NAME

       SSL_CTX_set_async_callback, SSL_CTX_set_async_callback_arg, SSL_set_async_callback,
       SSL_set_async_callback_arg, SSL_get_async_status, SSL_async_callback_fn - manage asynchronous operations

SYNOPSIS

        #include <openssl/ssl.h>

        typedef int (*SSL_async_callback_fn)(SSL *s, void *arg);
        int SSL_CTX_set_async_callback(SSL_CTX *ctx, SSL_async_callback_fn callback);
        int SSL_CTX_set_async_callback_arg(SSL_CTX *ctx, void *arg);
        int SSL_set_async_callback(SSL *s, SSL_async_callback_fn callback);
        int SSL_set_async_callback_arg(SSL *s, void *arg);
        int SSL_get_async_status(SSL *s, int *status);

DESCRIPTION

       SSL_CTX_set_async_callback() sets an asynchronous callback function. All SSL objects generated based on
       this SSL_CTX will get this callback. If an engine supports the callback mechanism, it will be
       automatically called if SSL_MODE_ASYNC has been set and an asynchronous capable engine completes a
       cryptography operation to notify the application to resume the paused work flow.

       SSL_CTX_set_async_callback_arg() sets the callback argument.

       SSL_set_async_callback() allows an application to set a callback in an asynchronous SSL object, so that
       when an engine completes a cryptography operation, the callback will be called to notify the application
       to resume the paused work flow.

       SSL_set_async_callback_arg() sets an argument for the SSL object when the above callback is called.

       SSL_get_async_status() returns the engine status. This function facilitates the communication from the
       engine to the application. During an SSL session, cryptographic operations are dispatched to an engine.
       The engine status is very useful for an application to know if the operation has been successfully
       dispatched. If the engine does not support this additional callback method, ASYNC_STATUS_UNSUPPORTED will
       be returned. See ASYNC_WAIT_CTX_set_status() for a description of all of the status values.

       An example of the above functions would be the following:

       1.  Application   sets   the   async  callback  and  callback  data  on  an  SSL  connection  by  calling
           SSL_set_async_callback().

       2.  Application sets SSL_MODE_ASYNC and makes an asynchronous SSL call

       3.  OpenSSL submits the asynchronous request to the engine. If a retry occurs  at  this  point  then  the
           status  within  the ASYNC_WAIT_CTX would be set and the async callback function would be called (goto
           Step 7).

       4.  The OpenSSL engine pauses the  current  job  and  returns,  so  that  the  application  can  continue
           processing other connections.

       5.  At  a  future  point  in  time (probably via a polling mechanism or via an interrupt) the engine will
           become aware that the asynchronous request has finished processing.

       6.  The engine will call the application's callback passing the callback data as a parameter.

       7.  The callback function should then run. Note: it is a requirement that the callback function is  small
           and nonblocking as it will be run in the context of a polling mechanism or an interrupt.

       8.  It  is  the  application's responsibility via the callback function to schedule recalling the OpenSSL
           asynchronous function and to continue processing.

       9.  The callback function has the option to check  the  status  returned  via  SSL_get_async_status()  to
           determine  whether  a  retry  happened  instead  of  the  request being submitted, allowing different
           processing if required.

RETURN VALUES

       SSL_CTX_set_async_callback(),         SSL_set_async_callback(),         SSL_CTX_set_async_callback_arg(),
       SSL_CTX_set_async_callback_arg() and SSL_get_async_status() return 1 on success or 0 on error.

SEE ALSO

       ssl(7)

HISTORY

       SSL_CTX_set_async_callback(),         SSL_CTX_set_async_callback_arg(),         SSL_set_async_callback(),
       SSL_set_async_callback_arg() and SSL_get_async_status() were first added to OpenSSL 3.0.

COPYRIGHT

       Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file  except  in  compliance
       with  the  License.   You  can  obtain  a  copy  in  the  file  LICENSE  in the source distribution or at
       <https://www.openssl.org/source/license.html>.

3.0.13                                             2025-02-05                       SSL_SET_ASYNC_CALLBACK(3SSL)