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