SSL_SET_ASYNC_CALLBACK(3ossl)

💅 NAME 💅

Okay so this is the 3ossl manual page for SSL_SET_ASYNC_CALLBACK, and it's basically the group chat for asynchronous operations. The whole squad is here: SSL_CTX_set_async_callback, SSL_CTX_set_async_callback_arg, SSL_set_async_callback, SSL_set_async_callback_arg, SSL_get_async_status, and the typedef SSL_async_callback_fn. Together they 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 🌸

Okay bestie, here's the tea, function by function:

• 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. Simple as that. 🎀
• 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. That's the list exactly as the page gives it, bestie. 💅

🦋 SEE ALSO 🦋

ssl(7) — that's the one. 💖

🌈 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. Periodt. 👑

👑 COPYRIGHT 👑

Copyright 2019-2020 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.

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Okay that's the whole page, bestie. This tea is from OpenSSL 3.5.0, dated 2026-09-25, and it's the SSL_SET_ASYNC_CALLBACK(3ossl) manual. Mwah 💋✨