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