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1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4 TD {font-family: Verdana,Arial,Helvetica} 5 BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6 H1 {font-family: Verdana,Arial,Helvetica} 7 H2 {font-family: Verdana,Arial,Helvetica} 8 H3 {font-family: Verdana,Arial,Helvetica} 9 A:link, A:visited, A:active { text-decoration: underline } 10 </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" 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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="http://bugzilla.gnome.org/buglist.cgi?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> 11 <li><a href="#General3">General overview</a></li> 12 <li><a href="#setting">Setting libxml2 set of memory routines</a></li> 13 <li><a href="#cleanup">Cleaning up after using the library</a></li> 14 <li><a href="#Debugging">Debugging routines</a></li> 15 <li><a href="#General4">General memory requirements</a></li> 16 <li><a href="#Compacting">Returning memory to the kernel</a></li> 17 </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> 18 provides the interfaces to the libxml2 memory system:</p><ul> 19 <li>libxml2 does not use the libc memory allocator directly but xmlFree(), 20 xmlMalloc() and xmlRealloc()</li> 21 <li>those routines can be reallocated to a specific set of routine, by 22 default the libc ones i.e. free(), malloc() and realloc()</li> 23 <li>the xmlmemory.c module includes a set of debugging routine</li> 24 </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 25 debugging, analysis or to implement a specific behaviour on memory management 26 (like on embedded systems). Two function calls are available to do so:</p><ul> 27 <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet 28 ()</a> which return the current set of functions in use by the parser</li> 29 <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a> 30 which allow to set up a new set of memory allocation functions</li> 31 </ul><p>Of course a call to xmlMemSetup() should probably be done before calling 32 any other libxml2 routines (unless you are sure your allocations routines are 33 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 34 allocation before the parser is fully functional (some encoding structures 35 for example). This also mean that once parsing is finished there is a tiny 36 amount of memory (a few hundred bytes) which can be recollected if you don't 37 reuse the library or any document built with it:</p><ul> 38 <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser 39 ()</a> is a centralized routine to free the library state and data. Note 40 that it won't deallocate any produced tree if any (use the xmlFreeDoc() 41 and related routines for this). This should be called only when the library 42 is not used anymore.</li> 43 <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser 44 ()</a> is the dual routine allowing to preallocate the parsing state 45 which can be useful for example to avoid initialization reentrancy 46 problems when using libxml2 in multithreaded applications</li> 47 </ul><p>Generally xmlCleanupParser() is safe assuming no parsing is ongoing and 48 no document is still being used, if needed the state will be rebuild at the 49 next invocation of parser routines (or by xmlInitParser()), but be careful 50 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 51 a set of memory allocation debugging routines keeping track of all allocated 52 blocks and the location in the code where the routine was called. A couple of 53 other debugging routines allow to dump the memory allocated infos to a file 54 or call a specific routine when a given block number is allocated:</p><ul> 55 <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a> 56 <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a> 57 and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a> 58 are the memory debugging replacement allocation routines</li> 59 <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump 60 ()</a> dumps all the information about the allocated memory block lefts 61 in the <code>.memdump</code> file</li> 62 </ul><p>When developing libxml2 memory debug is enabled, the tests programs call 63 xmlMemoryDump () and the "make test" regression tests will check for any 64 memory leak during the full regression test sequence, this helps a lot 65 ensuring that libxml2 does not leak memory and bullet proof memory 66 allocations use (some libc implementations are known to be far too permissive 67 resulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function and 68 also tries to give some information about the content and structure of the 69 allocated blocks left. This is sufficient in most cases to find the culprit, 70 but not always. Assuming the allocation problem is reproducible, it is 71 possible to find more easily:</p><ol> 72 <li>write down the block number xxxx not allocated</li> 73 <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest 74 when using GDB is to simply give the command 75 <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p> 76 <p>before running the program.</p> 77 </li> 78 <li>run the program under a debugger and set a breakpoint on 79 xmlMallocBreakpoint() a specific function called when this precise block 80 is allocated</li> 81 <li>when the breakpoint is reached you can then do a fine analysis of the 82 allocation an step to see the condition resulting in the missing 83 deallocation.</li> 84 </ol><p>I used to use a commercial tool to debug libxml2 memory problems but after 85 noticing that it was not detecting memory leaks that simple mechanism was 86 used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some 87 success, it is tied to the i386 architecture since it works by emulating the 88 processor and instruction set, it is slow but extremely efficient, i.e. it 89 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 90 of a number of things:</p><ul> 91 <li>the parser itself should work in a fixed amount of memory, except for 92 information maintained about the stacks of names and entities locations. 93 The I/O and encoding handlers will probably account for a few KBytes. 94 This is true for both the XML and HTML parser (though the HTML parser 95 need more state).</li> 96 <li>If you are generating the DOM tree then memory requirements will grow 97 nearly linear with the size of the data. In general for a balanced 98 textual document the internal memory requirement is about 4 times the 99 size of the UTF8 serialization of this document (example the XML-1.0 100 recommendation is a bit more of 150KBytes and takes 650KBytes of main 101 memory when parsed). Validation will add a amount of memory required for 102 maintaining the external Dtd state which should be linear with the 103 complexity of the content model defined by the Dtd</li> 104 <li>If you need to work with fixed memory requirements or don't need the 105 full DOM tree then using the <a href="xmlreader.html">xmlReader 106 interface</a> is probably the best way to proceed, it still allows to 107 validate or operate on subset of the tree if needed.</li> 108 <li>If you don't care about the advanced features of libxml2 like 109 validation, DOM, XPath or XPointer, don't use entities, need to work with 110 fixed memory requirements, and try to get the fastest parsing possible 111 then the SAX interface should be used, but it has known restrictions.</li> 112 </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 113 reduced memory usage although you freed the trees. This is because 114 libxml2 allocates memory in a number of small chunks. When freeing one 115 of those chunks, the OS may decide that giving this little memory back 116 to the kernel will cause too much overhead and delay the operation. As 117 all chunks are this small, they get actually freed but not returned to 118 the kernel. On systems using glibc, there is a function call 119 "malloc_trim" from malloc.h which does this missing operation (note that 120 it is allowed to fail). Thus, after freeing your tree you may simply try 121 "malloc_trim(0);" to really get the memory back. If your OS does not 122 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>