/p>

<p><b>ASYNC_WAIT_CTX</b>s also have a &quot;callback&quot; mechanism to notify applications. The callback is set by an application, and it will be automatically called when an engine completes a cryptography operation, so that the application can resume the paused work flow without polling. An engine could be written to look whether the callback has been set. If it has then it would use the callback mechanism in preference to the file descriptor notifications. If a callback is not set then the engine m