MPI_Cancel(3OpenMPI) MPI_Cancel(3OpenMPI)
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 cancel-lation, cancellation,
lation, 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 guar-anteed guaranteed
anteed 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 commu-nication, communication,
nication, 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 mes-sage. message.
sage.
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
Open MPI 1.2 September 2006 MPI_Cancel(3OpenMPI)
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