SSL_SET_BIO(3ossl)

💖 SSL_set_bio — the "adopt a BIO" manual 💖

okay bestie, welcome to SSL_SET_BIO(3ossl), the OpenSSL page that's literally just about handing your BIOs over to an SSL object and praying you got the ownership math right. spoiler: you're gonna want to use the set0 functions and not the big one. let's get into it 🌸

---

✨ NAME ✨

SSL_set_bio, SSL_set0_rbio, SSL_set0_wbio — connect the SSL object with a BIO 🫶

---

✨ SYNOPSIS ✨

#include <openssl/ssl.h>

void SSL_set_bio(SSL *ssl, BIO *rbio, BIO *wbio);
void SSL_set0_rbio(SSL *s, BIO *rbio);
void SSL_set0_wbio(SSL *s, BIO *wbio);

---

✨ DESCRIPTION ✨

🎀 the set0 girlies

SSL_set0_rbio() connects the BIO rbio for the read operations of the ssl object. the SSL engine inherits the behaviour of rbio — so if the BIO is nonblocking, then the ssl object is gonna be nonblocking too. it's giving inherited trauma but make it I/O 💅

this function transfers ownership of rbio to ssl. it gets automatically freed using BIO_free_all(3) when the ssl is freed. and the moment you call this function, any existing rbio that was previously set also gets freed via a call to BIO_free_all(3) — and yes, that includes the case where the rbio is set to the same value as previously. so don't be cute and re-set the same pointer thinking it's a no-op, because it is NOT 🙈

if you're using a custom BIO, rbio must implement either BIO_meth_set_read_ex(3) or BIO_meth_set_read(3).

SSL_set0_wbio() works in the same way as SSL_set0_rbio() except that it connects the BIO wbio for the write operations of the ssl object. and here's the plot twist, bestie: if the rbio and wbio are the same, then SSL_set0_rbio() and SSL_set0_wbio() each take ownership of one reference. so it may be necessary to increment the number of references available using BIO_up_ref(3) before calling the set0 functions. count your refs like you count your blessings, fr fr 💀

if you're using a custom BIO, wbio must implement BIO_meth_set_write_ex(3) or BIO_meth_set_write(3). it additionally must implement BIO_flush(3) using BIO_CTRL_FLUSH and BIO_meth_set_ctrl(3). and if flushing is unnecessary with wbio, then BIO_flush(3) should return one and do nothing. no notes, iconic behaviour ✨

🦋 the main character: SSL_set_bio()

SSL_set_bio() is similar to SSL_set0_rbio() and SSL_set0_wbio() except that it connects both the rbio and the wbio at the same time, and transfers the ownership of rbio and wbio to ssl according to the following set of rules 👑:

• If neither the rbio or wbio have changed from their previous values then nothing is done.
• If the rbio and wbio parameters are different and both are different to their previously set values then one reference is consumed for the rbio and one reference is consumed for the wbio.
• If the rbio and wbio parameters are the same and the rbio is not the same as the previously set value then one reference is consumed.
• If the rbio and wbio parameters are the same and the rbio is the same as the previously set value, then no additional references are consumed.
• If the rbio and wbio parameters are different and the rbio is the same as the previously set value then one reference is consumed for the wbio and no references are consumed for the rbio.
• If the rbio and wbio parameters are different and the wbio is the same as the previously set value and the old rbio and wbio values were the same as each other then one reference is consumed for the rbio and no references are consumed for the wbio.
• If the rbio and wbio parameters are different and the wbio is the same as the previously set value and the old rbio and wbio values were different to each other, then one reference is consumed for the rbio and one reference is consumed for the wbio.

🚨 because of this complexity, this function should be avoided; use SSL_set0_rbio() and SSL_set0_wbio() instead. like, genuinely. that list is a whole vibe check and nobody passes it.

🌈 one more thing, QUIC edition

where a new BIO is set on a QUIC connection SSL object, blocking mode will be disabled on that SSL object if the BIO cannot support blocking mode. and if another BIO is subsequently set on the SSL object which can support blocking mode, blocking mode will not be automatically re-enabled. so it stays off, respectfully. for more information, see SSL_set_blocking_mode(3).

---

✨ RETURN VALUES ✨

SSL_set_bio(), SSL_set0_rbio() and SSL_set0_wbio() cannot fail. no return values to check, no error handling arc, just vibes 💜

---

✨ SEE ALSO ✨

SSL_get_rbio(3), SSL_connect(3), SSL_accept(3), SSL_shutdown(3), ssl(7), bio(7)

---

✨ HISTORY ✨

SSL_set0_rbio() and SSL_set0_wbio() were added in OpenSSL 1.1.0. glow up happened, periodt 🎀

---

✨ COPYRIGHT ✨

Copyright 2000-2023 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>.

---

okay that's the whole page bestie, you're officially a BIO-ownership expert 🧚 this was OpenSSL 3.5.9, dated 2026-09-29, and it's the SSL_SET_BIO(3ossl) page — go forth and BIO_up_ref(3) responsibly, mwah 💅💖