BIO_ctrl.3

.\" $OpenBSD: BIO_ctrl.3,v 1.25 2023/11/16 20:19:23 schwarze Exp $
.\" full merge up to: OpenSSL 24a535eaf Tue Sep 22 13:14:20 2020 +0100
.\" selective merge up to: OpenSSL 0c5bc96f Tue Mar 15 13:57:22 2022 +0000
.\"
.\" This file is a derived work.
.\" The changes are covered by the following Copyright and license:
.\"
.\" Copyright (c) 2023 Ingo Schwarze <schwarze@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\" The original file was written by Dr. Stephen Henson <steve@openssl.org>.
.\" Copyright (c) 2000, 2016 The OpenSSL Project.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in
.\"    the documentation and/or other materials provided with the
.\"    distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\"    software must display the following acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\"    endorse or promote products derived from this software without
.\"    prior written permission. For written permission, please contact
.\"    openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\"    nor may "OpenSSL" appear in their names without prior written
.\"    permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\"    acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: November 16 2023 $
.Dt BIO_CTRL 3
.Os
.Sh NAME
.Nm BIO_ctrl ,
.Nm BIO_callback_ctrl ,
.Nm BIO_ptr_ctrl ,
.Nm BIO_int_ctrl ,
.Nm BIO_reset ,
.Nm BIO_seek ,
.Nm BIO_tell ,
.Nm BIO_flush ,
.Nm BIO_eof ,
.Nm BIO_set_close ,
.Nm BIO_get_close ,
.Nm BIO_pending ,
.Nm BIO_wpending ,
.Nm BIO_ctrl_pending ,
.Nm BIO_ctrl_wpending ,
.Nm BIO_get_info_callback ,
.Nm BIO_set_info_callback ,
.Nm BIO_info_cb ,
.Nm bio_info_cb
.Nd BIO control operations
.Sh SYNOPSIS
.In openssl/bio.h
.Ft long
.Fo BIO_ctrl
.Fa "BIO *b"
.Fa "int cmd"
.Fa "long larg"
.Fa "void *parg"
.Fc
.Ft long
.Fo BIO_callback_ctrl
.Fa "BIO *b"
.Fa "int cmd"
.Fa "BIO_info_cb *cb"
.Fc
.Ft char *
.Fo BIO_ptr_ctrl
.Fa "BIO *b"
.Fa "int cmd"
.Fa "long larg"
.Fc
.Ft long
.Fo BIO_int_ctrl
.Fa "BIO *b"
.Fa "int cmd"
.Fa "long larg"
.Fa "int iarg"
.Fc
.Ft int
.Fo BIO_reset
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_seek
.Fa "BIO *b"
.Fa "int ofs"
.Fc
.Ft int
.Fo BIO_tell
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_flush
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_eof
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_set_close
.Fa "BIO *b"
.Fa "long flag"
.Fc
.Ft int
.Fo BIO_get_close
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_pending
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_wpending
.Fa "BIO *b"
.Fc
.Ft size_t
.Fo BIO_ctrl_pending
.Fa "BIO *b"
.Fc
.Ft size_t
.Fo BIO_ctrl_wpending
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_get_info_callback
.Fa "BIO *b"
.Fa "BIO_info_cb **cbp"
.Fc
.Ft int
.Fo BIO_set_info_callback
.Fa "BIO *b"
.Fa "BIO_info_cb *cb"
.Fc
.Ft typedef int
.Fo BIO_info_cb
.Fa "BIO *b"
.Fa "int state"
.Fa "int res"
.Fc
.Ft typedef int
.Fo bio_info_cb
.Fa "BIO *b"
.Fa "int state"
.Fa "int res"
.Fc
.Sh DESCRIPTION
.Fn BIO_ctrl ,
.Fn BIO_callback_ctrl ,
.Fn BIO_ptr_ctrl ,
and
.Fn BIO_int_ctrl
are BIO "control" operations taking arguments of various types.
These functions are not normally called directly -
various macros are used instead.
The standard macros are described below.
Macros specific to a particular type of BIO
are described in the specific BIO's manual page
as well as any special features of the standard calls.
.Pp
Depending on the
.Fa cmd
and on the type of
.Fa b ,
.Fn BIO_ctrl
may have a read-only effect on
.Fa b
or change data in
.Fa b
or in its sub-structures.
It may also have a side effect of changing the memory pointed to by
.Fa parg .
.Pp
.Fn BIO_callback_ctrl
does not call
.Fn BIO_ctrl
but instead requires that the BIO type of
.Fa b
provides a dedicated
.Fa callback_ctrl
function pointer, which is built into the library for some standard BIO
types and can be provided with
.Xr BIO_meth_set_callback_ctrl 3
for application-defined BIO types.
The only
.Fa cmd
supported by
.Fn BIO_callback_ctrl
is
.Dv BIO_CTRL_SET_CALLBACK .
.Pp
.Fn BIO_ptr_ctrl
calls
.Fn BIO_ctrl
with
.Fa parg
pointing to the location of a temporary pointer variable initialized to
.Dv NULL .
.Pp
.Fn BIO_int_ctrl
calls
.Fn BIO_ctrl
with
.Fa parg
pointing to the location of a temporary
.Vt int
variable initialized to
.Fa iarg .
If
.Fn BIO_ctrl
changes the value stored at
.Pf * Fa parg ,
the new value is ignored.
.Pp
.Fn 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.
.Pp
.Fn BIO_seek
resets a file related BIO's (that is file descriptor and
FILE BIOs) file position pointer to
.Fa ofs
bytes from start of file.
.Pp
.Fn BIO_tell
returns the current file position of a file related BIO.
.Pp
.Fn BIO_flush
normally writes out any internally buffered data.
In some cases it is used to signal EOF and that no more data will be written.
.Pp
.Fn BIO_eof
returns 1 if the BIO has read EOF.
The precise meaning of "EOF" varies according to the BIO type.
.Pp
.Fn BIO_set_close
sets the BIO
.Fa b
close flag to
.Fa flag .
.Fa flag
can take the value
.Dv BIO_CLOSE
or
.Dv BIO_NOCLOSE .
Typically
.Dv 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.
.Pp
.Fn BIO_get_close
returns the BIO's close flag.
.Pp
.Fn BIO_pending ,
.Fn BIO_ctrl_pending ,
.Fn BIO_wpending ,
and
.Fn BIO_ctrl_wpending
return the number of pending characters in the BIO's read and write buffers.
Not all BIOs support these calls.
.Fn BIO_ctrl_pending
and
.Fn BIO_ctrl_wpending
return a
.Vt size_t
type and are functions.
.Pp
.Fn BIO_set_info_callback
installs the function pointer
.Fa cb
as an info callback in
.Fa b
by calling
.Fn BIO_callback_ctrl
with a command of
.Dv BIO_CTRL_SET_CALLBACK .
Among the BIO types built into the library, only
.Xr BIO_s_connect 3
and
.Xr BIO_f_ssl 3
support this functionality.
Some filter BIO types forward this control call
to the next BIO in the chain instead of processing it themselves.
.Pp
.Fn BIO_get_info_callback
places the function pointer to the info callback into
.Pf * Fa cbp
if any was installed using
.Fn BIO_set_info_callback
or
.Fn BIO_callback_ctrl .
If the type of
.Fa b
supports setting an info callback but none was installed, it stores a
.Dv NULL
pointer in
.Pf * Fa cbp .
.Pp
The function type name
.Vt bio_info_cb
is a deprecated synonym for
.Vt BIO_info_cb
provided for backward compatibility with some existing application software.
.Pp
The following
.Fa cmd
constants correspond to macros:
.Bl -column BIO_C_SET_SSL_RENEGOTIATE_TIMEOUT BIO_set_ssl_renegotiate_timeout(3)
.It Fa cmd No constant                   Ta corresponding macro
.It Dv BIO_C_DESTROY_BIO_PAIR            Ta Xr BIO_destroy_bio_pair 3
.It Dv BIO_C_DO_STATE_MACHINE            Ta Xr BIO_do_handshake 3
.It Dv BIO_C_FILE_SEEK                   Ta Fn BIO_seek
.It Dv BIO_C_FILE_TELL                   Ta Fn BIO_tell
.It Dv BIO_C_GET_ACCEPT                  Ta Xr BIO_get_accept_port 3
.It Dv BIO_C_GET_BIND_MODE               Ta Xr BIO_get_bind_mode 3
.It Dv BIO_C_GET_BUF_MEM_PTR             Ta Xr BIO_get_mem_ptr 3
.It Dv BIO_C_GET_BUFF_NUM_LINES          Ta Xr BIO_get_buffer_num_lines 3
.It Dv BIO_C_GET_CIPHER_CTX              Ta Xr BIO_get_cipher_ctx 3
.It Dv BIO_C_GET_CIPHER_STATUS           Ta Xr BIO_get_cipher_status 3
.It Dv BIO_C_GET_FD                      Ta Xr BIO_get_fd 3
.It Dv BIO_C_GET_FILE_PTR                Ta Xr BIO_get_fp 3
.It Dv BIO_C_GET_MD                      Ta Xr BIO_get_md 3
.It Dv BIO_C_GET_MD_CTX                  Ta Xr BIO_get_md_ctx 3
.It Dv BIO_C_GET_READ_REQUEST            Ta Xr BIO_get_read_request 3
.It Dv BIO_C_GET_SSL                     Ta Xr BIO_get_ssl 3
.It Dv BIO_C_GET_SSL_NUM_RENEGOTIATES    Ta Xr BIO_get_num_renegotiates 3
.It Dv BIO_C_GET_WRITE_BUF_SIZE          Ta Xr BIO_get_write_buf_size 3
.It Dv BIO_C_GET_WRITE_GUARANTEE         Ta Xr BIO_get_write_guarantee 3
.It Dv BIO_C_MAKE_BIO_PAIR               Ta Xr BIO_make_bio_pair 3
.It Dv BIO_C_RESET_READ_REQUEST          Ta Xr BIO_ctrl_reset_read_request 3
.It Dv BIO_C_SET_BIND_MODE               Ta Xr BIO_set_bind_mode 3
.It Dv BIO_C_SET_BUF_MEM                 Ta Xr BIO_set_mem_buf 3
.It Dv BIO_C_SET_BUF_MEM_EOF_RETURN      Ta Xr BIO_set_mem_eof_return 3
.It Dv BIO_C_SET_BUFF_READ_DATA          Ta Xr BIO_set_buffer_read_data 3
.It Dv BIO_C_SET_FD                      Ta Xr BIO_set_fd 3
.It Dv BIO_C_SET_FILE_PTR                Ta Xr BIO_set_fp 3
.It Dv BIO_C_SET_MD                      Ta Xr BIO_set_md 3
.It Dv BIO_C_SET_MD_CTX                  Ta Xr BIO_set_md_ctx 3
.It Dv BIO_C_SET_NBIO                    Ta Xr BIO_set_nbio 3
.It Dv BIO_C_SET_SSL                     Ta Xr BIO_set_ssl 3
.It Dv BIO_C_SET_SSL_RENEGOTIATE_BYTES   Ta Xr BIO_set_ssl_renegotiate_bytes 3
.It Dv BIO_C_SET_SSL_RENEGOTIATE_TIMEOUT Ta Xr BIO_set_ssl_renegotiate_timeout 3
.It Dv BIO_C_SET_WRITE_BUF_SIZE          Ta Xr BIO_set_write_buf_size 3
.It Dv BIO_C_SHUTDOWN_WR                 Ta Xr BIO_shutdown_wr 3
.It Dv BIO_C_SSL_MODE                    Ta Xr BIO_set_ssl_mode 3
.It Dv BIO_CTRL_DGRAM_CONNECT            Ta Xr BIO_ctrl_dgram_connect 3
.It Dv BIO_CTRL_DGRAM_GET_PEER           Ta Xr BIO_dgram_get_peer 3
.It Dv BIO_CTRL_DGRAM_GET_RECV_TIMER_EXP Ta Xr BIO_dgram_recv_timedout 3
.It Dv BIO_CTRL_DGRAM_GET_SEND_TIMER_EXP Ta Xr BIO_dgram_send_timedout 3
.It Dv BIO_CTRL_DGRAM_SET_CONNECTED      Ta Xr BIO_ctrl_set_connected 3
.It Dv BIO_CTRL_DGRAM_SET_PEER           Ta Xr BIO_dgram_set_peer 3
.It Dv BIO_CTRL_DUP                      Ta Xr BIO_dup_state 3
.It Dv BIO_CTRL_EOF                      Ta Fn BIO_eof
.It Dv BIO_CTRL_FLUSH                    Ta Fn BIO_flush
.It Dv BIO_CTRL_GET_CALLBACK             Ta Fn BIO_get_info_callback
.It Dv BIO_CTRL_GET_CLOSE                Ta Fn BIO_get_close
.It Dv BIO_CTRL_INFO                     Ta Xr BIO_get_mem_data 3
.It Dv BIO_CTRL_PENDING                  Ta Fn BIO_pending
.It Dv BIO_CTRL_RESET                    Ta Fn BIO_reset
.It Dv BIO_CTRL_SET_CALLBACK             Ta Fn BIO_set_info_callback
.It Dv BIO_CTRL_SET_CLOSE                Ta Fn BIO_set_close
.It Dv BIO_CTRL_WPENDING                 Ta Fn BIO_wpending
.El
.Pp
A few
.Fa cmd
constants serve more than one macro each
and are documented in the following manual pages:
.Bl -column BIO_C_SET_BUFF_SIZE BIO_s_connect(3) -offset 3n
.It Fa cmd No constant     Ta manual page
.It Dv BIO_C_GET_CONNECT   Ta Xr BIO_s_connect 3
.It Dv BIO_C_SET_ACCEPT    Ta Xr BIO_s_accept 3
.It Dv BIO_C_SET_BUFF_SIZE Ta Xr BIO_f_buffer 3
.It Dv BIO_C_SET_CONNECT   Ta Xr BIO_s_connect 3
.It Dv BIO_C_SET_FILENAME  Ta Xr BIO_s_file 3
.El
.Pp
Some
.Fa cmd
constants are not associated with any macros.
They are documented in the following manual pages:
.Bl -column BIO_CTRL_DGRAM_SET_RECV_TIMEOUT BIO_dgram_recv_timedout(3)\
 -offset 3n
.It Fa cmd No constant                 Ta manual page
.\" The following constants are intentionally undocumented because
.\" BIO_f_asn1 has been removed from the public API.
.\" .It Dv BIO_C_GET_EX_ARG                Ta Xr BIO_f_asn1 3
.\" .It Dv BIO_C_SET_EX_ARG                Ta Xr BIO_f_asn1 3
.It Dv BIO_CTRL_DGRAM_GET_FALLBACK_MTU Ta Xr BIO_dgram_set_peer 3
.It Dv BIO_CTRL_DGRAM_GET_MTU          Ta Xr BIO_dgram_set_peer 3
.It Dv BIO_CTRL_DGRAM_GET_RECV_TIMEOUT Ta Xr BIO_dgram_recv_timedout 3
.It Dv BIO_CTRL_DGRAM_GET_SEND_TIMEOUT Ta Xr BIO_dgram_send_timedout 3
.It Dv BIO_CTRL_DGRAM_SET_MTU          Ta Xr BIO_dgram_set_peer 3
.It Dv BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT Ta Xr BIO_dgram_recv_timedout 3
.It Dv BIO_CTRL_DGRAM_SET_RECV_TIMEOUT Ta Xr BIO_dgram_recv_timedout 3
.It Dv BIO_CTRL_DGRAM_SET_SEND_TIMEOUT Ta Xr BIO_dgram_send_timedout 3
.It Dv BIO_CTRL_DGRAM_MTU_EXCEEDED     Ta Xr BIO_s_datagram 3
.It Dv BIO_CTRL_POP                    Ta Xr BIO_pop 3
.It Dv BIO_CTRL_PUSH                   Ta Xr BIO_push 3
.El
.Sh RETURN VALUES
The meaning of the return values of
.Fn BIO_ctrl ,
.Fn BIO_callback_ctrl ,
and
.Fn BIO_int_ctrl
depends on both the type of
.Fa b
and on the
.Fa cmd .
If
.Fa b
is a
.Dv NULL
pointer, no action occurs and 0 is returned.
The return value \-2 usually indicates a fatal error.
In particular, it is returned if the
.Fa cmd
is unsupported by the type of
.Fa b .
.Pp
.Fn BIO_callback_ctrl
and
.Fn BIO_set_info_callback
return 1 on success, 0 if
.Fa b
is
.Dv NULL
or to indicate failure of a valid
.Fa cmd ,
or \-2 if the
.Fa cmd
is not supported by
.Fa b .
.Pp
.Fn BIO_ptr_ctrl
returns
.Dv NULL
if the
.Fn BIO_ctrl
call returns a negative value or does not change
.Pf * Fa parg ,
or the pointer it puts into
.Pf * Fa parg
otherwise.
.Pp
.Fn BIO_int_ctrl
returns the return value of
.Fn BIO_ctrl .
.Pp
.Fn BIO_reset
normally returns 1 for success and 0 or -1 for failure.
File BIOs are an exception, returning 0 for success and -1 for failure.
.Pp
.Fn BIO_seek
and
.Fn BIO_tell
both return the current file position on success
and -1 for failure, except file BIOs which for
.Fn BIO_seek
always return 0 for success and -1 for failure.
.Pp
.Fn BIO_flush
returns 1 for success and 0 or -1 for failure.
.Pp
.Fn BIO_eof
returns 1 if EOF has been reached or 0 otherwise.
.Pp
.Fn BIO_set_close
always returns 1.
.Pp
.Fn BIO_get_close
returns the close flag value
.Dv BIO_CLOSE
or
.Dv BIO_NOCLOSE .
.Pp
.Fn BIO_pending ,
.Fn BIO_ctrl_pending ,
.Fn BIO_wpending ,
and
.Fn BIO_ctrl_wpending
return the amount of pending data.
.Pp
.Fn BIO_get_info_callback
returns 1 on success, including when the type of
.Fa b
supports an info callback but none is installed,
0 if
.Fa b
is
.Dv NULL
or \-2 if the type of
.Fa b
does not support an info callback.
.Pp
If a callback was installed in
.Fa b
using
.Xr BIO_set_callback_ex 3
or
.Xr BIO_set_callback 3 ,
it can modify the return values of all these functions.
.Sh NOTES
Because it can write data,
.Fn BIO_flush
may return 0 or -1 indicating that the call should be retried later
in a similar manner to
.Xr BIO_write 3 .
The
.Xr BIO_should_retry 3
call should be used and appropriate action taken if the call fails.
.Pp
The return values of
.Fn BIO_pending
and
.Fn 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
.Vt FILE
structure's internal buffers but it is not possible
to determine this in a portable way.
For other types of BIO they may not be supported.
.Pp
If they do not internally handle a particular
.Fn BIO_ctrl
operation, filter BIOs 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: it can be called 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
.Fn BIO_seek ,
but this may still succeed if the chain ends
in a FILE or file descriptor BIO.
.Pp
Source/sink BIOs return a 0 if they do not recognize the
.Fn BIO_ctrl
operation.
.Sh SEE ALSO
.Xr BIO_meth_new 3 ,
.Xr BIO_new 3
.Sh HISTORY
.Fn BIO_ctrl ,
.Fn BIO_reset ,
.Fn BIO_flush ,
.Fn BIO_eof ,
.Fn BIO_set_close ,
.Fn BIO_get_close ,
and
.Fn BIO_pending
first appeared in SSLeay 0.6.0.
.Fn BIO_wpending
first appeared in SSLeay 0.8.1.
.Fn BIO_ptr_ctrl ,
.Fn BIO_int_ctrl ,
.Fn BIO_get_info_callback
and
.Fn BIO_set_info_callback
first appeared in SSLeay 0.9.0.
All these functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_seek
and
.Fn BIO_tell
first appeared in SSLeay 0.9.1.
.Fn BIO_ctrl_pending
and
.Fn BIO_ctrl_wpending
first appeared in OpenSSL 0.9.4.
These functions have been available since
.Ox 2.6 .
.Pp
.Fn BIO_callback_ctrl
first appeared in OpenSSL 0.9.5 and has been available since
.Ox 2.7 .
.Pp
.Fn bio_info_cb
first appeared with a more complicated prototype in OpenSSL 0.9.6
and has been available since
.Ox 2.9 .
.Pp
.Fn BIO_info_cb
first appeared in OpenSSL 1.1.0h and has been available since
.Ox 6.3 .
.Sh 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
.Fn BIO_seek
on a file BIO for a successful operation.