debugging.html.en revision 96d3308828b430916b4629d503c4878fad28773d
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
This file is generated from xml source: DO NOT EDIT
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-->
<title>Debugging Memory Allocation in APR - Apache HTTP Server</title>
<link href="/style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
<link href="/style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
<body id="manual-page"><div id="page-header">
<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.html">Directives</a> | <a href="/faq/">FAQ</a> | <a href="/glossary.html">Glossary</a> | <a href="/sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.1</p>
<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-project/">Documentation</a> > <a href="../">Version 2.1</a></div><div id="page-content"><div id="preamble"><h1>Debugging Memory Allocation in APR</h1>
<p>The allocation mechanism's 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>
</ul></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>
<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 class="section">
<h2><a name="howto" id="howto">Activating Debugging Options</a></h2>
<p>The various options for debugging memory are now enabled in
enabled by uncommenting the define for the option you wish to
use. The section of the code currently looks like this
<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 id="footer">
<p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p>
<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.html">Directives</a> | <a href="/faq/">FAQ</a> | <a href="/glossary.html">Glossary</a> | <a href="/sitemap.html">Sitemap</a></p></div>
</body></html>