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BN_CTX_start.3
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.\" $OpenBSD: BN_CTX_start.3,v 1.8 2019/08/20 10:59:09 schwarze Exp $
.\" full merge up to: OpenSSL 35fd9953 May 28 14:49:38 2019 +0200
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>.
.\" Copyright (c) 2000 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
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.\"
.Dd $Mdocdate: August 20 2019 $
.Dt BN_CTX_START 3
.Os
.Sh NAME
.Nm BN_CTX_start ,
.Nm BN_CTX_get ,
.Nm BN_CTX_end
.Nd use temporary BIGNUM variables
.Sh SYNOPSIS
.In openssl/bn.h
.Ft void
.Fo BN_CTX_start
.Fa "BN_CTX *ctx"
.Fc
.Ft BIGNUM *
.Fo BN_CTX_get
.Fa "BN_CTX *ctx"
.Fc
.Ft void
.Fo BN_CTX_end
.Fa "BN_CTX *ctx"
.Fc
.Sh DESCRIPTION
These functions are used to obtain temporary
.Vt BIGNUM
variables from a
.Vt BN_CTX
(which can be created using
.Xr BN_CTX_new 3 )
in order to save the overhead of repeatedly creating and freeing
.Vt BIGNUM Ns s
in functions that are called from inside a loop.
.Pp
A function must call
.Fn BN_CTX_start
first.
Then,
.Fn BN_CTX_get
may be called repeatedly to obtain temporary
.Vt BIGNUM Ns s .
All
.Fn BN_CTX_get
calls must be made before calling any other functions that use the
.Fa ctx
as an argument.
.Pp
Finally,
.Fn BN_CTX_end
must be called before returning from the function.
When
.Fn BN_CTX_end
is called, the
.Vt BIGNUM
pointers obtained from
.Fn BN_CTX_get
become invalid.
If
.Fa ctx
is
.Dv NULL ,
no action occurs.
.Sh RETURN VALUES
.Fn BN_CTX_get
returns a pointer to the
.Vt BIGNUM ,
or
.Dv NULL
on error.
Once
.Fn BN_CTX_get
has failed, the subsequent calls will return
.Dv NULL
as well, so it is sufficient to check the return value of the last
.Fn BN_CTX_get
call.
In case of an error, an error code is set which can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr BN_CTX_new 3 ,
.Xr BN_new 3
.Sh HISTORY
.Fn BN_CTX_start ,
.Fn BN_CTX_get ,
and
.Fn BN_CTX_end
first appeared in OpenSSL 0.9.5 and have been available since
.Ox 2.7 .