BIO_CTRL(3SSL)

📦

version 4.0.3 · 14 versions

💖 BIO_CTRL(3SSL) — the OpenSSL one, bestie 💖

okay so this is the OpenSSL girlies' page for all the BIO control operations, and honestly it's a whole vibe. it's filed under BIO_CTRL(3SSL) and it's giving "I hold the entire I/O stack together and I still look cute doing it." ✨

🎀 NAME 🎀

BIO_ctrl, BIO_callback_ctrl, BIO_ptr_ctrl, BIO_int_ctrl, BIO_reset, BIO_seek, BIO_tell, BIO_flush, BIO_eof, BIO_set_close, BIO_get_close, BIO_pending, BIO_wpending, BIO_ctrl_pending, BIO_ctrl_wpending, BIO_get_info_callback, BIO_set_info_callback, BIO_info_cb, BIO_get_ktls_send, BIO_get_ktls_recv, BIO_set_conn_mode, BIO_get_conn_mode, BIO_set_tfo — all of them, together, doing BIO control operations. periodt. 💅

🦋 SYNOPSIS 🦋

no cap, here's the whole cast list:

#include <openssl/bio.h>

typedef int BIO_info_cb(BIO *b, int state, int res);

long BIO_ctrl(BIO *bp, int cmd, long larg, void *parg);
long BIO_callback_ctrl(BIO *b, int cmd, BIO_info_cb *cb);
void *BIO_ptr_ctrl(BIO *bp, int cmd, long larg);
long BIO_int_ctrl(BIO *bp, int cmd, long larg, int iarg);

int BIO_reset(BIO *b);
int BIO_seek(BIO *b, int ofs);
int BIO_tell(BIO *b);
int BIO_flush(BIO *b);
int BIO_eof(BIO *b);
int BIO_set_close(BIO *b, long flag);
int BIO_get_close(BIO *b);
int BIO_pending(BIO *b);
int BIO_wpending(BIO *b);
size_t BIO_ctrl_pending(BIO *b);
size_t BIO_ctrl_wpending(BIO *b);

int BIO_get_info_callback(BIO *b, BIO_info_cb **cbp);
int BIO_set_info_callback(BIO *b, BIO_info_cb *cb);

int BIO_get_ktls_send(BIO *b);
int BIO_get_ktls_recv(BIO *b);

int BIO_set_conn_mode(BIO *b, int mode);
int BIO_get_conn_mode(BIO *b);

int BIO_set_tfo(BIO *b, int onoff);

🌸 DESCRIPTION 🌸

BIO_ctrl(), BIO_callback_ctrl(), BIO_ptr_ctrl() and BIO_int_ctrl() are the BIO "control" operations, and they take arguments of various types because why be boring. tbh you don't normally call these directly — various macros get used instead. The standard macros are described below, and the macros specific to a particular type of BIO get described in that specific BIO's manual page, plus any special features of the standard calls. iykyk. 🫶

BIO_reset() typically resets a BIO to some initial state — in the case of file related BIOs, for example, it rewinds the file pointer to the start of the file. iconic behavior honestly.

BIO_seek() resets a file related BIO's (that is file descriptor and FILE BIOs) file position pointer to ofs bytes from start of file. 🎯

BIO_tell() returns the current file position of a file related BIO.

BIO_flush() normally writes out any internally buffered data. in some cases it's used to signal EOF and that no more data will be written. it's giving "last call at the function." 😭

BIO_eof() returns 1 if the BIO has reached end-of-file as a result of the most recent read operation. The precise meaning of "EOF" varies according to the BIO type. And listen — the function reports the result of the previous read attempt and does not update this state based on subsequent operations. so don't come for it. 🙈

BIO_set_close() sets the BIO b close flag to flag. flag can take the value BIO_CLOSE or BIO_NOCLOSE. Typically BIO_CLOSE is used in a source/sink BIO to indicate that the underlying I/O stream should be closed when the BIO is freed. ✨

BIO_get_close() returns the BIOs close flag.

BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending() return the number of pending characters in the BIOs read and write buffers. not all BIOs support these calls bestie, be warned. BIO_ctrl_pending() and BIO_ctrl_wpending() return a size_t type and are functions, while BIO_pending() and BIO_wpending() are macros which call BIO_ctrl(). two icons, two delivery mechanisms. 💅

BIO_get_ktls_send() returns 1 if the BIO is using the Kernel TLS data-path for sending. Otherwise, it returns zero. BIO_get_ktls_recv() returns 1 if the BIO is using the Kernel TLS data-path for receiving. Otherwise, it returns zero. ✨

BIO_get_conn_mode() returns the BIO connection mode. BIO_set_conn_mode() sets the BIO connection mode. main character energy fr fr. 👑

BIO_set_tfo() disables TCP Fast Open when onoff is 0, and enables TCP Fast Open when onoff is nonzero. Setting the value to 1 is equivalent to setting BIO_SOCK_TFO in BIO_set_conn_mode(). 🔥

⭐ RETURN VALUES ⭐

ugh okay here's the receipts, and some of them are ambiguous so pay attention:

BIO_reset() normally returns 1 for success and <=0 for failure. File BIOs are an exception — they return 0 for success and -1 for failure. (yes really. the audacity.) 😭
BIO_seek() and BIO_tell() both return the current file position on success and -1 for failure, except file BIOs which for BIO_seek() always return 0 for success and -1 for failure.
BIO_flush() returns 1 for success and <=0 for failure.
BIO_eof() returns 1 if EOF has been reached, 0 if not, or negative values for failure.
BIO_set_close() returns 1 on success or <=0 for failure.
BIO_get_close() returns the close flag value: BIO_CLOSE or BIO_NOCLOSE. It also returns other negative values if an error occurs.
BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending() return the amount of pending data. BIO_pending() and BIO_wpending() return negative value or 0 on error. BIO_ctrl_pending() and BIO_ctrl_wpending() return 0 on error.
BIO_get_ktls_send() returns 1 if the BIO is using the Kernel TLS data-path for sending. Otherwise, it returns zero. BIO_get_ktls_recv() returns 1 if the BIO is using the Kernel TLS data-path for receiving. Otherwise, it returns zero.
BIO_set_conn_mode() returns 1 for success and 0 for failure. BIO_get_conn_mode() returns the current connection mode. Which may contain the bitwise-or of the following flags:

BIO_SOCK_REUSEADDR
BIO_SOCK_V6_ONLY
BIO_SOCK_KEEPALIVE
BIO_SOCK_NONBLOCK
BIO_SOCK_NODELAY
BIO_SOCK_TFO

BIO_set_tfo() returns 1 for success, and 0 for failure.

👑 NOTES 👑

BIO_flush(), because it can write data, may return 0 or -1 indicating that the call should be retried later in a similar manner to BIO_write_ex(). The BIO_should_retry() call should be used and appropriate action taken is the call fails. so bestie, always check before you assume it ate. 🍓

The return values of BIO_pending() and BIO_wpending() may not reliably determine the amount of pending data in all cases. For example in the case of a file BIO some data may be available in the FILE structures internal buffers but it is not possible to determine this in a portably way. For other types of BIO they may not be supported. it's giving "trust me bro" and honestly, no. 💀

Filter BIOs — if they do not internally handle a particular BIO_ctrl() operation — usually pass the operation to the next BIO in the chain. This often means there is no need to locate the required BIO for a particular operation, because you can call it on a chain and it will be automatically passed to the relevant BIO. however, this can cause unexpected results: for example no current filter BIOs implement BIO_seek(), but this may still succeed if the chain ends in a FILE or file descriptor BIO. the relay race is real. 🏃♀️

Source/sink BIOs return an 0 if they do not recognize the BIO_ctrl() operation.

💀 BUGS 💀

Some of the return values are ambiguous and care should be taken. In particular a return value of 0 can be returned if an operation is not supported, if an error occurred, if EOF has not been reached and in the case of BIO_seek() on a file BIO for a successful operation. yes bestie, 0 means like four different things. we don't make the rules. 😭

In older versions of OpenSSL the BIO_ctrl_pending() and BIO_ctrl_wpending() could return values greater than INT_MAX on error.

🌈 HISTORY 🌈

The BIO_get_ktls_send() and BIO_get_ktls_recv() macros were added in OpenSSL 3.0. They were modified to never return -1 in OpenSSL 3.0.4. glow-up era. ✨

The BIO_get_conn_mode(), BIO_set_conn_mode() and BIO_set_tfo() functions were added in OpenSSL 3.2. new girls on the block. 💖

🎀 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 tea, bestie 🫶 this whole page is from OpenSSL version 4.0.3, dated 2026-09-29, and it lives under the name BIO_CTRL(3SSL). go forth and control your BIOs. slay. 💅🌸🦋

📦 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