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