NAME

       MPI_Cancel - Cancels a communication request.

SYNTAX

C Syntax

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

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_CANCEL(REQUEST, IERROR)
            INTEGER   REQUEST, IERROR

C++ Syntax

       #include <mpi.h>
       void Request::Cancel() const

INPUT PARAMETER

       request   Communication request (handle).

OUTPUT PARAMETER

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       The  MPI_Cancel  operation  allows pending communications to be canceled. This is required
       for cleanup. Posting a send or a receive ties up user resources (send or receive buffers),
       and a cancel may be needed to free these resources gracefully.

       A call to MPI_Cancel marks for cancellation a pending, nonblocking communication operation
       (send or receive). The cancel call is local. It returns immediately, possibly  before  the
       communication is actually canceled. It is still necessary to complete a communication that
       has been marked for cancellation, using a call to MPI_Request_free, MPI_Wait, or  MPI_Test
       (or any of the derived operations).

       If   a  communication  is  marked  for  cancellation,  then  an  MPI_Wait  call  for  that
       communication is guaranteed to return, irrespective of the activities of  other  processes
       (i.e.,  MPI_Wait  behaves as a local function); similarly if MPI_Test is repeatedly called
       in a busy wait loop for  a  canceled  communication,  then  MPI_Test  will  eventually  be
       successful.

       MPI_Cancel  can  be  used  to  cancel  a communication that uses a persistent request (see
       Section 3.9 in the MPI-1 Standard, "Persistent Communication Requests") in the same way it
       is  used  for  nonpersistent  requests.  A  successful  cancellation  cancels  the  active
       communication, but not the request itself. After the call to MPI_Cancel and the subsequent
       call  to MPI_Wait or MPI_Test, the request becomes inactive and can be activated for a new
       communication.

       The successful cancellation of a buffered send frees the  buffer  space  occupied  by  the
       pending message.

       Either the cancellation succeeds or the communication succeeds, but not both. If a send is
       marked for cancellation, then it must be the case that either the send completes normally,
       in which case the message sent is received at the destination process, or that the send is
       successfully canceled,  in  which  case  no  part  of  the  message  is  received  at  the
       destination.  Then, any matching receive has to be satisfied by another send. If a receive
       is marked for cancellation, then it must be the case that  either  the  receive  completes
       normally,  or  that  the  receive  is  successfully canceled, in which case no part of the
       receive buffer is altered. Then, any matching send has to be satisfied by another receive.

       If the operation has been canceled, then information to that effect will  be  returned  in
       the status argument of the operation that completes the communication.

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 requests  may  remain;
       using MPI_Cancel allows the user to cancel these unsatisfied requests.

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran routines in the last argument. C++ functions do not return errors. If the  default
       error  handler  is  set  to  MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
       mechanism will be used to throw an MPI::Exception object.

       Before the error value is returned, the current MPI error handler is called.  By  default,
       this  error  handler aborts the MPI job, except for I/O function errors. The error handler
       may   be   changed   with   MPI_Comm_set_errhandler;   the   predefined   error    handler
       MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
       guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Probe
       MPI_Iprobe
       MPI_Test_cancelled
       MPI_Cart_coords