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258 lines
6.8 KiB
HTML
258 lines
6.8 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Debugging Memory Allocation in APR</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1 align="center">Debugging Memory Allocation in APR<br />
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</h1>
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<p>The allocation mechanism's within APR have a number of
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debugging modes that can be used to assist in finding memory
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problems. This document describes the modes available and gives
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instructions on activating them.</p>
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<ul>
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<li><a href="#options">Available debugging options</a></li>
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<li><a href="#combo">Allowable combinations</a></li>
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<li><a href="#howto">How to activate debugging</a></li>
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</ul>
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<hr />
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<h2>Allocation Debugging</h2>
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<h3>ALLOC_DEBUG</h3>
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<p><em>Debugging support: Define this to enable code which
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helps detect re-use of freed memory and other such
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nonsense.</em></p>
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<p>The theory is simple. The FILL_BYTE (0xa5) is written over
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all malloc'd memory as we receive it, and is written over
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everything that we free up during a clear_pool. We check that
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blocks on the free list always have the FILL_BYTE in them, and
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we check during palloc() that the bytes still have FILL_BYTE in
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them. If you ever see garbage URLs or whatnot containing lots
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of 0xa5s then you know something used data that's been freed or
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uninitialized.</p>
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<h2>Malloc Support</h2>
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<h3>ALLOC_USE_MALLOC</h3>
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<p><em>If defined all allocations will be done with malloc and
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free()d appropriately at the end.</em></p>
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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 ALLOC_DEBUG.
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But don't add in ALLOC_DEBUG if you're using Purify because
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ALLOC_DEBUG would hide all the uninitialized read errors that
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Purify can diagnose.</p>
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<h2>Pool Debugging</h2>
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<h3>POOL_DEBUG</h3>
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<p><em>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.</em></p>
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<p>In particular, it causes the table_{set,add,merge}n routines
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to check that their arguments are safe for the apr_table_t
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they're being placed in. It currently only works with the unix
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multiprocess model, but could be extended to others.</p>
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<h2>Table Debugging</h2>
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<h3>MAKE_TABLE_PROFILE</h3>
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<p><em>Provide diagnostic information about make_table() calls
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which are possibly too small.</em></p>
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<p>This requires a recent gcc which supports
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__builtin_return_address(). The error_log output will be a
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message such as:</p>
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<pre>
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table_push: apr_table_t created by 0x804d874 hit limit of 10
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</pre>
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<p>Use "<em><strong>l *0x804d874</strong></em>" to find the
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source that corresponds to. It indicates that a apr_table_t
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allocated by a call at that address has possibly too small an
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initial apr_table_t size guess.</p>
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<h2>Allocation Statistics</h2>
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<h3>ALLOC_STATS</h3>
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<p><em>Provide some statistics on the cost of
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allocations.</em></p>
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<p>This requires a bit of an understanding of how alloc.c
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works.</p>
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<hr />
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<h2>Allowable Combinations</h2>
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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 width="80%">
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<tr>
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<th width="25%">Option 1</th>
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<th width="15%">ALLOC<br />
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DEBUG</th>
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<th width="15%">ALLOC<br />
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USE<br />
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MALLOC</th>
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<th width="15%">POOL<br />
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DEBUG</th>
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<th width="15%">MAKE<br />
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TABLE<br />
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PROFILE</th>
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<th width="15%">ALLOC<br />
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STATS</th>
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</tr>
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<tr>
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<td>ALLOC_DEBUG</td>
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<td bgcolor="#ff0000"> </td>
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<td align="center">No</td>
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<td align="center">Yes</td>
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<td align="center">Yes</td>
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<td align="center">Yes</td>
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</tr>
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<tr>
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<td>ALLOC_USE<br />
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MALLOC</td>
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<td align="center">No</td>
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<td bgcolor="#ff0000"> </td>
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<td align="center">No</td>
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<td align="center">No</td>
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<td align="center">No</td>
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</tr>
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<tr>
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<td>POOL_DEBUG</td>
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<td align="center">Yes</td>
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<td align="center">No</td>
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<td bgcolor="#ff0000"> </td>
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<td align="center">Yes</td>
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<td align="center">Yes</td>
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</tr>
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<tr>
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<td>MAKE_TABLE<br />
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PROFILE</td>
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<td align="center">Yes</td>
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<td align="center">No</td>
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<td align="center">Yes</td>
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<td bgcolor="#ff0000"> </td>
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<td align="center">Yes</td>
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</tr>
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<tr>
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<td>ALLOC_STATS</td>
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<td align="center">Yes</td>
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<td align="center">No</td>
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<td align="center">Yes</td>
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<td align="center">Yes</td>
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<td bgcolor="#ff0000"> </td>
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</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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<hr />
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<h2>Activating Debugging Options</h2>
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<p>The various options for debugging memory are now enabled in
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the apr_general.h 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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<pre>
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/*
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#define ALLOC_DEBUG
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#define POOL_DEBUG
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#define ALLOC_USE_MALLOC
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#define MAKE_TABLE_PROFILE
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#define ALLOC_STATS
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*/
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typedef struct ap_pool_t {
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union block_hdr *first;
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union block_hdr *last;
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struct cleanup *cleanups;
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struct process_chain *subprocesses;
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struct ap_pool_t *sub_pools;
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struct ap_pool_t *sub_next;
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struct ap_pool_t *sub_prev;
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struct ap_pool_t *parent;
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char *free_first_avail;
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#ifdef ALLOC_USE_MALLOC
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void *allocation_list;
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#endif
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#ifdef POOL_DEBUG
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struct ap_pool_t *joined;
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#endif
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int (*apr_abort)(int retcode);
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struct datastruct *prog_data;
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}ap_pool_t;
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</pre>
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<p>To enable allocation debugging simply move the #define
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ALLOC_DEBUG above the start of the comments block and rebuild
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the server.</p>
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<strong>NB. In order to use the various options the server MUST
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be rebuilt after editing the header file.</strong>
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</body>
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</html>
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