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       MPI_Cancel -  Cancels a communication request


       #include <mpi.h>
       int MPI_Cancel(MPI_Request *preq)


       preq   - communication request (handle)


       Cancel  has  only  been implemented for receive requests; it is a no-op
       for send requests.  The primary expected use of MPI_Cancel is in multi-
       buffering  schemes,  where  speculative MPI_Irecv calls are made.  When
       the computation completes, some of these receive requests  may  remain;
       using  MPI_Cancel allows the user to cancel these unsatisfied requests.

       Cancelling a send operation is  much  more  difficult,  in  large  part
       because  the  send  will  usually  be  at least partially complete (the
       information on the tag, size, and source are usually  sent  immediately
       to  the  destination).   As  such,  LAM/MPI  has  not  implemented  the
       canceling of send operations.


       The MPI 1.1 specification, in the section on opaque objects, explicitly
       disallows freeing a null communicator.  The text from the standard is

       "A null handle argument is an erroneous IN argument in MPI calls,
       unless an exception is explicitly stated in the text that defines the
       function. Such exception is allowed for handles to request objects in
       Wait and Test calls (sections Communication Completion and Multiple
       Completions ). Otherwise, a null handle can only be passed to a
       function that allocates a new object and returns a reference to it in
       the handle."


       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.
              -  Invalid  MPI_Request  ; either null or, MPI_REQUEST_NULL , in
              the case of a MPI_Start  or  MPI_Startall  ,  not  a  persistent
              -  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.


       For more information, please see the official MPI Forum web site, which
       contains the text  of  both  the  MPI-1  and  MPI-2  standards.   These
       documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).


       The LAM Team would like the thank the MPICH Team for the handy  program
       to        generate        man        pages        ("doctext"       from    ),    the     initial
       formatting, and some initial text for most of the MPI-1 man pages.