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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Using APR Pools</title>
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<body>
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Last modified at [$Date: 2004-11-24 17:51:51 -0500 (Wed, 24 Nov 2004) $]
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<h1>Using APR Pools</h1>
<p>
From <a href="http://subversion.tigris.org/">Subversion</a>, we
have learned a <em>lot</em> about how to use pools in a heavily
structured/object-based environment.
<a href="http://httpd.apache.org/">Apache httpd</a> is a
completely different beast: "allocate a request pool. use
it. destroy it."
</p>
<p>
In a complex app, that request-style of behavior is not
present. Luckily, the "proper" use of pools can be described in
just a few rules:
</p>
<ul>
<li>
Objects should not have their own pools. An object is
allocated into a pool defined by the constructor's caller. The
<strong>caller</strong> knows the lifetime of the object and
will manage it via the pool. Generally, this also means that
objects will not have a "close" or a "free" since those
operations will happen implicitly as part of the destruction
of the pool the objects live within.
</li>
<li>
<p>
Functions should not create/destroy pools for their
operation; they should use a pool provided by the
caller. Again, the <strong>caller</strong> knows more about
how the function will be used, how often, how many times,
etc. Thus, it should be in charge of the function's memory
usage.
</p>
<p>
As an example, the caller might know that the app will exit
upon the function's return. Thus, the function would be
creating extra work if it built and destroyed a
pool. Instead, it should use the passed-in pool, which the
caller is going to be tossing as part of app-exit anyways.
</p>
</li>
<li>
<p>
Whenever an unbounded iteration occurs, a subpool should be
used. The general pattern is:
</p>
<blockquote>
<pre>
subpool = apr_create_subpool(pool);
for (i = 0; i < n; ++i) {
apr_pool_clear(subpool);
do_operation(..., subpool);
}
apr_pool_destroy(subpool);</pre>
</blockquote>
<p>
This pattern prevents the 'pool' from growing unbounded and
consuming all of memory. Note that it is slightly more
optimal to clear the pool on loop-entry. This pattern also
allows for a '<tt>continue</tt>' to occur within the loop,
yet still ensure the pool will be cleared.
</p>
</li>
<li>
Given all of the above, it is pretty well mandatory to pass a
pool to <em>every</em> function. Since objects are not
recording pools for themselves, and the caller is always
supposed to be managing memory, then each function needs a
pool, rather than relying on some hidden magic pool. In
limited cases, objects may record the pool used for their
construction so that they can construct sub-parts, but these
cases should be examined carefully. Internal pools can lead to
unbounded pool usage if the object is not careful.
</li>
</ul>
<hr>
<address>Greg Stein</address>
<!-- Created: Wed Jun 25 14:39:57 PDT 2003 -->
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Last modified: Wed Jun 25 14:50:19 PDT 2003
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