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10</style><title>Memory Management</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 Logo" /></a><a href="http://www.w3.org/Status"><img src="w3c.png" alt="W3C Logo" /></a><a href="http://www.redhat.com/"><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-180x168.gif" alt="Made with Libxml2 Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XML C parser and toolkit of Gnome</h1><h2>Memory 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cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Table of Content:</p><ol>
11  <li><a href="#General3">General overview</a></li>
12  <li><a href="#setting">Setting libxml2 set of memory routines</a></li>
13  <li><a href="#cleanup">Cleaning up after using the library</a></li>
14  <li><a href="#Debugging">Debugging routines</a></li>
15  <li><a href="#General4">General memory requirements</a></li>
16  <li><a href="#Compacting">Returning memory to the kernel</a></li>
17</ol><h3><a name="General3" id="General3">General overview</a></h3><p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlmemory.h</a></code>
18provides the interfaces to the libxml2 memory system:</p><ul>
19  <li>libxml2 does not use the libc memory allocator directly but xmlFree(),
20    xmlMalloc() and xmlRealloc()</li>
21  <li>those routines can be reallocated to a specific set of routine, by
22    default the libc ones i.e. free(), malloc() and realloc()</li>
23  <li>the xmlmemory.c module includes a set of debugging routine</li>
24</ul><h3><a name="setting" id="setting">Setting libxml2 set of memory routines</a></h3><p>It is sometimes useful to not use the default memory allocator, either for
25debugging, analysis or to implement a specific behaviour on memory management
26(like on embedded systems). Two function calls are available to do so:</p><ul>
27  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet
28    ()</a> which return the current set of functions in use by the parser</li>
29  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>
30    which allow to set up a new set of memory allocation functions</li>
31</ul><p>Of course a call to xmlMemSetup() should probably be done before calling
32any other libxml2 routines (unless you are sure your allocations routines are
33compatibles).</p><h3><a name="cleanup" id="cleanup">Cleaning up after using the library</a></h3><p>Libxml2 is not stateless, there is a few set of memory structures needing
34allocation before the parser is fully functional (some encoding structures
35for example). This also mean that once parsing is finished there is a tiny
36amount of memory (a few hundred bytes) which can be recollected if you don't
37reuse the library or any document built with it:</p><ul>
38  <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
39    ()</a> is a centralized routine to free the library state and data. Note
40    that it won't deallocate any produced tree if any (use the xmlFreeDoc()
41    and related routines for this). This should be called only when the library
42    is not used anymore.</li>
43  <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
44    ()</a> is the dual routine allowing to preallocate the parsing state
45    which can be useful for example to avoid initialization reentrancy
46    problems when using libxml2 in multithreaded applications</li>
47</ul><p>Generally xmlCleanupParser() is safe assuming no parsing is ongoing and
48no document is still being used, if needed the state will be rebuild at the
49next invocation of parser routines (or by xmlInitParser()), but be careful
50of the consequences in multithreaded applications.</p><h3><a name="Debugging" id="Debugging">Debugging routines</a></h3><p>When configured using --with-mem-debug flag (off by default), libxml2 uses
51a set of memory allocation debugging routines keeping track of all allocated
52blocks and the location in the code where the routine was called. A couple of
53other debugging routines allow to dump the memory allocated infos to a file
54or call a specific routine when a given block number is allocated:</p><ul>
55  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a>
56    <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
57    and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
58    are the memory debugging replacement allocation routines</li>
59  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
60    ()</a> dumps all the information about the allocated memory block lefts
61    in the <code>.memdump</code> file</li>
62</ul><p>When developing libxml2 memory debug is enabled, the tests programs call
63xmlMemoryDump () and the "make test" regression tests will check for any
64memory leak during the full regression test sequence, this helps a lot
65ensuring that libxml2  does not leak memory and bullet proof memory
66allocations use (some libc implementations are known to be far too permissive
67resulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function and
68also tries to give some information about the content and structure of the
69allocated blocks left. This is sufficient in most cases to find the culprit,
70but not always. Assuming the allocation problem is reproducible, it is
71possible to find more easily:</p><ol>
72  <li>write down the block number xxxx not allocated</li>
73  <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest
74    when using GDB is to simply give the command
75    <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p>
76    <p>before running the program.</p>
77  </li>
78  <li>run the program under a debugger and set a breakpoint on
79    xmlMallocBreakpoint() a specific function called when this precise block
80    is allocated</li>
81  <li>when the breakpoint is reached you can then do a fine analysis of the
82    allocation an step  to see the condition resulting in the missing
83    deallocation.</li>
84</ol><p>I used to use a commercial tool to debug libxml2 memory problems but after
85noticing that it was not detecting memory leaks that simple mechanism was
86used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some
87success, it is tied to the i386 architecture since it works by emulating the
88processor and instruction set, it is slow but  extremely efficient, i.e. it
89spot memory usage errors in a very precise way.</p><h3><a name="General4" id="General4">General memory requirements</a></h3><p>How much libxml2 memory require ? It's hard to tell in average it depends
90of a number of things:</p><ul>
91  <li>the parser itself should work  in a fixed amount of memory, except for
92    information maintained about the stacks of names and  entities locations.
93    The I/O and encoding handlers will probably account for a few KBytes.
94    This is true for both the XML and HTML parser (though the HTML parser
95    need more state).</li>
96  <li>If you are generating the DOM tree then memory requirements will grow
97    nearly linear with the size of the data. In general for a balanced
98    textual document the internal memory requirement is about 4 times the
99    size of the UTF8 serialization of this document (example the XML-1.0
100    recommendation is a bit more of 150KBytes and takes 650KBytes of main
101    memory when parsed). Validation will add a amount of memory required for
102    maintaining the external Dtd state which should be linear with the
103    complexity of the content model defined by the Dtd</li>
104  <li>If you need to work with fixed memory requirements or don't need the
105    full DOM tree then using the <a href="xmlreader.html">xmlReader
106    interface</a> is probably the best way to proceed, it still allows to
107    validate or operate on subset of the tree if needed.</li>
108  <li>If you don't care about the advanced features of libxml2 like
109    validation, DOM, XPath or XPointer, don't use entities, need to work with
110    fixed memory requirements, and try to get the fastest parsing possible
111    then the SAX interface should be used, but it has known restrictions.</li>
112</ul><p></p><h3><a name="Compacting" id="Compacting">Returning memory to the kernel</a></h3><p>You may encounter that your process using libxml2 does not have a
113reduced memory usage although you freed the trees. This is because
114libxml2 allocates memory in a number of small chunks. When freeing one
115of those chunks, the OS may decide that giving this little memory back
116to the kernel will cause too much overhead and delay the operation. As
117all chunks are this small, they get actually freed but not returned to
118the kernel. On systems using glibc, there is a function call
119"malloc_trim" from malloc.h which does this missing operation (note that
120it is allowed to fail). Thus, after freeing your tree you may simply try
121"malloc_trim(0);" to really get the memory back. If your OS does not
122provide malloc_trim, try searching for a similar function.</p><p></p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>
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