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195 lines
7.3 KiB
XML
195 lines
7.3 KiB
XML
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="debugging.xml.meta">
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<parentdocument href="./">Developer Documentation</parentdocument>
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<title>Debugging Memory Allocation in APR</title>
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<summary>
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<p>The allocation mechanisms within APR have a number of debugging modes
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that can be used to assist in finding memory problems. This document
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describes the modes available and gives instructions on activating
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them.</p>
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</summary>
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<section id="options"><title>Available debugging options</title>
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<section id="alloc_debug">
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<title>Allocation Debugging - ALLOC_DEBUG</title>
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<note>Debugging support: Define this to enable code which
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helps detect re-use of <code>free()</code>d memory and other such
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nonsense.</note>
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<p>The theory is simple. The <code>FILL_BYTE</code> (<code>0xa5</code>)
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is written over all <code>malloc</code>'d memory as we receive it, and
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is written over everything that we free up during a
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<code>clear_pool</code>. We check that blocks on the free list always
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have the <code>FILL_BYTE</code> in them, and we check during
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<code>palloc()</code> that the bytes still have <code>FILL_BYTE</code>
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in them. If you ever see garbage URLs or whatnot containing lots
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of <code>0xa5</code>s then you know something used data that's been
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freed or uninitialized.</p>
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</section>
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<section id="alloc_use_malloc">
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<title>Malloc Support - ALLOC_USE_MALLOC</title>
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<note>If defined all allocations will be done with
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<code>malloc()</code> and <code>free()</code>d appropriately at the
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end.</note>
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<p>This is intended to be used with something like Electric
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Fence or Purify to help detect memory problems. Note that if
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you're using efence then you should also add in <code>ALLOC_DEBUG</code>.
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But don't add in <code>ALLOC_DEBUG</code> if you're using Purify because
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<code>ALLOC_DEBUG</code> would hide all the uninitialized read errors
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that Purify can diagnose.</p>
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</section>
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<section id="pool_debug"><title>Pool Debugging - POOL_DEBUG</title>
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<note>This is intended to detect cases where the wrong pool is
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used when assigning data to an object in another pool.</note>
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<p>In particular, it causes the <code>table_{set,add,merge}n</code>
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routines to check that their arguments are safe for the
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<code>apr_table_t</code> they're being placed in. It currently only works
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with the unix multiprocess model, but could be extended to others.</p>
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</section>
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<section id="make_table_profile">
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<title>Table Debugging - MAKE_TABLE_PROFILE</title>
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<note>Provide diagnostic information about make_table() calls
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which are possibly too small.</note>
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<p>This requires a recent gcc which supports
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<code>__builtin_return_address()</code>. The error_log output will be a
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message such as:</p>
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<example>
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table_push: apr_table_t created by 0x804d874 hit limit of 10
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</example>
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<p>Use <code>l *0x804d874</code> to find the
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source that corresponds to. It indicates that a <code>apr_table_t</code>
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allocated by a call at that address has possibly too small an
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initial <code>apr_table_t</code> size guess.</p>
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</section>
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<section id="alloc_stats">
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<title>Allocation Statistics - ALLOC_STATS</title>
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<note>Provide some statistics on the cost of allocations.</note>
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<p>This requires a bit of an understanding of how <code>alloc.c</code>
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works.</p>
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</section>
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</section>
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<section id="combo"><title>Allowable Combinations</title>
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<p>Not all the options outlined above can be activated at the
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same time. the following table gives more information.</p>
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<table border="1" style="zebra">
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<tr><th></th>
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<th>ALLOC DEBUG</th>
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<th>ALLOC USE MALLOC</th>
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<th>POOL DEBUG</th>
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<th>MAKE TABLE PROFILE</th>
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<th>ALLOC STATS</th></tr>
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<tr><th>ALLOC DEBUG</th>
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<td>-</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td></tr>
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<tr><th>ALLOC USE MALLOC</th>
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<td>No</td><td>-</td><td>No</td><td>No</td><td>No</td></tr>
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<tr><th>POOL DEBUG</th>
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<td>Yes</td><td>No</td><td>-</td><td>Yes</td><td>Yes</td></tr>
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<tr><th>MAKE TABLE PROFILE</th>
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<td>Yes</td><td>No</td><td>Yes</td><td>-</td><td>Yes</td></tr>
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<tr><th>ALLOC STATS</th>
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<td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>-</td></tr>
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</table>
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<p>Additionally the debugging options are not suitable for
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multi-threaded versions of the server. When trying to debug
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with these options the server should be started in single
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process mode.</p>
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</section>
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<section id="howto"><title>Activating Debugging Options</title>
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<p>The various options for debugging memory are now enabled in
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the <code>apr_general.h</code> header file in APR. The various options are
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enabled by uncommenting the define for the option you wish to
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use. The section of the code currently looks like this
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(<em>contained in srclib/apr/include/apr_pools.h</em>)</p>
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<example>
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/*<br />
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#define ALLOC_DEBUG<br />
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#define POOL_DEBUG<br />
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#define ALLOC_USE_MALLOC<br />
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#define MAKE_TABLE_PROFILE<br />
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#define ALLOC_STATS<br />
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*/<br />
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<br />
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typedef struct ap_pool_t {<br />
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<indent>
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union block_hdr *first;<br />
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union block_hdr *last;<br />
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struct cleanup *cleanups;<br />
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struct process_chain *subprocesses;<br />
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struct ap_pool_t *sub_pools;<br />
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struct ap_pool_t *sub_next;<br />
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struct ap_pool_t *sub_prev;<br />
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struct ap_pool_t *parent;<br />
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char *free_first_avail;<br />
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</indent>
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#ifdef ALLOC_USE_MALLOC<br />
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<indent>
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void *allocation_list;<br />
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</indent>
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#endif<br />
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#ifdef POOL_DEBUG<br />
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<indent>
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struct ap_pool_t *joined;<br />
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</indent>
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#endif<br />
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<indent>
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int (*apr_abort)(int retcode);<br />
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struct datastruct *prog_data;<br />
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</indent>
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} ap_pool_t;
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</example>
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<p>To enable allocation debugging simply move the <code>#define
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ALLOC_DEBUG</code> above the start of the comments block and rebuild
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the server.</p>
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<note><title>Note</title>
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<p>In order to use the various options the server <strong>must</strong>
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be rebuilt after editing the header file.</p>
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</note>
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</section>
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</manualpage>
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