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<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>
+ <p>
+ This document has been removed.
+ </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>
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