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<?xml version="1.0" encoding="ISO-8859-1"?>
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<title>Debugging Memory Allocation in APR - Apache HTTP Server</title>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<div id="path">
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Debugging Memory Allocation in APR</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/developer/debugging.html" title="English"> en </a></p>
</div>
<p>The allocation mechanisms within APR have a number of debugging modes
that can be used to assist in finding memory problems. This document
describes the modes available and gives instructions on activating
them.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#options">Available debugging options</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#combo">Allowable Combinations</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#howto">Activating Debugging Options</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="options" id="options">Available debugging options</a></h2>
<h3><a name="alloc_debug" id="alloc_debug">Allocation Debugging - ALLOC_DEBUG</a></h3>
<div class="note">Debugging support: Define this to enable code which
helps detect re-use of <code>free()</code>d memory and other such
nonsense.</div>
<p>The theory is simple. The <code>FILL_BYTE</code> (<code>0xa5</code>)
is written over all <code>malloc</code>'d memory as we receive it, and
is written over everything that we free up during a
<code>clear_pool</code>. We check that blocks on the free list always
have the <code>FILL_BYTE</code> in them, and we check during
<code>palloc()</code> that the bytes still have <code>FILL_BYTE</code>
in them. If you ever see garbage URLs or whatnot containing lots
of <code>0xa5</code>s then you know something used data that's been
freed or uninitialized.</p>
<h3><a name="alloc_use_malloc" id="alloc_use_malloc">Malloc Support - ALLOC_USE_MALLOC</a></h3>
<div class="note">If defined all allocations will be done with
<code>malloc()</code> and <code>free()</code>d appropriately at the
end.</div>
<p>This is intended to be used with something like Electric
Fence or Purify to help detect memory problems. Note that if
you're using efence then you should also add in <code>ALLOC_DEBUG</code>.
But don't add in <code>ALLOC_DEBUG</code> if you're using Purify because
<code>ALLOC_DEBUG</code> would hide all the uninitialized read errors
that Purify can diagnose.</p>
<h3><a name="pool_debug" id="pool_debug">Pool Debugging - POOL_DEBUG</a></h3>
<div class="note">This is intended to detect cases where the wrong pool is
used when assigning data to an object in another pool.</div>
<p>In particular, it causes the <code>table_{set,add,merge}n</code>
routines to check that their arguments are safe for the
<code>apr_table_t</code> they're being placed in. It currently only works
with the unix multiprocess model, but could be extended to others.</p>
<h3><a name="make_table_profile" id="make_table_profile">Table Debugging - MAKE_TABLE_PROFILE</a></h3>
<div class="note">Provide diagnostic information about make_table() calls
which are possibly too small.</div>
<p>This requires a recent gcc which supports
<code>__builtin_return_address()</code>. The error_log output will be a
message such as:</p>
<div class="example"><p><code>
table_push: apr_table_t created by 0x804d874 hit limit of 10
</code></p></div>
<p>Use <code>l *0x804d874</code> to find the
source that corresponds to. It indicates that a <code>apr_table_t</code>
allocated by a call at that address has possibly too small an
initial <code>apr_table_t</code> size guess.</p>
<h3><a name="alloc_stats" id="alloc_stats">Allocation Statistics - ALLOC_STATS</a></h3>
<div class="note">Provide some statistics on the cost of allocations.</div>
<p>This requires a bit of an understanding of how <code>alloc.c</code>
works.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="combo" id="combo">Allowable Combinations</a></h2>
<p>Not all the options outlined above can be activated at the
same time. the following table gives more information.</p>
<table class="bordered"><tr class="header"><th />
<th>ALLOC DEBUG</th>
<th>ALLOC USE MALLOC</th>
<th>POOL DEBUG</th>
<th>MAKE TABLE PROFILE</th>
<th>ALLOC STATS</th></tr>
<tr><th>ALLOC DEBUG</th>
<td>-</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td></tr>
<tr class="odd"><th>ALLOC USE MALLOC</th>
<td>No</td><td>-</td><td>No</td><td>No</td><td>No</td></tr>
<tr><th>POOL DEBUG</th>
<td>Yes</td><td>No</td><td>-</td><td>Yes</td><td>Yes</td></tr>
<tr class="odd"><th>MAKE TABLE PROFILE</th>
<td>Yes</td><td>No</td><td>Yes</td><td>-</td><td>Yes</td></tr>
<tr><th>ALLOC STATS</th>
<td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>-</td></tr>
</table>
<p>Additionally the debugging options are not suitable for
multi-threaded versions of the server. When trying to debug
with these options the server should be started in single
process mode.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="howto" id="howto">Activating Debugging Options</a></h2>
<p>The various options for debugging memory are now enabled in
the <code>apr_general.h</code> header file in APR. The various options are
enabled by uncommenting the define for the option you wish to
use. The section of the code currently looks like this
(<em>contained in srclib/apr/include/apr_pools.h</em>)</p>
<div class="example"><p><code>
/*<br />
#define ALLOC_DEBUG<br />
#define POOL_DEBUG<br />
#define ALLOC_USE_MALLOC<br />
#define MAKE_TABLE_PROFILE<br />
#define ALLOC_STATS<br />
*/<br />
<br />
typedef struct ap_pool_t {<br />
<span class="indent">
union block_hdr *first;<br />
union block_hdr *last;<br />
struct cleanup *cleanups;<br />
struct process_chain *subprocesses;<br />
struct ap_pool_t *sub_pools;<br />
struct ap_pool_t *sub_next;<br />
struct ap_pool_t *sub_prev;<br />
struct ap_pool_t *parent;<br />
char *free_first_avail;<br />
</span>
#ifdef ALLOC_USE_MALLOC<br />
<span class="indent">
void *allocation_list;<br />
</span>
#endif<br />
#ifdef POOL_DEBUG<br />
<span class="indent">
struct ap_pool_t *joined;<br />
</span>
#endif<br />
<span class="indent">
int (*apr_abort)(int retcode);<br />
struct datastruct *prog_data;<br />
</span>
} ap_pool_t;
</code></p></div>
<p>To enable allocation debugging simply move the <code>#define
ALLOC_DEBUG</code> above the start of the comments block and rebuild
the server.</p>
<div class="note"><h3>Note</h3>
<p>In order to use the various options the server <strong>must</strong>
be rebuilt after editing the header file.</p>
</div>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/developer/debugging.html" title="English"> en </a></p>
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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