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       MPI_Cancel -  Cancels a communication request


       int MPI_Cancel(MPI_Request * request)


              - communication request (handle)


       The  primary  expected  use of MPI_Cancel is in multi-buffering schemes, where speculative
       MPI_Irecvs 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).  Users are advised that cancelling a send,
       while a local operation (as defined by the  MPI  standard),  is  likely  to  be  expensive
       (usually generating one or more internal messages).


       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
       malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.


       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.


       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 (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.  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  MPI_Comm_set_errhandler (for
       communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler  (for  RMA
       windows).   The  MPI-1  routine  MPI_Errhandler_set may be used but its use is deprecated.
       The predefined error handler MPI_ERRORS_RETURN may be used to cause  error  values  to  be
       returned.   Note  that  MPI  does  not  guarentee that an MPI program can continue past an
       error; however, MPI implementations will attempt to continue whenever possible.

              - No error; MPI routine completed successfully.
              - Invalid MPI_Request .  Either null or, in the case of a MPI_Start or MPI_Startall
              , not a persistent request.
              -  Invalid  argument.  Some argument is invalid and is not identified by a specific
              error class (e.g., MPI_ERR_RANK ).

                                            11/21/2018                              MPI_Cancel(3)