rq - request (handle)
The MPI_Cancel() function is used to attempt to cancel a pending request, rq .
If the request is already complete, then nothing happens, and this function will return MPI_SUCCESS. If the request is still in progress an attempt is made to cancel it. If cancellation succeeds, then the request will complete with a status of MPI_ERR_CANCEL. This may be tested with the MPI_Test_cancelled() function.
A successfully cancelled send is not ever successfully matched to a receive at the destination. A successfully cancelled receive never matches any messages.
Note that some in-progress requests are impossible to cancel. MPI_Cancel() will silently fail, returning MPI_SUCCESS even though those requests will complete successfully.
The request must be a valid one, not MPI_REQUEST_NULL. PMPI_Cancel() is the profiling version of this function.
All MPI routines except for MPI_Wtime and MPI_Wtick return an error code. The the current MPI error handler is invoked if the return value is not MPI_SUCCESS. The default error handler aborts, but this may be changed with by using the MPI_Errhandler_set() function. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarentee that an MPI program can continue past an error. In this implementation, all errors except MPI_ERR_INTERN or MPI_ERR_OTHER should always be recoverable.
In C, the error code is passed as the return value. In FORTRAN, all functions have an parameter ierr which returns the error code. MPI C++ functions do not directly return an error code. However, C++ users may want to use the MPI::ERRORS_THROW_EXCEPTIONS handler. This will throw an MPI::Exception with the corresponding error code. To prevent exceptions from being raised from within C and Fortran code, they will see all error return values as MPI_ERR_PENDING when this handler is chosen. In this implementation, call MPI::throw_exception() to throw the correct exception if this occurs.
MPI_SUCCESS - No error;
MPI_ERR_PENDING - Pending exception;
MPI_ERR_REQUEST - Invalid request;
MPI_ERR_INTERN - Out of Memory.
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