documentations: - doc/xml.html doc/xmlmem.html: added a module describing
documentations:
- doc/xml.html doc/xmlmem.html: added a module describing memory
interfaces and use, updated the main page.
Daniel
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
+ "http://www.w3.org/TR/REC-html40/loose.dtd">
+<html>
+<head>
+ <title>Libxml memory management</title>
+ <meta name="GENERATOR" content="amaya V3.2">
+ <meta http-equiv="Content-Type" content="text/html">
+</head>
+
+<body bgcolor="#ffffff">
+<h1 align="center">Libxml memory management</h1>
+
+<p>Location: <a
+href="http://xmlsoft.org/xmlmem.html">http://xmlsoft.org/xmlmem.html</a></p>
+
+<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>
+
+<p>Mailing-list archive: <a
+href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>
+
+<p>Version: $Revision$</p>
+
+<p>Table of Content:</p>
+<ol>
+ <li><a href="#General">General overview</a></li>
+ <li><a href="#setting">Setting libxml set of memory routines</a></li>
+ <li><a href="#cleanup">Cleaning up after parsing</a></li>
+ <li><a href="#Debugging">Debugging routines</a></li>
+ <li><a href="#General">General memory requirements</a></li>
+</ol>
+
+<h2><a name="General">General overview</a></h2>
+
+<p>The module <code><a
+href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlmemory.h</a></code>
+provides the interfaces to the libxml memory system:</p>
+<ul>
+ <li>libxml does not use the libc memory allocator directly but xmlFree(),
+ xmlMalloc() and xmlRealloc()</li>
+ <li>those routines can be reallocated to a specific set of routine, by
+ default the libc ones i.e. free(), malloc() and realloc()</li>
+ <li>the xmlmemory.c module includes a set of debugging routine</li>
+</ul>
+
+<h2><a name="setting">Setting libxml set of memory routines</a></h2>
+
+<p>It is sometimes useful to not use the default memory allocator, either for
+debugging, analysis or to implement a specific behaviour on memory management
+(like on embedded systems). Two function calls are available to do so:</p>
+<ul>
+ <li><a href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlMemGet
+ ()</a> which return the current set of functions in use by the parser</li>
+ <li><a
+ href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlMemSetup()</a>
+ which allow to set up a new set of memory allocation functions</li>
+</ul>
+
+<p>Of course a call to xmlMemSetup() should probably be done before calling
+any other libxml routines (unless you are sure your allocations routines are
+compatibles).</p>
+
+<h2><a name="cleanup">Cleaning up after parsing</a></h2>
+
+<p>Libxml is not stateless, there is a few set of memory structures needing
+allocation before the parser is fully functionnal (some encoding structures
+for example). This also mean that once parsing is finished there is a tiny
+amount of memory (a few hundred bytes) which can be recollected if you don't
+reuse the parser immediately:</p>
+<ul>
+ <li><a href="http://xmlsoft.org/html/gnome-xml-parser.html">xmlCleanupParser
+ ()</a> is a centralized routine to free the parsing states. Note that it
+ won't deallocate any produced tree if any (use the xmlFreeDoc() and
+ related routines for this).</li>
+ <li><a href="http://xmlsoft.org/html/gnome-xml-parser.html">xmlInitParser
+ ()</a> is the dual routine allowing to preallocate the parsing state which
+ can be useful for example to avoid initialization reentrancy problems when
+ using libxml in multithreaded applications</li>
+</ul>
+
+<p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
+at the next invocation of parser routines, but be careful of the consequences
+in multithreaded applications.</p>
+
+<h2><a name="Debugging">Debugging routines</a></h2>
+
+<p>When configured using --with-mem-debug flag (off by default), libxml uses a
+set of memory allocation debugging routineskeeping track of all allocated
+blocks and the location in the code where the routine was called. A couple of
+other debugging routines allow to dump the memory allocated infos to a file or
+call a specific routine when a given block number is allocated:</p>
+<ul>
+ <li><a
+ href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlMallocLoc()</a>
+ <a
+ href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlReallocLoc()</a>
+ and <a
+ href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlMemStrdupLoc()</a>
+ are the memory debugging replacement allocation routines</li>
+ <li><a href="http://xmlsoft.org/html/gnome-xml-xmlmemory.html">xmlMemoryDump
+ ()</a> dumps all the informations about the allocated memory block lefts
+ in the <code>.memdump</code> file</li>
+</ul>
+
+<p> When developping libxml memory debug is enabled, the tests programs call
+xmlMemoryDump () and the "make test" regression tests will check for any
+memory leak during the full regression test sequence, this helps a lot
+ensuring that libxml does not leak memory and bullet proof memory allocations
+use (some libc implementations are known to be far too permissive resulting in
+major portability problems!). </p>
+
+<p>If the .memdump reports a leak, it displays the allocation function and
+also tries to give some informations about the content and structure of the
+allocated blocks left. This is sufficient in most cases to find the culprit,
+but not always. Assuming the allocation problem is reproductible, it is
+possible to find more easilly:</p>
+<ol>
+ <li>write down the block number xxxx not allocated</li>
+ <li>export the environement variable XML_MEM_BREAKPOINT=xxxx</li>
+ <li>run the program under a debugger and set a breakpoint on
+ xmlMallocBreakpoint() a specific function called when this precise block
+ is allocated</li>
+ <li>when the breakpoint is reached you can then do a fine analysis of the
+ allocation an step to see the condition resulting in the missing
+ deallocation.</li>
+</ol>
+
+<p> I used to use a commercial tool to debug libxml memory problems but after
+noticing that it was not detecting memory leaks that simple mechanism was used
+and proved extremely efficient until now.</p>
+
+<h2><a name="General">General memory requirements</a></h2>
+
+<p>How much libxml memory require ? It's hard to tell in average it depends of
+a number of things:</p>
+<ul>
+ <li>the parser itself should work in a fixed amout of memory, except for
+ information maintained about the stacks of names and entities locations.
+ The I/O and encoding handlers will probably account for a few KBytes. This
+ is true for both the XML and HTML parser (though the HTML parser need more
+ state).</li>
+ <li>If you are generating the DOM tree then memory requirements will grow
+ nearly lineary with the size of the data. In general for a balanced
+ textual document the internal memory requirement is about 4 times the size
+ of the UTF8 serialization of this document (exmple the XML-1.0
+ recommendation is a bit more of 150KBytes and takes 650KBytes of main
+ memory when parsed). Validation will add a amount of memory required for
+ maintaining the external Dtd state which should be linear with the
+ complexity of the content model defined by the Dtd </li>
+ <li>If you don't care about the advanced features of libxml like validation,
+ DOM, XPath or XPointer, but really need to work fixed memory requirements,
+ then the SAX interface should be used.</li>
+</ul>
+
+<p></p>
+
+<p><a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
+
+<p>$Id$</p>
+</body>
+</html>