Cancels a communication request


#include "mpi.h"
int MPI_Cancel( request )
MPI_Request *request;

Input Parameter

request - 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_Irecvs 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). MPICH will support this once other enhancements are completed; however, users are advised that cancelling a send, while a local operation, is likely to be expensive (usually generating one or more internal messages).

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.

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.


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. Before the value is returned, the current MPI error handler is called. 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. Note that MPI does not guarentee that an MPI program can continue past an error.

MPI_SUCCESS - No error; MPI routine completed successfully.
MPI_ERR_REQUEST - Invalid MPI_Request. Either null or, 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 (e.g., MPI_ERR_RANK).