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123 lines
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123 lines
14 KiB
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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</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 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" style="font-weight:bold">Main Menu</a></li><li><a href="html/index.html" style="font-weight:bold">Reference Manual</a></li><li><a href="examples/index.html" style="font-weight:bold">Code Examples</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="ChangeLog.html">ChangeLog</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://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="http://libxmlplusplus.sourceforge.net/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="https://gitlab.gnome.org/GNOME/libxml2/issues">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>
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<li><a href="#General3">General overview</a></li>
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<li><a href="#setting">Setting libxml2 set of memory routines</a></li>
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<li><a href="#cleanup">Cleaning up after using the library</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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<li><a href="#Compacting">Returning memory to the kernel</a></li>
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</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>
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provides the interfaces to the libxml2 memory system:</p><ul>
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<li>libxml2 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><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
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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><ul>
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<li><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><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><p>Of course a call to xmlMemSetup() should probably be done before calling
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any other libxml2 routines (unless you are sure your allocations routines are
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compatibles).</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
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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 library or any document built with it:</p><ul>
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<li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
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()</a> is a centralized routine to free the library state and data. Note
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that it won't deallocate any produced tree if any (use the xmlFreeDoc()
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and related routines for this). This should be called only when the library
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is not used anymore.</li>
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<li><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 libxml2 in multithreaded applications</li>
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</ul><p>Generally xmlCleanupParser() is safe assuming no parsing is ongoing and
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no document is still being used, if needed the state will be rebuild at the
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next invocation of parser routines (or by xmlInitParser()), but be careful
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of 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
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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><ul>
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<li><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><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
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()</a> dumps all the information about the allocated memory block lefts
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in the <code>.memdump</code> file</li>
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</ul><p>When developing libxml2 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 libxml2 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><p>If the .memdump reports a leak, it displays the allocation function and
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also tries to give some information 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><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><p>I used to use a commercial tool to debug libxml2 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><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
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of a number of things:</p><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 libxml2 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><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
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reduced memory usage although you freed the trees. This is because
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libxml2 allocates memory in a number of small chunks. When freeing one
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of those chunks, the OS may decide that giving this little memory back
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to the kernel will cause too much overhead and delay the operation. As
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all chunks are this small, they get actually freed but not returned to
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the kernel. On systems using glibc, there is a function call
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"malloc_trim" from malloc.h which does this missing operation (note that
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it is allowed to fail). Thus, after freeing your tree you may simply try
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"malloc_trim(0);" to really get the memory back. If your OS does not
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provide 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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