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       MPIL_Trace_on -  LAM/MPI-specific function to enable run-time tracing


       #include <mpi.h>
       int MPIL_Trace_on(void)


       These functions give the application the flexibility to generate traces
       only during certain interesting phases of the application's  execution.
       This  technique can considerably reduce the size of the trace files and
       burden of displaying them.

       Both functions are collective over the MPI_COMM_WORLD communicator.  In
       typical  usage,  the -toff option of mpirun (1) would be used to enable
       tracing, but start with the runtime switch in the off position.  At the
       beginning  of  an interesting phase of program execution, MPIL_Trace_on
       would be called.  MPIL_Trace_off would be called after the  interesting
       phase.   Tracing  can  be turned on and off many times.  Each period of
       tracing eventually forms a trace segment in the trace file extracted by
       lamtrace  (1).   If  the on/off functions are never used and tracing is
       enabled with the -ton option of mpirun (1), a single trace  segment  is

       The on/off functions have no effect if tracing is not enabled by mpirun
       (1) with either the -ton or -toff switches.  Thus, an  application  can
       be  littered  with these functions but run without trace collection and
       very little additional overhead due to the no-operation function calls.

       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


       After the volume of generated traces exceeds a preset limit, the oldest
       traces are discarded in  favour  of  new  traces.   Avoiding  discarded
       traces is further incentive to use MPIL_Trace_on and MPIL_Trace_off .


       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


       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.
              - No error; MPI routine completed successfully.
              -  Other  error;  use  MPI_Error_string  to get more information
              about this error code.


       MPI_Trace_off, mpirun