# MPI_Comm_split

Creates new communicators based on colors and keys

## Synopsis

    #include "mpi.h"
    int MPI_Comm_split ( MPI_Comm comm, int color, int key, MPI_Comm *comm_out )

## 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 assigment (integer)

## Output Parameter

newcomm
  new communicator (handle)

## Notes

The `color` must be non-negative or `MPI_UNDEFINED`.

## 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

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

     1) A table is built of colors, and keys (has a next field also).
     2) The tables of all processes are merged using 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 MPI_UNDEFINED, it can return
         a 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.

## 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_Errhandler_set`; 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.

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

Location:comm_split.c