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
- 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