BIO_SET_CALLBACK(3SSL)

📦

version 4.0.3 · 14 versions

💖 NAME 💖

bestie, gather round — this page is all about the BIO callback functions, and it's a whole squad:

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 — the BIO callback functions, periodt. 👑

✨ SYNOPSIS ✨

okay so first, you gotta include the header:

#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);

🫠 the deprecated era (no cap, these are old)

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 — see openssl_user_macros(7) for the tea:

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;

🌸 DESCRIPTION 🌸

BIO_set_callback_ex() and BIO_get_callback_ex() set and retrieve the BIO callback. the callback gets called during most high-level BIO operations. it's giving debugging tool, bestie — you can use it to trace what a BIO is doing, or even to modify its operation. 💅

BIO_set_callback() and BIO_get_callback() set and retrieve the old format BIO callback. new code should NOT use these functions, but they're still hanging around for backwards compatibility reasons. also important: any callback set via BIO_set_callback_ex() will get called in preference to any set by BIO_set_callback(). ex wins, every time. ✨

BIO_set_callback_arg() and BIO_get_callback_arg() are macros — use them to set and retrieve an argument for use in the callback.

BIO_debug_callback_ex() is a standard debugging callback that prints out info relating to each BIO operation. if the callback argument is set, it gets interpreted as a BIO to send the info to — otherwise it goes to stderr. the BIO_debug_callback() function 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 described below, so lock in 🔒:

b — the BIO the callback is attached to gets passed in as b.
oper — oper is set to the operation being performed. for some operations the callback is called twice — once before and once after the actual operation — and in the latter case, oper has BIO_CB_RETURN or'ed with it.
len — the length of the data requested to be read or written. this is only useful if oper is BIO_CB_READ, BIO_CB_WRITE or BIO_CB_GETS.
argp, argi, argl — the meaning of the arguments argp, argi and argl depends on the value of oper, that is, the operation being performed.
processed — a pointer to a location which will be 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 — ret is 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. in the case of callbacks called before the actual BIO operation, 1 is placed in ret — and if the return value is not positive, it will be immediately returned to the application and the BIO operation will not be performed. 🫢

the callback should normally just return ret when it's done processing… unless it specifically wants to modify the value returned to the application. character development, but make it optional. 💖

🎀 CALLBACK OPERATIONS 🎀

in the notes below, callback defers to the actual callback function that is called. iykyk.

`BIO_free(b)`

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

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)

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)

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)

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)

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)

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)

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);

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)

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)

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)

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, 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)

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)

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. and plot twist 🌪️ — the return value of BIO_sendmmsg() is altered to the value returned by the BIO_CB_SENDMMSG | BIO_CB_RETURN call.

`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. twins but make it mmsg 💅

💜 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.

🦋 EXAMPLES 🦋

BIO_debug_callback_ex() is an example — its source lives in crypto/bio/bio_cb.c. go read it, bestie, it's lowkey iconic 📖

🌈 HISTORY 🌈

the BIO_debug_callback_ex() function 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. it's giving glow-up ✨

👑 COPYRIGHT 👑

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>.

and that's the whole page bestie — this was OpenSSL, version 4.0.3, dated 2026-09-29, filed under BIO_SET_CALLBACK(3SSL). 🫶🎀 slay, go set some callbacks, no cap 💖

📦 where this page comes from

ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms

BIO_SET_CALLBACK(3SSL)

💖 NAME 💖

✨ SYNOPSIS ✨

🫠 the deprecated era (no cap, these are old)

🌸 DESCRIPTION 🌸

🎀 CALLBACK OPERATIONS 🎀

BIO_free(b)

BIO_read_ex(b, data, dlen, readbytes)

BIO_write(b, data, dlen, written)

BIO_gets(b, buf, size)

BIO_puts(b, buf)

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

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

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

💜 RETURN VALUES 💜

🦋 EXAMPLES 🦋

🌈 HISTORY 🌈

👑 COPYRIGHT 👑

`BIO_free(b)`

`BIO_read_ex(b, data, dlen, readbytes)`

`BIO_write(b, data, dlen, written)`

`BIO_gets(b, buf, size)`

`BIO_puts(b, buf)`

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

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

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