MPI_Cancel(3)                      Open MPI                      MPI_Cancel(3)



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




1.4.1                            Jan 14, 2010                    MPI_Cancel(3)
