=pod

=head1 NAME

bio - Basic I/O abstraction

=head1 SYNOPSIS

=for openssl generic

 #include <openssl/bio.h>

=head1 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 I<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 I<BIO_s_> and
a filter BIO with I<BIO_f_>.

=head2 TCP Fast Open

TCP Fast Open (RFC7413), abbreviated "TFO", is supported by the BIO
interface since OpenSSL 3.1. TFO is supported in the following operating systems:

=over 4

=item * Linux kernel 3.13 and later, where TFO is enabled by default.

=item * Linux kernel 4.11 and later, using TCP_FASTOPEN_CONNECT.

=item * FreeBSD 10.3 to 11.4, supports server TFO only.

=item * FreeBSD 12.0 and later, supports both client and server TFO.

=item * macOS 10.14 and later.

=back

Each operating system has a slightly different API for TFO. Please
refer to the operating systems' API documentation when using
sockets directly.

=head1 EXAMPLES

Create a memory BIO:

 BIO *mem = BIO_new(BIO_s_mem());

=head1 SEE ALSO

L<BIO_ctrl(3)>,
L<BIO_f_base64(3)>, L<BIO_f_buffer(3)>,
L<BIO_f_cipher(3)>, L<BIO_f_md(3)>,
L<BIO_f_null(3)>, L<BIO_f_ssl(3)>,
L<BIO_f_readbuffer(3)>,
L<BIO_find_type(3)>,
L<BIO_get_conn_mode(3)>,
L<BIO_new(3)>,
L<BIO_new_bio_pair(3)>,
L<BIO_push(3)>, L<BIO_read_ex(3)>,
L<BIO_s_accept(3)>, L<BIO_s_bio(3)>,
L<BIO_s_connect(3)>, L<BIO_s_fd(3)>,
L<BIO_s_file(3)>, L<BIO_s_mem(3)>,
L<BIO_s_null(3)>, L<BIO_s_socket(3)>,
L<BIO_set_callback(3)>,
L<BIO_set_conn_mode(3)>,
L<BIO_set_tfo(3)>,
L<BIO_set_tfo_accept(3)>,
L<BIO_should_retry(3)>

=head1 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
L<https://www.openssl.org/source/license.html>.

=cut