apache/docs/manual/developer/debugging.xml

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<parentdocument href="./">Developer Documentation</parentdocument>

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

<summary>
    <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>
</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 alloc.c 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>