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MPIL_Comm_gps(3)			     LAM/MPI				 MPIL_Comm_gps(3)

       MPIL_Comm_gps  -   LAM/MPI-specific function to return the GPS information of a given rank
       from a communicator

       #include <mpi.h>
       int MPIL_Comm_gps(MPI_Comm comm, int rank, int *pnid,
	   int *ppid)

       comm   - communicator (handle)
       rank   - rank of process to query (integer)

       pnid   - LAM node ID
       ppid   - LAM process ID

       In the LAM implementation of MPI, each communicator has an integer context  ID  associated
       with  it  for synchronizing on different contexts.  This ID is global to all the processes
       in the communicator's group, and uniquely identifies that communicator for  each  process.
       These properties allow the processes to safely exchange messages without interference from
       operations on other communicators.

       The MPI standard does not provide a way to access/view this implementation-dependent  syn-
       chronization  since  communicators  are opaque objects.	Users do not need such access for
       normal MPI operations.  On the other hand, when debugging  MPI  applications,  the  opaque
       nature  of  communicators  hinders  the	user's efforts.  This is especially true on fully
       observable systems such as LAM, where users can monitor the full state  of  the	processes
       and message queues, which includes the context ID (see mpitask (1) and mpimsg (1)).

       LAM supplementary functions do not operate with communicators and ranks but with nodes and
       process identifiers.  MPIL_Comm_gps accepts the MPI values and returns the LAM values.  In
       the  case  of  an  intercommunicator the values returned are those of the process with the
       given rank in the remote group.

       This is a LAM/MPI-specific function and is intended mainly for debugging.  If  this  func-
       tion is used, it should be used in conjunction with the LAM_MPI C preprocessor macro

       #if LAM_MPI
       int nid, pid;
       MPIL_Comm_gps(MPI_COMM_WORLD, 0, &nid, &pid);

       All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have an additional argu-
       ment 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.

       If an error occurs in an MPI function, the current MPI error handler is called  to  handle
       it.   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 (in C and Fortran; this error handler is less useful in
       with the C++ MPI bindings.   The  predefined  error  handler  MPI::ERRORS_THROW_EXCEPTIONS
       should  be  used in C++ if the error value needs to be recovered).  Note that MPI does not
       guarantee that an MPI program can continue past an error.

       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.  The C++ bindings for
       MPI do not return error values; instead, error values are communicated by throwing  excep-
       tions  of  type	MPI::Exception	(but  not by default).	Exceptions are only thrown if the
       error value is not MPI::SUCCESS

       Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return
       upon an error, there will be no way to recover what the actual error value was.
	      - No error; MPI routine completed successfully.
	      -  Invalid  communicator.   A  common error is to use a null communicator in a call
	      (not even allowed in MPI_Comm_rank ).
	      - Invalid source or destination rank.  Ranks must be between zero and the  size  of
	      the  communicator  minus	one;  ranks  in  a  receive  (	MPI_Recv  ,  MPI_Irecv	,
	      MPI_Sendrecv , etc.) may also be MPI_ANY_SOURCE
	      - Invalid argument.  Some argument is invalid and is not identified by  a  specific
	      error class.  This is typically a NULL pointer or other such error.

       MPIL_Comm_id, MPIL_Type_id


LAM/MPI 6.5.8				    11/10/2002				 MPIL_Comm_gps(3)
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