH implementation is unable to acquire memory.

SEE ALSO

       MPI_Comm_free

                                                   11/12/2019                                  MPI_Comm_split(3)