SSL_CTX_SET_READ_AHEAD(3ossl)

💅 NAME 💅

okay so this whole page is about SSL_CTX_set_read_ahead, SSL_CTX_get_read_ahead, SSL_set_read_ahead, SSL_get_read_ahead, and SSL_CTX_get_default_read_ahead — aka the little squad that manages whether to read as many input bytes as possible. that's it, that's the vibe. ✨

🌸 SYNOPSIS 🌸

#include <openssl/ssl.h>

void SSL_set_read_ahead(SSL *s, int yes);
int SSL_get_read_ahead(const SSL *s);

SSL_CTX_set_read_ahead(SSL_CTX *ctx, int yes);
long SSL_CTX_get_read_ahead(SSL_CTX *ctx);
long SSL_CTX_get_default_read_ahead(SSL_CTX *ctx);

🦋 DESCRIPTION 🦋

okay bestie, here's the tea. SSL_CTX_set_read_ahead() and SSL_set_read_ahead() set whether we should read as many input bytes as possible (this is a thing for nonblocking reads) or not. 🫶

like, picture this: if x bytes are currently required by OpenSSL, but y bytes are available from the underlying BIO (where y > x), then OpenSSL will read all y bytes into its buffer (providing that the buffer is large enough) if reading ahead is on, or x bytes otherwise. literally that simple. 💅

setting the parameter yes to 0 turns reading ahead off, and any other values turn it on. ✨

also, SSL_CTX_set_default_read_ahead() is identical to SSL_CTX_set_read_ahead(). no cap.

SSL_CTX_get_read_ahead() and SSL_get_read_ahead() tell you whether reading ahead has been set or not. and SSL_CTX_get_default_read_ahead() is identical to SSL_CTX_get_read_ahead(). twins fr fr. 👯

🚨 QUIC check: these functions cannot be used with QUIC SSL objects. and SSL_set_read_ahead() has no effect if called on a QUIC SSL object. so don't even try it. 💀

⭐ NOTES ⭐

okay important stuff, listen up:

• these functions have no impact when used with DTLS. also the return values for SSL_CTX_get_read_head() and SSL_get_read_ahead() are undefined for DTLS. (yes, that's get_read_head with the typo — copying it exactly, bestie, don't come for me 🙈)
• setting read_ahead can impact the behaviour of the SSL_pending() function (see SSL_pending(3)).
• since SSL_read() can return SSL_ERROR_WANT_READ for non-application data records, and SSL_has_pending() can't tell the difference between processed and unprocessed data, it's recommended that if read ahead is turned on, then SSL_MODE_AUTO_RETRY is not turned off using SSL_CTX_clear_mode(). that will prevent getting SSL_ERROR_WANT_READ when there is still a complete record available that hasn't been processed. 🔥
• if the application wants to continue to use the underlying transport (e.g. TCP connection) after the SSL connection is finished using SSL_shutdown(), reading ahead should be turned off. otherwise the SSL structure might read data that it shouldn't. 😭 so like... turn it off, okay?

👑 RETURN VALUES 👑

SSL_get_read_ahead() and SSL_CTX_get_read_ahead() return 0 if reading ahead is off, and non zero otherwise. slay, that's the whole return section. ✨

🌈 SEE ALSO 🌈

ssl(7), SSL_pending(3)

🎀 COPYRIGHT 🎀

Copyright 2015-2023 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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and that's the page, bestie 💖 this iconic little manual page is from OpenSSL 3.5.0, dated 2026-09-25, and it still answers to the name SSL_CTX_SET_READ_AHEAD(3ossl). mwah 💋