SilcBool silc_async_init(SilcAsyncOperation op,
                             SilcAsyncOperationAbort abort_cb,
                             SilcAsyncOperationPause pause_cb,
                             void *context);

    Initializes and starts a pre-allocated asynchronous operation context,
    and assigns `abort_cb' callback for it, which can be used by some upper
    layer to abort the asynchronous operation, by calling the
    silc_async_abort.  Since this use pre-allocated context, the function
    silc_async_free need not be called.  This function is equivalent
    to silc_async_alloc except this does not allocate any memory.  The `op'
    needs not be uninitialized.

    If the `pause_cb' is provided then the upper layer may also halt and
    then later resume the execution of the operation, by calling the
    silc_async_halt and silc_async_resume respectively.  If `pause_cb' is
    not provided then these functions has no effect for this operation.