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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 com-
munication operation (send or receive). The cancel call is local. It
returns immediately, possibly before the communication is actually can-
celed. 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 func-
tion); similarly if MPI_Test is repeatedly called in a busy wait loop
for a canceled communication, then MPI_Test will eventually be success-
ful.
MPI_Cancel can be used to cancel a communication that uses a persistent
request (see Section 3.9 in the MPI-1 Standard, "Persistent Communica-
tion 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
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 suc-
cessfully 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++ func-
tions 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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