Ubuntu Manpage: MPI_Cancel - Cancels a communication request.

Provided by: openmpi-doc_4.1.2-2ubuntu1_all

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.