BIO_PUSH(3SSL)

✨ BIO_PUSH 💅 (okay so it's BIO_push, BIO_pop, BIO_set_next, and it lives in OpenSSL's section 3SSL bestie)

🌸 NAME

BIO_push, BIO_pop, BIO_set_next — the lil trio that adds and removes BIOs from a chain 🦋

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💖 SYNOPSIS

#include <openssl/bio.h>

BIO *BIO_push(BIO *b, BIO *next);
BIO *BIO_pop(BIO *b);
void BIO_set_next(BIO *b, BIO *next);

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👑 DESCRIPTION

okay so let's get into it 💅

BIO_push() pushes b on next. If b is NULL, bestie, the function literally does nothing and just returns next — no drama, no notes 🫶. Otherwise it prepends b (which may be a single BIO or a whole chain of BIOs, she can be either, main character energy ✨) to next — unless next is NULL, in which case, well, anyway. Then it makes a control call on b and returns b 🎀

BIO_pop() removes the BIO b from whatever chain she's part of. If b is NULL the function does nothing and returns NULL 💀. Otherwise it makes a control call on b and returns the next BIO in the chain, or NULL if there is no next BIO. The removed BIO becomes a single BIO with no association with the original chain — so she can be freed, or she can go join a totally different chain. Glow up, honestly 🌈

BIO_set_next() replaces the existing next BIO in a chain with the BIO pointed to by next. The new chain may include some of the same BIOs from the old chain, or it may be completely different. Mix and match, it's giving customization 🎀

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⭐ NOTES

ngl the names of these functions are perhaps a little misleading 😭 BIO_push() joins two BIO chains whereas BIO_pop() deletes a single BIO from a chain — and the deleted BIO does not need to be at the end of a chain. She can leave from the middle, icon behaviour 💅

The process of calling BIO_push() and BIO_pop() on a BIO may have additional consequences (a control call is made to the affected BIOs). Any effects will be noted in the descriptions of individual BIOs, so go read those, iykyk 📖

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🫶 RETURN VALUES

• BIO_push() returns the head of the chain, which usually is b, or next if b is NULL.
• BIO_pop() returns the next BIO in the chain, or NULL if there is no next BIO.

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🔥 EXAMPLES

For these examples suppose md1 and md2 are digest BIOs, b64 is a base64 BIO and f is a file BIO. (Yes, those names are exactly as iconic as they sound ✨)

If the call:

BIO_push(b64, f);

is made, then the new chain will be b64-f. After making the calls:

BIO_push(md2, b64);
BIO_push(md1, md2);

the new chain is md1-md2-b64-f. Data written to md1 will be digested by md1 and md2, base64 encoded, and finally written to f. A whole pipeline, no cap 🧚

It should be noted that reading causes data to pass in the reverse direction, that is data is read from f, base64 decoded, and digested by md2 and then md1. Reversible? We love to see it 💜

The call:

BIO_pop(md2);

will return b64 and the new chain will be md1-b64-f. Data can be written to and read from md1 as before, except that md2 will no more be applied. Bye md2, you ate while you were here 🙈

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🌈 SEE ALSO

• bio(7)

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💀 HISTORY

The BIO_set_next() function was added in OpenSSL 1.1.0. Vintage, lowkey 🎀

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📜 COPYRIGHT

Copyright 2000-2026 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 whole page, bestie 🫶 this was version 4.0.3 from 2026-09-29 — yeah, we're fancy, we named ourselves BIO_PUSH(3SSL) with the manual name folded right on in 💅 slay, go build a chain ✨🦋🌸