| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd"> |
| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> |
| <link rel="SHORTCUT ICON" href="/favicon.ico"> |
| <style type="text/css"><!-- |
| TD {font-family: Verdana,Arial,Helvetica} |
| BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} |
| H1 {font-family: Verdana,Arial,Helvetica} |
| H2 {font-family: Verdana,Arial,Helvetica} |
| H3 {font-family: Verdana,Arial,Helvetica} |
| A:link, A:visited, A:active { text-decoration: underline } |
| --></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 library for 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>Main Menu</b></center></td></tr> |
| <tr><td bgcolor="#fffacd"><ul> |
| <li><a href="index.html">Home</a></li> |
| <li><a href="intro.html">Introduction</a></li> |
| <li><a href="FAQ.html">FAQ</a></li> |
| <li><a href="docs.html">Documentation</a></li> |
| <li><a href="bugs.html">Reporting bugs and getting help</a></li> |
| <li><a href="help.html">How to help</a></li> |
| <li><a href="downloads.html">Downloads</a></li> |
| <li><a href="news.html">News</a></li> |
| <li><a href="XMLinfo.html">XML</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">libxml 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="xmldtd.html">Validation & DTDs</a></li> |
| <li><a href="xmlmem.html">Memory Management</a></li> |
| <li><a href="encoding.html">Encodings support</a></li> |
| <li><a href="xmlio.html">I/O Interfaces</a></li> |
| <li><a href="catalog.html">Catalog support</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="contribs.html">Contributions</a></li> |
| <li><a href="tutorial/index.html">Tutorial</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.fh-frankfurt.de/~igor/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> |
| <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="#General4">General memory requirements</a></li> |
| </ol> |
| <h3><a name="General3">General overview</a></h3> |
| <p>The module <code><a href="http://xmlsoft.org/html/libxml-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> |
| <h3><a name="setting">Setting libxml set of memory routines</a></h3> |
| <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/libxml-xmlmemory.html">xmlMemGet |
| ()</a> which return the current set of functions in use by the parser</li> |
| <li> |
| <a href="http://xmlsoft.org/html/libxml-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> |
| <h3><a name="cleanup">Cleaning up after parsing</a></h3> |
| <p>Libxml is not stateless, there is a few set of memory structures needing |
| allocation before the parser is fully functional (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/libxml-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/libxml-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> |
| <h3><a name="Debugging">Debugging routines</a></h3> |
| <p>When configured using --with-mem-debug flag (off by default), libxml uses |
| a set of memory allocation debugging routines keeping 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/libxml-xmlmemory.html">xmlMallocLoc()</a> |
| <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a> |
| and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a> |
| are the memory debugging replacement allocation routines</li> |
| <li> |
| <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump |
| ()</a> dumps all the informations about the allocated memory block lefts |
| in the <code>.memdump</code> file</li> |
| </ul> |
| <p>When developing 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 reproducible, it is |
| possible to find more easily:</p> |
| <ol> |
| <li>write down the block number xxxx not allocated</li> |
| <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest |
| when using GDB is to simply give the command |
| <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p> |
| <p>before running the program.</p> |
| </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. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some |
| success, it is tied to the i386 architecture since it works by emulating the |
| processor and instruction set, it is slow but extremely efficient, i.e. it |
| spot memory usage errors in a very precise way.</p> |
| <h3><a name="General4">General memory requirements</a></h3> |
| <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 amount 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 linear 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 (example 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><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> |