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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 2.1</p> |
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| <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div> |
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| <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> > <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 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> |
| <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 alloc.c 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> |
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| <p><span>Available Languages: </span><a href="../en/developer/debugging.html" title="English"> en </a></p> |
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| <p class="apache">Copyright 1999-2004 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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