oracular (3) MPI_Cancel.openmpi.3.gz

NAME

       MPI_Cancel - Cancels a communication request.

SYNTAX

C Syntax

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

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_CANCEL(REQUEST, IERROR)
            INTEGER   REQUEST, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Cancel(request, ierror)
            TYPE(MPI_Request), INTENT(IN) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: 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