NAME
MPI_Cancel - Cancels a communication request
SYNOPSIS
#include <mpi.h>
int MPI_Cancel(MPI_Request *preq)
INPUT PARAMETER
preq - communication request (handle)
NOTES
Cancel has only been implemented for receive requests; it is a no-op
for send requests. The primary expected use of MPI_Cancel is in multi-
buffering schemes, where speculative MPI_Irecv calls are made. When
the computation completes, some of these receive requests may remain;
using MPI_Cancel allows the user to cancel these unsatisfied requests.
Cancelling a send operation is much more difficult, in large part
because the send will usually be at least partially complete (the
information on the tag, size, and source are usually sent immediately
to the destination). As such, LAM/MPI has not implemented the
canceling of send operations.
NULL HANDLES
The MPI 1.1 specification, in the section on opaque objects, explicitly
disallows freeing a null communicator. The text from the standard is
"A null handle argument is an erroneous IN argument in MPI calls,
unless an exception is explicitly stated in the text that defines the
function. Such exception is allowed for handles to request objects in
Wait and Test calls (sections Communication Completion and Multiple
Completions ). Otherwise, a null handle can only be passed to a
function that allocates a new object and returns a reference to it in
the handle."
NOTES FOR FORTRAN
All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
an additional argument ierr at the end of the argument list. ierr is
an integer and has the same meaning as the return value of the routine
in C. In Fortran, MPI routines are subroutines, and are invoked with
the call statement.
All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in
Fortran.
ERRORS
If an error occurs in an MPI function, the current MPI error handler is
called to handle it. By default, this error handler aborts the MPI
job. The error handler may be changed with MPI_Errhandler_set ; the
predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned (in C and Fortran; this error handler is less
useful in with the C++ MPI bindings. The predefined error handler
MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value
needs to be recovered). Note that MPI does not guarantee that an MPI
program can continue past an error.
All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error
value; C routines as the value of the function and Fortran routines in
the last argument. The C++ bindings for MPI do not return error
values; instead, error values are communicated by throwing exceptions
of type MPI::Exception (but not by default). Exceptions are only
thrown if the error value is not MPI::SUCCESS .
Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI
functions will return upon an error, there will be no way to recover
what the actual error value was.
MPI_SUCCESS
- No error; MPI routine completed successfully.
MPI_ERR_REQUEST
- Invalid MPI_Request ; either null or, MPI_REQUEST_NULL , in
the case of a MPI_Start or MPI_Startall , not a persistent
request.
MPI_ERR_ARG
- Invalid argument. Some argument is invalid and is not
identified by a specific error class. This is typically a NULL
pointer or other such error.
MORE INFORMATION
For more information, please see the official MPI Forum web site, which
contains the text of both the MPI-1 and MPI-2 standards. These
documents contain detailed information about each MPI function (most of
which is not duplicated in these man pages).
http://www.mpi-forum.org/
ACKNOWLEDGEMENTS
The LAM Team would like the thank the MPICH Team for the handy program
to generate man pages ("doctext" from
ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial
formatting, and some initial text for most of the MPI-1 man pages.
LOCATION
cancel.c