<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML3.2 EN"> <HTML> <HEAD> <META NAME="GENERATOR" CONTENT="DOCTEXT"> <TITLE>MPI_Comm_split</TITLE> </HEAD> <BODY BGCOLOR="FFFFFF"> <A NAME="MPI_Comm_split"><H1>MPI_Comm_split</H1></A> Creates new communicators based on colors and keys <H2>Synopsis</H2> <PRE> int MPI_Comm_split(MPI_Comm comm, int color, int key, MPI_Comm *newcomm) </PRE> <H2>Input Parameters</H2> <DL> <DT><B>comm </B><DD>communicator (handle) <DT><B>color </B><DD>control of subset assignment (nonnegative integer). Processes with the same color are in the same new communicator <DT><B>key </B><DD>control of rank assigment (integer) </DL> <P> <H2>Output Parameter</H2> <DL><DT><B>newcomm </B> <DD> new communicator (handle) </DL> <P> <H2>Notes</H2> The <TT>color</TT> must be non-negative or <TT>MPI_UNDEFINED</TT>. <P> <H2>Thread and Interrupt Safety</H2> <P> 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 <TT>malloc </TT>or other non-MPICH runtime routines that are themselves not interrupt-safe. <P> <H2>Notes for Fortran</H2> All MPI routines in Fortran (except for <TT>MPI_WTIME</TT> and <TT>MPI_WTICK</TT>) have an additional argument <TT>ierr</TT> at the end of the argument list. <TT>ierr </TT>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 <TT>call</TT> statement. <P> All MPI objects (e.g., <TT>MPI_Datatype</TT>, <TT>MPI_Comm</TT>) are of type <TT>INTEGER </TT>in Fortran. <P> <H2>Algorithm</H2> <PRE> 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 <TT>MPI_UNDEFINED</TT> as the color, create a process with a single member. 3. Use key to order the ranks 4. Set the VCRs using the ordered key values </PRE> <P> <H2>Errors</H2> <P> All MPI routines (except <TT>MPI_Wtime</TT> and <TT>MPI_Wtick</TT>) 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 <TT>MPI_Comm_set_errhandler</TT> (for communicators), <TT>MPI_File_set_errhandler</TT> (for files), and <TT>MPI_Win_set_errhandler</TT> (for RMA windows). The MPI-1 routine <TT>MPI_Errhandler_set</TT> may be used but its use is deprecated. The predefined error handler <TT>MPI_ERRORS_RETURN</TT> may be used to cause error values to be returned. Note that MPI does <EM>not</EM> guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. <P> <DL><DT><B>MPI_SUCCESS </B> <DD> No error; MPI routine completed successfully. </DL> <DL><DT><B>MPI_ERR_COMM </B> <DD> Invalid communicator. A common error is to use a null communicator in a call (not even allowed in <TT>MPI_Comm_rank</TT>). </DL> <DL><DT><B>MPI_ERR_INTERN </B> <DD> This error is returned when some part of the MPICH implementation is unable to acquire memory. </DL> <P> <H2>See Also</H2> MPI_Comm_free <BR><P><B>Location:</B>comm_split.c<P> </BODY></HTML>