rq - request (handle)
rq - request is set to MPI_REQUEST_NULL (handle)
The MPI_Request_free() function is used to free a request, rq that is no longer required. Normally, a call to a function similar to MPI_Wait() will free a request when it is complete. However, a persistent request can only be freed by this function.
If the status of a request is not required, then this function may be used to dispose of a request handle. Note that some other sychronization method may then need to be used to determine when that request is completed and buffers allowable to be reused.
If the request is successfully freed, then it will be set to MPI_REQUEST_NULL.
The request must be a valid one, not MPI_REQUEST_NULL. PMPI_Request_free() 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.
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