BIO_SET_CALLBACK(3ossl)

💅 BIO_SET_CALLBACK(3ossl) — the OpenSSL glow-up

okay bestie, so this whole page lives in the OpenSSL manual, section 3ossl, and it's all about BIO callback functions — aka the little spies that watch every BIO operation go by. no cap, it's giving backstage-pass energy. 🎀

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✨ NAME ✨

BIO_set_callback_ex, BIO_get_callback_ex, BIO_set_callback, BIO_get_callback, BIO_set_callback_arg, BIO_get_callback_arg, BIO_debug_callback, BIO_debug_callback_ex, BIO_callback_fn_ex, BIO_callback_fn — all of these are, collectively, the BIO callback functions. that's the whole squad 💜

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🦋 SYNOPSIS 🦋

#include <openssl/bio.h>

typedef long (*BIO_callback_fn_ex)(BIO *b, int oper, const char *argp,
                                   size_t len, int argi,
                                   long argl, int ret, size_t *processed);

void BIO_set_callback_ex(BIO *b, BIO_callback_fn_ex callback);
BIO_callback_fn_ex BIO_get_callback_ex(const BIO *b);

void BIO_set_callback_arg(BIO *b, char *arg);
char *BIO_get_callback_arg(const BIO *b);

long BIO_debug_callback_ex(BIO *bio, int oper, const char *argp, size_t len,
                           int argi, long argl, int ret, size_t *processed);

so here's the tea 🫖 — the following functions have been deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value (go peek at openssl_user_macros(7) if you want the deets):

typedef long (*BIO_callback_fn)(BIO *b, int oper, const char *argp, int argi,
                                long argl, long ret);
void BIO_set_callback(BIO *b, BIO_callback_fn cb);
BIO_callback_fn BIO_get_callback(const BIO *b);
long BIO_debug_callback(BIO *bio, int cmd, const char *argp, int argi,
                        long argl, long ret);

typedef struct bio_mmsg_cb_args_st {
    BIO_MSG    *msg;
    size_t      stride, num_msg;
    uint64_t    flags;
    size_t     *msgs_processed;
} BIO_MMSG_CB_ARGS;

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🌸 DESCRIPTION 🌸

BIO_set_callback_ex() and BIO_get_callback_ex() are the set-and-get duo for the BIO callback. the callback gets called during most high-level BIO operations — it's literally main character energy for debugging 🕵️‍♀️. use it to trace what a BIO is doing, or to actually modify how it operates. iconic.

BIO_set_callback() and BIO_get_callback() do the same thing but for the old format BIO callback. new code should not use these, bestie — they're only hanging around for backwards compatibility. and important: any callback set via BIO_set_callback_ex() gets called in preference to any set by BIO_set_callback(). periodt.

BIO_set_callback_arg() and BIO_get_callback_arg() are macros — they set and retrieve an argument for use in the callback. handy little helpers 🎀

BIO_debug_callback_ex() is the standard debugging callback — it prints out info relating to each BIO operation. if the callback argument is set, it's interpreted as a BIO to send the info to; otherwise stderr is used. BIO_debug_callback() is the deprecated version of the same callback, for use with the old callback format BIO_set_callback() function.

BIO_callback_fn_ex is the type of the callback function, and BIO_callback_fn is the type of the old format callback function. the meaning of each argument is down below 👇

• b — the BIO the callback is attached to gets passed in as b.
• oper — set to the operation being performed. for some operations the callback is called twice — once before and once after the actual operation — and the second one has oper or'ed with BIO_CB_RETURN.
• len — the length of the data requested to be read or written. only useful if oper is BIO_CB_READ, BIO_CB_WRITE or BIO_CB_GETS.
• argp argi argl — the meaning of these three depends on the value of oper, i.e. the operation being performed. iykyk.
• processed — a pointer to a location that gets updated with the amount of data that was actually read or written. only used for BIO_CB_READ, BIO_CB_WRITE, BIO_CB_GETS and BIO_CB_PUTS.
• ret — the return value that would be returned to the application if no callback were present. the actual value returned is the return value of the callback itself. for callbacks called before the actual BIO operation, 1 is placed in ret — and if the return value is not positive, it gets immediately returned to the application and the BIO operation will not be performed. important!! 🚨

the callback should normally just return ret when it's finished processing — unless it specifically wants to modify the value returned to the application. don't be doing extra out here without a reason 💅

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👑 CALLBACK OPERATIONS 👑

in these notes, callback refers to the actual callback function that gets called. buckle up, there's a lot of them and they all slay in their own way 🌈

💖 BIO_free(b)

callback_ex(b, BIO_CB_FREE, NULL, 0, 0, 0L, 1L, NULL)

or

callback(b, BIO_CB_FREE, NULL, 0L, 0L, 1L)

is called before the free operation.

💖 BIO_read_ex(b, data, dlen, readbytes)

callback_ex(b, BIO_CB_READ, data, dlen, 0, 0L, 1L, NULL)

or

callback(b, BIO_CB_READ, data, dlen, 0L, 1L)

is called before the read, and

callback_ex(b, BIO_CB_READ | BIO_CB_RETURN, data, dlen, 0, 0L, retvalue,
            &readbytes)

or

callback(b, BIO_CB_READ|BIO_CB_RETURN, data, dlen, 0L, retvalue)

after.

💖 BIO_write(b, data, dlen, written)

callback_ex(b, BIO_CB_WRITE, data, dlen, 0, 0L, 1L, NULL)

or

callback(b, BIO_CB_WRITE, datat, dlen, 0L, 1L)

is called before the write, and

callback_ex(b, BIO_CB_WRITE | BIO_CB_RETURN, data, dlen, 0, 0L, retvalue,
            &written)

or

callback(b, BIO_CB_WRITE|BIO_CB_RETURN, data, dlen, 0L, retvalue)

after.

💖 BIO_gets(b, buf, size)

callback_ex(b, BIO_CB_GETS, buf, size, 0, 0L, 1, NULL, NULL)

or

callback(b, BIO_CB_GETS, buf, size, 0L, 1L)

is called before the operation, and

callback_ex(b, BIO_CB_GETS | BIO_CB_RETURN, buf, size, 0, 0L, retvalue,
            &readbytes)

or

callback(b, BIO_CB_GETS|BIO_CB_RETURN, buf, size, 0L, retvalue)

after.

💖 BIO_puts(b, buf)

callback_ex(b, BIO_CB_PUTS, buf, 0, 0, 0L, 1L, NULL);

or

callback(b, BIO_CB_PUTS, buf, 0, 0L, 1L)

is called before the operation, and

callback_ex(b, BIO_CB_PUTS | BIO_CB_RETURN, buf, 0, 0, 0L, retvalue, &written)

or

callback(b, BIO_CB_PUTS|BIO_CB_RETURN, buf, 0, 0L, retvalue)

after.

💖 BIO_ctrl(BIO *b, int cmd, long larg, void *parg)

callback_ex(b, BIO_CB_CTRL, parg, 0, cmd, larg, 1L, NULL)

or

callback(b, BIO_CB_CTRL, parg, cmd, larg, 1L)

is called before the call, and

callback_ex(b, BIO_CB_CTRL | BIO_CB_RETURN, parg, 0, cmd, larg, ret, NULL)

or

callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, cmd, larg, ret)

after.

Note: cmd == BIO_CTRL_SET_CALLBACK is special, because parg is not the argument of type BIO_info_cb itself. in this case parg is a pointer to the actual call parameter — go see BIO_callback_ctrl 🙈

💖 BIO_sendmmsg(BIO *b, BIO_MSG *msg, size_t stride, size_t num_msg, uint64_t flags, size_t *msgs_processed)

callback_ex(b, BIO_CB_SENDMMSG, args, 0, 0, 0, 1, NULL)

or

callback(b, BIO_CB_SENDMMSG, args, 0, 0, 1)

is called before the call, and

callback_ex(b, BIO_CB_SENDMMSG | BIO_CB_RETURN, args, ret, 0, 0, ret, NULL)

or

callback(b, BIO_CB_SENDMMSG | BIO_CB_RETURN, args, ret, 0, 0, ret)

after.

args is a pointer to a BIO_MMSG_CB_ARGS structure containing the arguments passed to BIO_sendmmsg(). ret is the return value of the BIO_sendmmsg() call. the return value of BIO_sendmmsg() gets altered to whatever value the BIO_CB_SENDMMSG | BIO_CB_RETURN call returns. lowkey powerful 🔥

💖 BIO_recvmmsg(BIO *b, BIO_MSG *msg, size_t stride, size_t num_msg, uint64_t flags, size_t *msgs_processed)

see the documentation for BIO_sendmmsg(). BIO_recvmmsg() works identically, except that BIO_CB_RECVMMSG is used instead of BIO_CB_SENDMMSG. twin energy fr fr 🫶

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⭐ RETURN VALUES ⭐

BIO_get_callback_ex() and BIO_get_callback() return the callback function previously set by a call to BIO_set_callback_ex() and BIO_set_callback() respectively.

BIO_get_callback_arg() returns a char pointer to the value previously set via a call to BIO_set_callback_arg().

BIO_debug_callback() returns 1 or ret if it's called after specific BIO operations.

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🧚 EXAMPLES 🧚

BIO_debug_callback_ex() is literally an example — its source lives in crypto/bio/bio_cb.c. go read it, it's a whole vibe 📖✨

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🔥 HISTORY 🔥

• BIO_debug_callback_ex() was added in OpenSSL 3.0.
• BIO_set_callback(), BIO_get_callback(), and BIO_debug_callback() were deprecated in OpenSSL 3.0. use the non-deprecated _ex functions instead — they're the upgrade bestie, trust 💅

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🍓 COPYRIGHT 🍓

Copyright 2000-2021 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 thing! 🎀 this page is from OpenSSL 3.5.6, dated 2026-04-07, and it's the BIO_SET_CALLBACK(3ossl) manual page — go forth and trace those BIOs, bestie. mwah 💖💜🌈