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10</style><title>Memory Management</title></head><body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><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 Management</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Developer Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html">Home</a></li><li><a href="guidelines.html">XML Guidelines</a></li><li><a href="tutorial/index.html">Tutorial</a></li><li><a href="xmlreader.html">The Reader Interface</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="architecture.html">libxml2 architecture</a></li><li><a href="tree.html">The tree output</a></li><li><a href="interface.html">The SAX interface</a></li><li><a href="xmlmem.html">Memory Management</a></li><li><a href="xmlio.html">I/O Interfaces</a></li><li><a href="library.html">The parser interfaces</a></li><li><a href="entities.html">Entities or no entities</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="upgrade.html">Upgrading 1.x code</a></li><li><a href="threads.html">Thread safety</a></li><li><a href="DOM.html">DOM Principles</a></li><li><a href="example.html">A real example</a></li><li><a href="xml.html">flat page</a>, <a href="site.xsl">stylesheet</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li><li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&product=libxml2">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" 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><li><a href="#General3">General overview</a></li>
11  <li><a href="#setting">Setting libxml2 set of memory routines</a></li>
12  <li><a href="#cleanup">Cleaning up after parsing</a></li>
13  <li><a href="#Debugging">Debugging routines</a></li>
14  <li><a href="#General4">General memory requirements</a></li>
15</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>
16provides the interfaces to the libxml2 memory system:</p><ul><li>libxml2 does not use the libc memory allocator directly but xmlFree(),
17    xmlMalloc() and xmlRealloc()</li>
18  <li>those routines can be reallocated to a specific set of routine, by
19    default the libc ones i.e. free(), malloc() and realloc()</li>
20  <li>the xmlmemory.c module includes a set of debugging routine</li>
21</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
22debugging, analysis or to implement a specific behaviour on memory management
23(like on embedded systems). Two function calls are available to do so:</p><ul><li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet
24    ()</a> which return the current set of functions in use by the parser</li>
25  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>
26    which allow to set up a new set of memory allocation functions</li>
27</ul><p>Of course a call to xmlMemSetup() should probably be done before calling
28any other libxml2 routines (unless you are sure your allocations routines are
29compatibles).</p><h3><a name="cleanup" id="cleanup">Cleaning up after parsing</a></h3><p>Libxml2 is not stateless, there is a few set of memory structures needing
30allocation before the parser is fully functional (some encoding structures
31for example). This also mean that once parsing is finished there is a tiny
32amount of memory (a few hundred bytes) which can be recollected if you don't
33reuse the parser immediately:</p><ul><li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
34    ()</a> is a centralized routine to free the parsing states. Note that it
35    won't deallocate any produced tree if any (use the xmlFreeDoc() and
36    related routines for this).</li>
37  <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
38    ()</a> is the dual routine allowing to preallocate the parsing state
39    which can be useful for example to avoid initialization reentrancy
40    problems when using libxml2 in multithreaded applications</li>
41</ul><p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
42at the next invocation of parser routines, but be careful of the consequences
43in 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
44a set of memory allocation debugging routines keeping track of all allocated
45blocks and the location in the code where the routine was called. A couple of
46other debugging routines allow to dump the memory allocated infos to a file
47or call a specific routine when a given block number is allocated:</p><ul><li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a>
48    <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
49    and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
50    are the memory debugging replacement allocation routines</li>
51  <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
52    ()</a> dumps all the informations about the allocated memory block lefts
53    in the <code>.memdump</code> file</li>
54</ul><p>When developing libxml2 memory debug is enabled, the tests programs call
55xmlMemoryDump () and the &quot;make test&quot; regression tests will check for any
56memory leak during the full regression test sequence, this helps a lot
57ensuring that libxml2  does not leak memory and bullet proof memory
58allocations use (some libc implementations are known to be far too permissive
59resulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function and
60also tries to give some informations about the content and structure of the
61allocated blocks left. This is sufficient in most cases to find the culprit,
62but not always. Assuming the allocation problem is reproducible, it is
63possible to find more easily:</p><ol><li>write down the block number xxxx not allocated</li>
64  <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest
65    when using GDB is to simply give the command
66    <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p>
67    <p>before running the program.</p>
68  </li>
69  <li>run the program under a debugger and set a breakpoint on
70    xmlMallocBreakpoint() a specific function called when this precise block
71    is allocated</li>
72  <li>when the breakpoint is reached you can then do a fine analysis of the
73    allocation an step  to see the condition resulting in the missing
74    deallocation.</li>
75</ol><p>I used to use a commercial tool to debug libxml2 memory problems but after
76noticing that it was not detecting memory leaks that simple mechanism was
77used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some
78success, it is tied to the i386 architecture since it works by emulating the
79processor and instruction set, it is slow but  extremely efficient, i.e. it
80spot 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
81of a number of things:</p><ul><li>the parser itself should work  in a fixed amount of memory, except for
82    information maintained about the stacks of names and  entities locations.
83    The I/O and encoding handlers will probably account for a few KBytes.
84    This is true for both the XML and HTML parser (though the HTML parser
85    need more state).</li>
86  <li>If you are generating the DOM tree then memory requirements will grow
87    nearly linear with the size of the data. In general for a balanced
88    textual document the internal memory requirement is about 4 times the
89    size of the UTF8 serialization of this document (example the XML-1.0
90    recommendation is a bit more of 150KBytes and takes 650KBytes of main
91    memory when parsed). Validation will add a amount of memory required for
92    maintaining the external Dtd state which should be linear with the
93    complexity of the content model defined by the Dtd</li>
94  <li>If you need to work with fixed memory requirements or don't need the
95    full DOM tree then using the <a href="xmlreader.html">xmlReader
96    interface</a> is probably the best way to proceed, it still allows to
97    validate or operate on subset of the tree if needed.</li>
98  <li>If you don't care about the advanced features of libxml2 like
99    validation, DOM, XPath or XPointer, don't use entities, need to work with
100    fixed memory requirements, and try to get the fastest parsing possible
101    then the SAX interface should be used, but it has known restrictions.</li>
102</ul><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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