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<ul id="index">
  <li><a href="#NAME">NAME</a></li>
  <li><a href="#SYNOPSIS">SYNOPSIS</a></li>
  <li><a href="#DESCRIPTION">DESCRIPTION</a></li>
  <li><a href="#NOTES">NOTES</a></li>
  <li><a href="#RETURN-VALUES">RETURN VALUES</a></li>
  <li><a href="#EXAMPLES">EXAMPLES</a></li>
  <li><a href="#SEE-ALSO">SEE ALSO</a></li>
  <li><a href="#HISTORY">HISTORY</a></li>
  <li><a href="#COPYRIGHT">COPYRIGHT</a></li>
</ul>

<h1 id="NAME">NAME</h1>

<p>BIO_push, BIO_pop, BIO_set_next - add and remove BIOs from a chain</p>

<h1 id="SYNOPSIS">SYNOPSIS</h1>

<pre><code>#include &lt;openssl/bio.h&gt;

BIO *BIO_push(BIO *b, BIO *next);
BIO *BIO_pop(BIO *b);
void BIO_set_next(BIO *b, BIO *next);</code></pre>

<h1 id="DESCRIPTION">DESCRIPTION</h1>

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

<p>BIO_pop() removes the BIO <i>b</i> from any chain it is part of. If <i>b</i> is NULL the function does nothing and returns NULL. Otherwise it makes a control call on <i>b</i> 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.</p>

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

<h1 id="NOTES">NOTES</h1>

<p>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.</p>

<p>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.</p>

<h1 id="RETURN-VALUES">RETURN VALUES</h1>

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

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

<h1 id="EXAMPLES">EXAMPLES</h1>

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

<p>If the call:</p>

<pre><code>BIO_push(b64, f);</code></pre>

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

<pre><code>BIO_push(md2, b64);
BIO_push(md1, md2);</code></pre>

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

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

<p>The call:</p>

<pre><code>BIO_pop(md2);</code></pre>

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

<h1 id="SEE-ALSO">SEE ALSO</h1>

<p><a href="../man7/bio.html">bio(7)</a></p>

<h1 id="HISTORY">HISTORY</h1>

<p>The BIO_set_next() function was added in OpenSSL 1.1.0.</p>

<h1 id="COPYRIGHT">COPYRIGHT</h1>

<p>Copyright 2000-2026 The OpenSSL Project Authors. All Rights Reserved.</p>

<p>Licensed under the Apache License 2.0 (the &quot;License&quot;). 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 <a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>


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