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

                                            11/12/2016                          MPI_Comm_split(3)