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3    <title>Using APR Pools</title>
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7      Last modified at [$Date: 2004-11-24 17:51:51 -0500 (Wed, 24 Nov 2004) $]
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10    <h1>Using APR Pools</h1>
11
12    <p>
13      From <a href="http://subversion.tigris.org/">Subversion</a>, we
14      have learned a <em>lot</em> about how to use pools in a heavily
15      structured/object-based environment.
16      <a href="http://httpd.apache.org/">Apache httpd</a> is a
17      completely different beast: "allocate a request pool. use
18      it. destroy it."
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20
21    <p>
22      In a complex app, that request-style of behavior is not
23      present. Luckily, the "proper" use of pools can be described in
24      just a few rules:
25    </p>
26
27    <ul>
28      <li>
29        Objects should not have their own pools. An object is
30        allocated into a pool defined by the constructor's caller. The
31        <strong>caller</strong> knows the lifetime of the object and
32        will manage it via the pool. Generally, this also means that
33        objects will not have a "close" or a "free" since those
34        operations will happen implicitly as part of the destruction
35        of the pool the objects live within.
36      </li>
37
38      <li>
39        <p>
40          Functions should not create/destroy pools for their
41          operation; they should use a pool provided by the
42          caller. Again, the <strong>caller</strong> knows more about
43          how the function will be used, how often, how many times,
44          etc. Thus, it should be in charge of the function's memory
45          usage.
46        </p>
47        <p>
48          As an example, the caller might know that the app will exit
49          upon the function's return. Thus, the function would be
50          creating extra work if it built and destroyed a
51          pool. Instead, it should use the passed-in pool, which the
52          caller is going to be tossing as part of app-exit anyways.
53        </p>
54      </li>
55
56      <li>
57        <p>
58          Whenever an unbounded iteration occurs, a subpool should be
59          used. The general pattern is:
60        </p>
61        <blockquote>
62          <pre>
63subpool = apr_create_subpool(pool);
64for (i = 0; i < n; ++i) {
65  apr_pool_clear(subpool);
66
67  do_operation(..., subpool);
68}
69apr_pool_destroy(subpool);</pre>
70        </blockquote>
71        <p>
72          This pattern prevents the 'pool' from growing unbounded and
73          consuming all of memory. Note that it is slightly more
74          optimal to clear the pool on loop-entry. This pattern also
75          allows for a '<tt>continue</tt>' to occur within the loop,
76          yet still ensure the pool will be cleared.
77        </p>
78      </li>
79
80      <li>
81        Given all of the above, it is pretty well mandatory to pass a
82        pool to <em>every</em> function. Since objects are not
83        recording pools for themselves, and the caller is always
84        supposed to be managing memory, then each function needs a
85        pool, rather than relying on some hidden magic pool. In
86        limited cases, objects may record the pool used for their
87        construction so that they can construct sub-parts, but these
88        cases should be examined carefully. Internal pools can lead to
89        unbounded pool usage if the object is not careful.
90      </li>
91    </ul>
92
93    <hr>
94    <address>Greg Stein</address>
95    <!-- Created: Wed Jun 25 14:39:57 PDT 2003 -->
96    <!-- hhmts start -->
97Last modified: Wed Jun 25 14:50:19 PDT 2003
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