NAME

       MPIL_Comm_id -  LAM/MPI-specific function to return the context ID of the communicator

SYNOPSIS

       #include <mpi.h>
       int
       MPIL_Comm_id(MPI_Comm comm, int *pcid)

INPUT PARAMETER

       comm   - communicator (handle)

OUTPUT PARAMETER

       pcid   - communicator context id (integer)

NOTES

       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
       synchronization 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)).

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

       #if LAM_MPI
       int cid;
       MPIL_Comm_id(MPI_COMM_WORLD, &cid);
       #endif

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.

ERRORS

       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
       exceptions 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.
       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_ARG
              - 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.

SEE ALSO

       MPIL_Comm_gps, MPIL_Type_id

LOCATION

       mpil_id.c