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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><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>
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