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=head1 NAME

BIO_push, BIO_pop, BIO_set_next - add and remove BIOs from a chain

=head1 SYNOPSIS

 #include <openssl/bio.h>

 BIO *BIO_push(BIO *b, BIO *next);
 BIO *BIO_pop(BIO *b);
 void BIO_set_next(BIO *b, BIO *next);

=head1 DESCRIPTION

BIO_push() pushes I<b> on I<next>.
If I<b> is NULL the function does nothing and returns I<next>.
Otherwise it prepends I<b>, which may be a single BIO or a chain of BIOs,
to I<next> (unless I<next> is NULL).
It then makes a control call on I<b> and returns I<b>.

BIO_pop() removes the BIO I<b> from any chain is is part of.
If I<b> is NULL the function does nothing and returns NULL.
Otherwise it makes a control call on I<b> and
returns the next BIO in the chain, or NULL if there is no next BIO.
The removed BIO becomes a single BIO with no association with
the original chain, it can thus be freed or be made part of a different chain.

BIO_set_next() replaces the existing next BIO in a chain with the BIO pointed to
by I<next>. The new chain may include some of the same BIOs from the old chain
or it may be completely different.

=head1 NOTES

The names of these functions are perhaps a little misleading. BIO_push()
joins two BIO chains whereas BIO_pop() deletes a single BIO from a chain,
the deleted BIO does not need to be at the end of a chain.

The process of calling BIO_push() and BIO_pop() on a BIO may have additional
consequences (a control call is made to the affected BIOs).
Any effects will be noted in the descriptions of individual BIOs.

=head1 RETURN VALUES

BIO_push() returns the head of the chain,
which usually is I<b>, or I<next> if I<b> is NULL.

BIO_pop() returns the next BIO in the chain,
or NULL if there is no next BIO.

=head1 EXAMPLES

For these examples suppose I<md1> and I<md2> are digest BIOs,
I<b64> is a base64 BIO and I<f> is a file BIO.

If the call:

 BIO_push(b64, f);

is made then the new chain will be I<b64-f>. After making the calls

 BIO_push(md2, b64);
 BIO_push(md1, md2);

the new chain is I<md1-md2-b64-f>. Data written to I<md1> will be digested
by I<md1> and I<md2>, base64 encoded, and finally written to I<f>.

It should be noted that reading causes data to pass in the reverse
direction, that is data is read from I<f>, base64 decoded,
and digested by I<md2> and then I<md1>.

The call:

 BIO_pop(md2);

will return I<b64> and the new chain will be I<md1-b64-f>.
Data can be written to and read from I<md1> as before,
except that I<md2> will no more be applied.

=head1 SEE ALSO

L<bio>

=head1 HISTORY

The BIO_set_next() function was added in OpenSSL 1.1.0.

=head1 COPYRIGHT

Copyright 2000-2021 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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