MPI_Cancel(3) Cancels a communication request


#include <mpi.h>
int MPI_Cancel(MPI_Request *preq)


- communication request (handle)


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.


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."


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.


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.

- No error; MPI routine completed successfully.
- Invalid MPI_Request ; either null or, MPI_REQUEST_NULL , in the case of a MPI_Start or MPI_Startall , not a persistent request.
- 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.


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).


The LAM Team would like the thank the MPICH Team for the handy program to generate man pages ("doctext" from ), the initial formatting, and some initial text for most of the MPI-1 man pages.