BIO_F_BUFFER(3ossl)

💅 BIO_F_BUFFER — the buffering BIO, bestie edition ✨

okay so this is a page from the OpenSSL manual — BIO_F_BUFFER(3ossl), and it's literally all about the buffering BIO, the one that goes "bestie hold my data for a sec" 🫶. Sit down, grab a matcha, we're getting into it.

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✨ NAME ✨

BIO_get_buffer_num_lines, BIO_set_read_buffer_size, BIO_set_write_buffer_size, BIO_set_buffer_size, BIO_set_buffer_read_data, BIO_f_buffer — the whole buffering BIO fam 💖

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🌸 SYNOPSIS 🌸

#include <openssl/bio.h>

const BIO_METHOD *BIO_f_buffer(void);

long BIO_get_buffer_num_lines(BIO *b);
long BIO_set_read_buffer_size(BIO *b, long size);
long BIO_set_write_buffer_size(BIO *b, long size);
long BIO_set_buffer_size(BIO *b, long size);
long BIO_set_buffer_read_data(BIO *b, void *buf, long num);

copy it exactly, no cap, that's the whole cast of characters 💅

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🎀 DESCRIPTION 🎀

BIO_f_buffer() hands you back the buffering BIO method. That's it, that's the vibe. ✨

Data you write to a buffering BIO gets buffered (obviously) and then periodically yeeted onward to the next BIO in the chain. Data you read from a buffering BIO comes from an internal buffer, which itself gets filled up from the next BIO in the chain. Both BIO_gets() and BIO_puts() are supported — iconic, honestly 👑

If you call BIO_reset() on a buffering BIO, it clears out any buffered data. bye bye data 💀

BIO_get_buffer_num_lines() tells you how many lines are currently buffered. Like a little line counter, so useful.

BIO_set_read_buffer_size(), BIO_set_write_buffer_size() and BIO_set_buffer_size() set the read, the write, or both the read and write buffer sizes to size. The initial buffer size is DEFAULT_BUFFER_SIZE, which is currently 4096. Okay and listen — any attempt to shrink the buffer below DEFAULT_BUFFER_SIZE is ignored, like the function simply will not hear it 🙈. Also: any buffered data gets cleared when the buffer is resized. gone. vanished. periodt.

BIO_set_buffer_read_data() clears the read buffer and fills it with num bytes of buf. And if num is larger than the current buffer size, the buffer gets expanded — main character energy, it grows 🌱.

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🦋 NOTES 🦋

All of these functions, except BIO_f_buffer(), are implemented as macros. iykyk — that means no function pointer shenanigans for you.

Buffering BIOs implement BIO_read_ex() and BIO_gets() by doing BIO_read_ex() operations on the next BIO in the chain and stashing the result in an internal buffer, then handing bytes back to the caller as appropriate for the call. So: a BIO_gets() is guaranteed to give the caller a whole line, and a BIO_read_ex() is guaranteed to give the caller the number of bytes it asks for — unless there's an error or end of communication is reached in the next BIO 😭. Which means by prepending a buffering BIO to a chain, you can provide BIO_gets() or exact-size BIO_read_ex() functionality even if the following BIOs don't support it. glow-up via proxy, honestly 💅

⚠️ Do NOT add more than one BIO_f_buffer() to a BIO chain. fr fr. If you do, the result is a full read of the size of the internal buffer of the top BIO_f_buffer(), which is 4 KiB at a minimum. not the move 🙅‍♀️

Data is only written to the next BIO in the chain when the write buffer fills up, or when BIO_flush() is called. So it's important to call BIO_flush() whenever there's pending data that should be written — like when you're removing a buffering BIO using BIO_pop(). And heads up: BIO_flush() may need to be retried if the ultimate source/sink BIO is non blocking 🌀.

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🔥 RETURN VALUES 🔥

• BIO_f_buffer() returns the buffering BIO method. slay.
• BIO_get_buffer_num_lines() returns the number of lines buffered (may be 0) — or a negative value in case of errors 😬
• BIO_set_read_buffer_size(), BIO_set_write_buffer_size() and BIO_set_buffer_size() return 1 if the buffer was successfully resized, or <=0 for failure 💀
• BIO_set_buffer_read_data() returns 1 if the data was set correctly, or <=0 if there was an error 🙈

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👑 SEE ALSO 👑

bio(7), BIO_reset(3), BIO_flush(3), BIO_pop(3), BIO_ctrl(3). go read those next, bestie, they're all connected 🧚

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💜 COPYRIGHT 💜

Copyright 2000-2020 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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🌈 and that's the whole page! this one shipped as part of OpenSSL 3.5.0, dated 2026-09-25 — so if you're reading it in some other version, vibe check your install first 🎀💖✨