<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->

<!--
 Licensed to the Apache Software Foundation (ASF) under one or more
 contributor license agreements.  See the NOTICE file distributed with
 this work for additional information regarding copyright ownership.
 The ASF licenses this file to You under the Apache License, Version 2.0
 (the "License"); you may not use this file except in compliance with
 the License.  You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
-->

<manualpage metafile="debugging.xml.meta">
<parentdocument href="./">Developer Documentation</parentdocument>

<title>Debugging Memory Allocation in APR</title>

<summary>
    <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>
</summary>
  
<section id="options"><title>Available debugging options</title>
    <section id="alloc_debug">
    <title>Allocation Debugging - ALLOC_DEBUG</title>

      <note>Debugging support: Define this to enable code which
      helps detect re-use of <code>free()</code>d memory and other such
      nonsense.</note>

      <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>
    </section>

    <section id="alloc_use_malloc">
    <title>Malloc Support - ALLOC_USE_MALLOC</title>

      <note>If defined all allocations will be done with
      <code>malloc()</code> and <code>free()</code>d appropriately at the
      end.</note>

      <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>
    </section>

    <section id="pool_debug"><title>Pool Debugging - POOL_DEBUG</title>
      <note>This is intended to detect cases where the wrong pool is
      used when assigning data to an object in another pool.</note>

      <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>
    </section>

    <section id="make_table_profile">
    <title>Table Debugging - MAKE_TABLE_PROFILE</title>

      <note>Provide diagnostic information about make_table() calls
      which are possibly too small.</note>

      <p>This requires a recent gcc which supports
      <code>__builtin_return_address()</code>. The error_log output will be a
      message such as:</p>
      <example>
        table_push: apr_table_t created by 0x804d874 hit limit of 10
      </example>

      <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>
    </section>

    <section id="alloc_stats">
    <title>Allocation Statistics -  ALLOC_STATS</title>

      <note>Provide some statistics on the cost of allocations.</note>

      <p>This requires a bit of an understanding of how <code>alloc.c</code>
      works.</p>
    </section>
</section>

<section id="combo"><title>Allowable Combinations</title>

    <p>Not all the options outlined above can be activated at the
    same time. the following table gives more information.</p>

    <table border="1" style="zebra">
    <tr><th></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><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><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>
</section>

<section id="howto"><title>Activating Debugging Options</title>

    <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>

    <example>
      /*<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 />
      <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 />
      </indent>
      #ifdef ALLOC_USE_MALLOC<br />
      <indent>
        void *allocation_list;<br />
      </indent>
      #endif<br />
      #ifdef POOL_DEBUG<br />
      <indent>
        struct ap_pool_t *joined;<br />
      </indent>
      #endif<br />
      <indent>
        int (*apr_abort)(int retcode);<br />
        struct datastruct *prog_data;<br />
      </indent>
      } ap_pool_t;
    </example>

    <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>

    <note><title>Note</title>
    <p>In order to use the various options the server <strong>must</strong>
    be rebuilt after editing the header file.</p>
    </note>
</section>
</manualpage>

