#include "mpi.h" int MPI_Cancel( MPI_Request *request )
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 of version 1.2.0, MPICH supports cancelling of sends. Users are advised that cancelling a send, while a local operation (as defined by the MPI standard), is likely to be expensive (usually generating one or more internal messages).
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.
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.