NAME

       MPI_Cancel -  Cancels a communication request

SYNOPSIS

       int MPI_Cancel(MPI_Request *request)

INPUT PARAMETERS

       request
              - communication request (handle)

NOTES

       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).

THREAD AND INTERRUPT SAFETY

       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.

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.

NULL HANDLES

       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.

ERRORS

       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.

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

                                                   11/12/2016                                      MPI_Cancel(3)