BIO(7SSL) OpenSSL BIO(7SSL)
NAME
bio - Basic I/O abstraction
SYNOPSIS
#include <openssl/bio.h>
DESCRIPTION
A BIO is an I/O abstraction, it hides many of the underlying I/O details
from an application. If an application uses a BIO for its I/O it can
transparently handle SSL connections, unencrypted network connections
and file I/O.
There are two types of BIO, a source/sink BIO and a filter BIO.
As its name implies a source/sink BIO is a source and/or sink of data,
examples include a socket BIO and a file BIO.
A filter BIO takes data from one BIO and passes it through to another,
or the application. The data may be left unmodified (for example a
message digest BIO) or translated (for example an encryption BIO). The
effect of a filter BIO may change according to the I/O operation it is
performing: for example an encryption BIO will encrypt data if it is
being written to and decrypt data if it is being read from.
BIOs can be joined together to form a chain (a single BIO is a chain
with one component). A chain normally consists of one source/sink BIO
and one or more filter BIOs. Data read from or written to the first BIO
then traverses the chain to the end (normally a source/sink BIO).
Some BIOs (such as memory BIOs) can be used immediately after calling
BIO_new(). Others (such as file BIOs) need some additional
initialization, and frequently a utility function exists to create and
initialize such BIOs.
If BIO_free() is called on a BIO chain it will only free one BIO
resulting in a memory leak.
Calling BIO_free_all() on a single BIO has the same effect as calling
BIO_free() on it other than the discarded return value.
Normally the type argument is supplied by a function which returns a
pointer to a BIO_METHOD. There is a naming convention for such
functions: a source/sink BIO typically starts with BIO_s_ and a filter
BIO with BIO_f_.
TCP Fast Open
TCP Fast Open (RFC7413), abbreviated "TFO", is supported by the BIO
interface since OpenSSL 3.2. TFO is supported in the following operating
systems:
• Linux kernel 3.13 and later, where TFO is enabled by default.
• Linux kernel 4.11 and later, using TCP_FASTOPEN_CONNECT.
• FreeBSD 10.3 to 11.4, supports server TFO only.
• FreeBSD 12.0 and later, supports both client and server TFO.
• macOS 10.14 and later.
Each operating system has a slightly different API for TFO. Please refer
to the operating systems' API documentation when using sockets directly.
EXAMPLES
Create a memory BIO:
BIO *mem = BIO_new(BIO_s_mem());
SEE ALSO
BIO_ctrl(3), BIO_f_base64(3), BIO_f_buffer(3), BIO_f_cipher(3),
BIO_f_md(3), BIO_f_null(3), BIO_f_ssl(3), BIO_f_readbuffer(3),
BIO_find_type(3), BIO_get_conn_mode(3), BIO_new(3), BIO_new_bio_pair(3),
BIO_push(3), BIO_read_ex(3), BIO_s_accept(3), BIO_s_bio(3),
BIO_s_connect(3), BIO_s_fd(3), BIO_s_file(3), BIO_s_mem(3),
BIO_s_null(3), BIO_s_socket(3), BIO_set_callback(3),
BIO_set_conn_mode(3), BIO_set_tfo(3), BIO_set_tfo_accept(3),
BIO_should_retry(3)
COPYRIGHT
Copyright 2000-2022 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>.
3.5.7 2026-06-09 BIO(7SSL)
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