mirror of
https://gitlab.gnome.org/GNOME/libxml2.git
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ce192eb8a6
* relaxng.c xmllint.c: more work on RelaxNG streaming validation trying to improve the subset compiled, and more testing. * doc/downloads.html doc/xml.html doc/xmlmem.html: some updates on the documentation * test/relaxng/tutor11_1_3.xml: fixes the DTD path * result/relaxng/*.err: fix some of the outputs Daniel
232 lines
13 KiB
HTML
232 lines
13 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<link rel="SHORTCUT ICON" href="/favicon.ico">
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<style type="text/css"><!--
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TD {font-family: Verdana,Arial,Helvetica}
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BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
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H1 {font-family: Verdana,Arial,Helvetica}
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H2 {font-family: Verdana,Arial,Helvetica}
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H3 {font-family: Verdana,Arial,Helvetica}
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A:link, A:visited, A:active { text-decoration: underline }
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--></style>
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<title>Memory Management</title>
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</head>
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<body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000">
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<table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr>
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<td width="180">
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<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>
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</td>
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<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">
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<h1>The XML C library for Gnome</h1>
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<h2>Memory Management</h2>
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</td></tr></table></td></tr></table></td>
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</tr></table>
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<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>
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<td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
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<tr><td bgcolor="#fffacd">
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<form action="search.php" enctype="application/x-www-form-urlencoded" method="GET">
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<input name="query" type="TEXT" size="20" value=""><input name="submit" type="submit" value="Search ...">
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</form>
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<ul>
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<li><a href="index.html">Home</a></li>
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<li><a href="intro.html">Introduction</a></li>
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<li><a href="FAQ.html">FAQ</a></li>
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<li><a href="docs.html">Documentation</a></li>
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<li><a href="bugs.html">Reporting bugs and getting help</a></li>
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<li><a href="help.html">How to help</a></li>
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<li><a href="downloads.html">Downloads</a></li>
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<li><a href="news.html">News</a></li>
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<li><a href="XMLinfo.html">XML</a></li>
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<li><a href="XSLT.html">XSLT</a></li>
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<li><a href="python.html">Python and bindings</a></li>
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<li><a href="architecture.html">libxml architecture</a></li>
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<li><a href="tree.html">The tree output</a></li>
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<li><a href="interface.html">The SAX interface</a></li>
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<li><a href="xmldtd.html">Validation & DTDs</a></li>
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<li><a href="xmlmem.html">Memory Management</a></li>
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<li><a href="encoding.html">Encodings support</a></li>
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<li><a href="xmlio.html">I/O Interfaces</a></li>
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<li><a href="catalog.html">Catalog support</a></li>
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<li><a href="library.html">The parser interfaces</a></li>
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<li><a href="entities.html">Entities or no entities</a></li>
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<li><a href="namespaces.html">Namespaces</a></li>
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<li><a href="upgrade.html">Upgrading 1.x code</a></li>
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<li><a href="threads.html">Thread safety</a></li>
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<li><a href="DOM.html">DOM Principles</a></li>
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<li><a href="example.html">A real example</a></li>
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<li><a href="contribs.html">Contributions</a></li>
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<li><a href="xmlreader.html">The Reader Interface</a></li>
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<li><a href="tutorial/index.html">Tutorial</a></li>
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<li><a href="guidelines.html">XML Guidelines</a></li>
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<li>
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<a href="xml.html">flat page</a>, <a href="site.xsl">stylesheet</a>
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</li>
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</ul>
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</td></tr>
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</table>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
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<tr><td bgcolor="#fffacd"><ul>
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<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
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<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
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<li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li>
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<li><a href="ftp://xmlsoft.org/">FTP</a></li>
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<li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li>
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<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
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<li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li>
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<li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li>
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<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&product=libxml2">Bug Tracker</a></li>
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</ul></td></tr>
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</table>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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<tr><td bgcolor="#fffacd"><ul>
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<li><a href="APIchunk0.html">Alphabetic</a></li>
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<li><a href="APIconstructors.html">Constructors</a></li>
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<li><a href="APIfunctions.html">Functions/Types</a></li>
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<li><a href="APIfiles.html">Modules</a></li>
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<li><a href="APIsymbols.html">Symbols</a></li>
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</ul></td></tr>
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</table>
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</td></tr></table></td>
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<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">
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<p>Table of Content:</p>
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<ol>
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<li><a href="#General3">General overview</a></li>
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<li><a href="#setting">Setting libxml set of memory routines</a></li>
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<li><a href="#cleanup">Cleaning up after parsing</a></li>
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<li><a href="#Debugging">Debugging routines</a></li>
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<li><a href="#General4">General memory requirements</a></li>
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</ol>
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<h3><a name="General3">General overview</a></h3>
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<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlmemory.h</a></code>
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provides the interfaces to the libxml memory system:</p>
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<ul>
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<li>libxml does not use the libc memory allocator directly but xmlFree(),
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xmlMalloc() and xmlRealloc()</li>
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<li>those routines can be reallocated to a specific set of routine, by
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default the libc ones i.e. free(), malloc() and realloc()</li>
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<li>the xmlmemory.c module includes a set of debugging routine</li>
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</ul>
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<h3><a name="setting">Setting libxml set of memory routines</a></h3>
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<p>It is sometimes useful to not use the default memory allocator, either for
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debugging, analysis or to implement a specific behaviour on memory management
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(like on embedded systems). Two function calls are available to do so:</p>
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<ul>
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<li>
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<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet
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()</a> which return the current set of functions in use by the parser</li>
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<li>
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<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>
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which allow to set up a new set of memory allocation functions</li>
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</ul>
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<p>Of course a call to xmlMemSetup() should probably be done before calling
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any other libxml routines (unless you are sure your allocations routines are
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compatibles).</p>
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<h3><a name="cleanup">Cleaning up after parsing</a></h3>
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<p>Libxml is not stateless, there is a few set of memory structures needing
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allocation before the parser is fully functional (some encoding structures
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for example). This also mean that once parsing is finished there is a tiny
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amount of memory (a few hundred bytes) which can be recollected if you don't
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reuse the parser immediately:</p>
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<ul>
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<li>
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<a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
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()</a> is a centralized routine to free the parsing states. Note that it
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won't deallocate any produced tree if any (use the xmlFreeDoc() and
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related routines for this).</li>
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<li>
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<a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
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()</a> is the dual routine allowing to preallocate the parsing state
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which can be useful for example to avoid initialization reentrancy
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problems when using libxml in multithreaded applications</li>
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</ul>
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<p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
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at the next invocation of parser routines, but be careful of the consequences
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in multithreaded applications.</p>
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<h3><a name="Debugging">Debugging routines</a></h3>
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<p>When configured using --with-mem-debug flag (off by default), libxml uses
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a set of memory allocation debugging routines keeping track of all allocated
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blocks and the location in the code where the routine was called. A couple of
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other debugging routines allow to dump the memory allocated infos to a file
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or call a specific routine when a given block number is allocated:</p>
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<ul>
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<li>
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<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a>
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<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
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and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
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are the memory debugging replacement allocation routines</li>
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<li>
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<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
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()</a> dumps all the informations about the allocated memory block lefts
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in the <code>.memdump</code> file</li>
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</ul>
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<p>When developing libxml memory debug is enabled, the tests programs call
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xmlMemoryDump () and the "make test" regression tests will check for any
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memory leak during the full regression test sequence, this helps a lot
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ensuring that libxml does not leak memory and bullet proof memory
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allocations use (some libc implementations are known to be far too permissive
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resulting in major portability problems!).</p>
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<p>If the .memdump reports a leak, it displays the allocation function and
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also tries to give some informations about the content and structure of the
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allocated blocks left. This is sufficient in most cases to find the culprit,
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but not always. Assuming the allocation problem is reproducible, it is
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possible to find more easily:</p>
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<ol>
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<li>write down the block number xxxx not allocated</li>
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<li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest
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when using GDB is to simply give the command
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<p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p>
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<p>before running the program.</p>
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</li>
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<li>run the program under a debugger and set a breakpoint on
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xmlMallocBreakpoint() a specific function called when this precise block
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is allocated</li>
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<li>when the breakpoint is reached you can then do a fine analysis of the
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allocation an step to see the condition resulting in the missing
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deallocation.</li>
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</ol>
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<p>I used to use a commercial tool to debug libxml memory problems but after
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noticing that it was not detecting memory leaks that simple mechanism was
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used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some
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success, it is tied to the i386 architecture since it works by emulating the
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processor and instruction set, it is slow but extremely efficient, i.e. it
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spot memory usage errors in a very precise way.</p>
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<h3><a name="General4">General memory requirements</a></h3>
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<p>How much libxml memory require ? It's hard to tell in average it depends
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of a number of things:</p>
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<ul>
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<li>the parser itself should work in a fixed amount of memory, except for
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information maintained about the stacks of names and entities locations.
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The I/O and encoding handlers will probably account for a few KBytes.
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This is true for both the XML and HTML parser (though the HTML parser
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need more state).</li>
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<li>If you are generating the DOM tree then memory requirements will grow
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nearly linear with the size of the data. In general for a balanced
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textual document the internal memory requirement is about 4 times the
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size of the UTF8 serialization of this document (example the XML-1.0
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recommendation is a bit more of 150KBytes and takes 650KBytes of main
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memory when parsed). Validation will add a amount of memory required for
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maintaining the external Dtd state which should be linear with the
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complexity of the content model defined by the Dtd</li>
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<li>If you need to work with fixed memory requirements or don't need the
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full DOM tree then using the <a href="xmlreader.html">xmlReader
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interface</a> is probably the best way to proceed, it still allows to
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validate or operate on subset of the tree if needed.</li>
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<li>If you don't care about the advanced features of libxml like
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validation, DOM, XPath or XPointer, don't use entities, need to work with
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fixed memory requirements, and try to get the fastest parsing possible
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then the SAX interface should be used, but it has known restrictions.</li>
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</ul>
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<p></p>
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<p><a href="bugs.html">Daniel Veillard</a></p>
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</td></tr></table></td></tr></table></td></tr></table></td>
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</tr></table></td></tr></table>
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</body>
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</html>
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