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23 <manualpage metafile="debugging.xml.meta">
24 <parentdocument href="./">Developer Documentation</parentdocument>
26 <title>Debugging Memory Allocation in APR</title>
29 <p>The allocation mechanisms within APR have a number of debugging modes
30 that can be used to assist in finding memory problems. This document
31 describes the modes available and gives instructions on activating
35 <section id="options"><title>Available debugging options</title>
36 <section id="alloc_debug">
37 <title>Allocation Debugging - ALLOC_DEBUG</title>
39 <note>Debugging support: Define this to enable code which
40 helps detect re-use of <code>free()</code>d memory and other such
43 <p>The theory is simple. The <code>FILL_BYTE</code> (<code>0xa5</code>)
44 is written over all <code>malloc</code>'d memory as we receive it, and
45 is written over everything that we free up during a
46 <code>clear_pool</code>. We check that blocks on the free list always
47 have the <code>FILL_BYTE</code> in them, and we check during
48 <code>palloc()</code> that the bytes still have <code>FILL_BYTE</code>
49 in them. If you ever see garbage URLs or whatnot containing lots
50 of <code>0xa5</code>s then you know something used data that's been
51 freed or uninitialized.</p>
54 <section id="alloc_use_malloc">
55 <title>Malloc Support - ALLOC_USE_MALLOC</title>
57 <note>If defined all allocations will be done with
58 <code>malloc()</code> and <code>free()</code>d appropriately at the
61 <p>This is intended to be used with something like Electric
62 Fence or Purify to help detect memory problems. Note that if
63 you're using efence then you should also add in <code>ALLOC_DEBUG</code>.
64 But don't add in <code>ALLOC_DEBUG</code> if you're using Purify because
65 <code>ALLOC_DEBUG</code> would hide all the uninitialized read errors
66 that Purify can diagnose.</p>
69 <section id="pool_debug"><title>Pool Debugging - POOL_DEBUG</title>
70 <note>This is intended to detect cases where the wrong pool is
71 used when assigning data to an object in another pool.</note>
73 <p>In particular, it causes the <code>table_{set,add,merge}n</code>
74 routines to check that their arguments are safe for the
75 <code>apr_table_t</code> they're being placed in. It currently only works
76 with the unix multiprocess model, but could be extended to others.</p>
79 <section id="make_table_profile">
80 <title>Table Debugging - MAKE_TABLE_PROFILE</title>
82 <note>Provide diagnostic information about make_table() calls
83 which are possibly too small.</note>
85 <p>This requires a recent gcc which supports
86 <code>__builtin_return_address()</code>. The error_log output will be a
89 table_push: apr_table_t created by 0x804d874 hit limit of 10
92 <p>Use <code>l *0x804d874</code> to find the
93 source that corresponds to. It indicates that a <code>apr_table_t</code>
94 allocated by a call at that address has possibly too small an
95 initial <code>apr_table_t</code> size guess.</p>
98 <section id="alloc_stats">
99 <title>Allocation Statistics - ALLOC_STATS</title>
101 <note>Provide some statistics on the cost of allocations.</note>
103 <p>This requires a bit of an understanding of how <code>alloc.c</code>
108 <section id="combo"><title>Allowable Combinations</title>
110 <p>Not all the options outlined above can be activated at the
111 same time. the following table gives more information.</p>
113 <table border="1" style="zebra">
116 <th>ALLOC USE MALLOC</th>
118 <th>MAKE TABLE PROFILE</th>
119 <th>ALLOC STATS</th></tr>
120 <tr><th>ALLOC DEBUG</th>
121 <td>-</td><td>No</td><td>Yes</td><td>Yes</td><td>Yes</td></tr>
122 <tr><th>ALLOC USE MALLOC</th>
123 <td>No</td><td>-</td><td>No</td><td>No</td><td>No</td></tr>
124 <tr><th>POOL DEBUG</th>
125 <td>Yes</td><td>No</td><td>-</td><td>Yes</td><td>Yes</td></tr>
126 <tr><th>MAKE TABLE PROFILE</th>
127 <td>Yes</td><td>No</td><td>Yes</td><td>-</td><td>Yes</td></tr>
128 <tr><th>ALLOC STATS</th>
129 <td>Yes</td><td>No</td><td>Yes</td><td>Yes</td><td>-</td></tr>
132 <p>Additionally the debugging options are not suitable for
133 multi-threaded versions of the server. When trying to debug
134 with these options the server should be started in single
138 <section id="howto"><title>Activating Debugging Options</title>
140 <p>The various options for debugging memory are now enabled in
141 the <code>apr_general.h</code> header file in APR. The various options are
142 enabled by uncommenting the define for the option you wish to
143 use. The section of the code currently looks like this
144 (<em>contained in srclib/apr/include/apr_pools.h</em>)</p>
148 #define ALLOC_DEBUG<br />
149 #define POOL_DEBUG<br />
150 #define ALLOC_USE_MALLOC<br />
151 #define MAKE_TABLE_PROFILE<br />
152 #define ALLOC_STATS<br />
155 typedef struct ap_pool_t {<br />
157 union block_hdr *first;<br />
158 union block_hdr *last;<br />
159 struct cleanup *cleanups;<br />
160 struct process_chain *subprocesses;<br />
161 struct ap_pool_t *sub_pools;<br />
162 struct ap_pool_t *sub_next;<br />
163 struct ap_pool_t *sub_prev;<br />
164 struct ap_pool_t *parent;<br />
165 char *free_first_avail;<br />
167 #ifdef ALLOC_USE_MALLOC<br />
169 void *allocation_list;<br />
172 #ifdef POOL_DEBUG<br />
174 struct ap_pool_t *joined;<br />
178 int (*apr_abort)(int retcode);<br />
179 struct datastruct *prog_data;<br />
184 <p>To enable allocation debugging simply move the <code>#define
185 ALLOC_DEBUG</code> above the start of the comments block and rebuild
188 <note><title>Note</title>
189 <p>In order to use the various options the server <strong>must</strong>
190 be rebuilt after editing the header file.</p>