BIO_S_BIO(3SSL) OpenSSL BIO_S_BIO(3SSL)
NAME
BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr,
BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_new_bio_pair,
BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee,
BIO_get_read_request, BIO_ctrl_get_read_request,
BIO_ctrl_reset_read_request, BIO_nread0, BIO_nread, BIO_nwrite0,
BIO_nwrite - BIO pair BIO
SYNOPSIS
#include <openssl/bio.h>
const BIO_METHOD *BIO_s_bio(void);
int BIO_make_bio_pair(BIO *b1, BIO *b2);
int BIO_destroy_bio_pair(BIO *b);
int BIO_shutdown_wr(BIO *b);
int BIO_set_write_buf_size(BIO *b, long size);
size_t BIO_get_write_buf_size(BIO *b, long size);
int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
int BIO_get_write_guarantee(BIO *b);
size_t BIO_ctrl_get_write_guarantee(BIO *b);
int BIO_get_read_request(BIO *b);
size_t BIO_ctrl_get_read_request(BIO *b);
int BIO_ctrl_reset_read_request(BIO *b);
int BIO_nread0(BIO *bio, char **buf);
int BIO_nread(BIO *bio, char **buf, int num);
int BIO_nwrite0(BIO *bio, char **buf);
int BIO_nwrite(BIO *bio, char **buf, int num);
DESCRIPTION
BIO_s_bio() returns the method for a BIO pair. A BIO pair is a pair of
source/sink BIOs where data written to either half of the pair is
buffered and can be read from the other half. Both halves must usually
by handled by the same application thread since no locking is done on
the internal data structures.
Since BIO chains typically end in a source/sink BIO it is possible to
make this one half of a BIO pair and have all the data processed by the
chain under application control.
One typical use of BIO pairs is to place TLS/SSL I/O under application
control, this can be used when the application wishes to use a non
standard transport for TLS/SSL or the normal socket routines are
inappropriate.
Calls to BIO_read_ex() will read data from the buffer or request a retry
if no data is available.
Calls to BIO_write_ex() will place data in the buffer or request a retry
if the buffer is full.
The standard calls BIO_ctrl_pending() and BIO_ctrl_wpending() can be
used to determine the amount of pending data in the read or write
buffer.
BIO_reset() clears any data in the write buffer.
BIO_make_bio_pair() joins two separate BIOs into a connected pair.
BIO_destroy_pair() destroys the association between two connected BIOs.
Freeing up any half of the pair will automatically destroy the
association.
BIO_shutdown_wr() is used to close down a BIO b. After this call no
further writes on BIO b are allowed (they will return an error). Reads
on the other half of the pair will return any pending data or EOF when
all pending data has been read.
BIO_set_write_buf_size() sets the write buffer size of BIO b to size.
If the size is not initialized a default value is used. This is
currently 17K, sufficient for a maximum size TLS record.
BIO_get_write_buf_size() returns the size of the write buffer.
BIO_new_bio_pair() combines the calls to BIO_new(), BIO_make_bio_pair()
and BIO_set_write_buf_size() to create a connected pair of BIOs bio1,
bio2 with write buffer sizes writebuf1 and writebuf2. If either size is
zero then the default size is used. BIO_new_bio_pair() does not check
whether bio1 or bio2 do point to some other BIO, the values are
overwritten, BIO_free() is not called.
BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the
maximum length of data that can be currently written to the BIO. Writes
larger than this value will return a value from BIO_write_ex() less than
the amount requested or if the buffer is full request a retry.
BIO_ctrl_get_write_guarantee() is a function whereas
BIO_get_write_guarantee() is a macro.
BIO_get_read_request() and BIO_ctrl_get_read_request() return the amount
of data requested, or the buffer size if it is less, if the last read
attempt at the other half of the BIO pair failed due to an empty buffer.
This can be used to determine how much data should be written to the BIO
so the next read will succeed: this is most useful in TLS/SSL
applications where the amount of data read is usually meaningful rather
than just a buffer size. After a successful read this call will return
zero. It also will return zero once new data has been written
satisfying the read request or part of it. Note that
BIO_get_read_request() never returns an amount larger than that returned
by BIO_get_write_guarantee().
BIO_ctrl_reset_read_request() can also be used to reset the value
returned by BIO_get_read_request() to zero.
Non-copying Interface
BIO_nread0(), BIO_nread(), BIO_nwrite0(), and BIO_nwrite() provide a
non-copying interface for reading from and writing to BIO pairs. These
functions allow direct access to the internal buffer, avoiding the
overhead of copying data.
BIO_nread0() returns in *buf a pointer to the start of the available
data in the peer's write buffer and returns the number of bytes
available. This allows reading directly from the buffer without
copying. It does not consume the data; a subsequent call to BIO_nread()
is needed to advance the buffer position.
BIO_nread() is similar to BIO_nread0() but also advances the read
position by up to num bytes. The actual number of bytes consumed is
returned. The *buf pointer is set to the start of the data that was
consumed. Since the data is considered consumed after this call, the
pointer returned by BIO_nread() should not be used afterwards unless the
caller also controls the writing side. The typical pattern is to call
BIO_nread0() first, use the data, and then call BIO_nread() to consume
it.
BIO_nwrite0() returns in *buf a pointer to the start of the available
space in the write buffer and returns the number of bytes that can be
written. This allows writing directly to the buffer without copying.
It does not commit the data; a subsequent call to BIO_nwrite() is needed
to update the buffer length.
BIO_nwrite() is similar to BIO_nwrite0() but also commits up to num
bytes as written. The actual number of bytes committed is returned. The
*buf pointer is set to the start of the region that was committed.
BIO_nwrite() should only be called after the data has actually been
written to the buffer obtained from BIO_nwrite0(), since committing
signals data availability to the reading side.
Note that due to the ring buffer implementation, if wrapping around
would be required, BIO_nread0() and BIO_nwrite0() may return less than
the total available space. In such cases, a second call may be needed to
access the remaining data or space.
NOTES
Both halves of a BIO pair should be freed. That is even if one half is
implicit freed due to a BIO_free_all() or SSL_free() call the other half
needs to be freed.
When used in bidirectional applications (such as TLS/SSL) care should be
taken to flush any data in the write buffer. This can be done by calling
BIO_pending() on the other half of the pair and, if any data is pending,
reading it and sending it to the underlying transport. This must be done
before any normal processing (such as calling select() ) due to a
request and BIO_should_read() being true.
To see why this is important consider a case where a request is sent
using BIO_write_ex() and a response read with BIO_read_ex(), this can
occur during an TLS/SSL handshake for example. BIO_write_ex() will
succeed and place data in the write buffer. BIO_read_ex() will initially
fail and BIO_should_read() will be true. If the application then waits
for data to be available on the underlying transport before flushing the
write buffer it will never succeed because the request was never sent!
BIO_eof() is true if no data is in the peer BIO and the peer BIO has
been shutdown.
BIO_make_bio_pair(), BIO_destroy_bio_pair(), BIO_shutdown_wr(),
BIO_set_write_buf_size(), BIO_get_write_buf_size(),
BIO_get_write_guarantee(), and BIO_get_read_request() are implemented as
macros.
RETURN VALUES
BIO_new_bio_pair() returns 1 on success, with the new BIOs available in
bio1 and bio2, or 0 on failure, with NULL pointers stored into the
locations for bio1 and bio2. Check the error stack for more information.
[XXXXX: More return values need to be added here]
BIO_nread0() returns the number of bytes available for reading, 0 if the
peer has closed and no data remains (EOF), or -1 if no data is currently
available (retry may be appropriate). If the BIO is not initialized, -2
is returned.
BIO_nwrite0() returns the number of bytes of space available for
writing, or -1 if no space is currently available (retry may be
appropriate) or the BIO has been closed. If the BIO is not initialized,
-2 is returned.
BIO_nread() and BIO_nwrite() return the number of bytes consumed or
committed respectively, or the same error values as BIO_nread0() and
BIO_nwrite0().
EXAMPLES
The BIO pair can be used to have full control over the network access of
an application. The application can call select() on the socket as
required without having to go through the SSL-interface.
BIO *internal_bio, *network_bio;
...
BIO_new_bio_pair(&internal_bio, 0, &network_bio, 0);
SSL_set_bio(ssl, internal_bio, internal_bio);
SSL_operations(); /* e.g. SSL_read and SSL_write */
...
application | TLS-engine
| |
+----------> SSL_operations()
| /\ ||
| || \/
| BIO-pair (internal_bio)
| BIO-pair (network_bio)
| || /\
| \/ ||
+-----------< BIO_operations()
| |
| |
socket
...
SSL_free(ssl); /* implicitly frees internal_bio */
BIO_free(network_bio);
...
As the BIO pair will only buffer the data and never directly access the
connection, it behaves nonblocking and will return as soon as the write
buffer is full or the read buffer is drained. Then the application has
to flush the write buffer and/or fill the read buffer.
Use the BIO_ctrl_pending(), to find out whether data is buffered in the
BIO and must be transferred to the network. Use
BIO_ctrl_get_read_request() to find out, how many bytes must be written
into the buffer before the SSL_operation() can successfully be
continued.
A typical usage pattern for the non-copying write interface is:
int ret;
char *buf;
ret = BIO_nwrite0(bio, &buf);
if (ret > 0) {
/* write up to 'ret' bytes directly to 'buf' */
memcpy(buf, data, len);
BIO_nwrite(bio, &buf, len); /* commit the write */
}
A typical usage pattern for the non-copying read interface is:
int ret;
char *buf;
ret = BIO_nread0(bio, &buf);
if (ret > 0) {
/* read up to 'ret' bytes directly from 'buf' */
process_data(buf, ret);
BIO_nread(bio, &buf, ret); /* consume the data */
}
WARNINGS
As the data is buffered, SSL_operation() may return with an
ERROR_SSL_WANT_READ condition, but there is still data in the write
buffer. An application must not rely on the error value of
SSL_operation() but must assure that the write buffer is always flushed
first. Otherwise a deadlock may occur as the peer might be waiting for
the data before being able to continue.
SEE ALSO
SSL_set_bio(3), ssl(7), bio(7), BIO_should_retry(3), BIO_read_ex(3)
COPYRIGHT
Copyright 2000-2026 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_S_BIO(3SSL)
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