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| <manualpage metafile="debugging.xml.meta"> |
| <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> |
| |