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=head1 NAME

BN_CTX_start, BN_CTX_get, BN_CTX_end - use temporary BIGNUM variables

=head1 SYNOPSIS

 #include <openssl/bn.h>

 void BN_CTX_start(BN_CTX *ctx);

 BIGNUM *BN_CTX_get(BN_CTX *ctx);

 void BN_CTX_end(BN_CTX *ctx);

=head1 DESCRIPTION

These functions are used to obtain temporary B<BIGNUM> variables from
a B<BN_CTX> (which can been created by using L<BN_CTX_new(3)>)
in order to save the overhead of repeatedly creating and
freeing B<BIGNUM>s in functions that are called from inside a loop.

A function must call BN_CTX_start() first. Then, BN_CTX_get() may be
called repeatedly to obtain temporary B<BIGNUM>s. All BN_CTX_get()
calls must be made before calling any other functions that use the
B<ctx> as an argument.

Finally, BN_CTX_end() must be called before returning from the function.
If B<ctx> is NULL, nothing is done.
When BN_CTX_end() is called, the B<BIGNUM> pointers obtained from
BN_CTX_get() become invalid.

=head1 RETURN VALUES

BN_CTX_start() and BN_CTX_end() return no values.

BN_CTX_get() returns a pointer to the B<BIGNUM>, or B<NULL> on error.
Once BN_CTX_get() has failed, the subsequent calls will return B<NULL>
as well, so it is sufficient to check the return value of the last
BN_CTX_get() call. In case of an error, an error code is set, which
can be obtained by L<ERR_get_error(3)>.


=head1 SEE ALSO

L<BN_CTX_new(3)>

=head1 COPYRIGHT

Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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>.

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