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Daniel Veillardd012f482007-08-22 17:36:57 +0000[diff] [blame]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" 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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 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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>
Daniel Veillard8a469172003-06-12 16:05:07 +0000[diff] [blame]11 <li><a href="#setting">Setting libxml2 set of memory routines</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]12 <li><a href="#cleanup">Cleaning up after parsing</a></li>
13 <li><a href="#Debugging">Debugging routines</a></li>
14 <li><a href="#General4">General memory requirements</a></li>
Daniel Veillardad87d5e2008-02-04 16:50:03 +0000[diff] [blame]15 <li><a href="#Compacting">Returning memory to the kernel</a></li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]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>
17provides 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>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]21 <li>the xmlmemory.c module includes a set of debugging routine</li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]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
23debugging, 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
29any other libxml2 routines (unless you are sure your allocations routines are
30compatibles).</p><h3><a name="cleanup" id="cleanup">Cleaning up after parsing</a></h3><p>Libxml2 is not stateless, there is a few set of memory structures needing
31allocation before the parser is fully functional (some encoding structures
32for example). This also mean that once parsing is finished there is a tiny
33amount of memory (a few hundred bytes) which can be recollected if you don't
34reuse the parser immediately:</p><ul><li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
35 ()</a> is a centralized routine to free the parsing states. Note that it
36 won't deallocate any produced tree if any (use the xmlFreeDoc() and
37 related routines for this).</li>
38 <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
39 ()</a> is the dual routine allowing to preallocate the parsing state
40 which can be useful for example to avoid initialization reentrancy
41 problems when using libxml2 in multithreaded applications</li>
42</ul><p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
43at the next invocation of parser routines, but be careful of the consequences
44in 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
45a set of memory allocation debugging routines keeping track of all allocated
46blocks and the location in the code where the routine was called. A couple of
47other debugging routines allow to dump the memory allocated infos to a file
48or 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>
49 <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
50 and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
51 are the memory debugging replacement allocation routines</li>
52 <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
William M. Brack43a87292007-02-15 20:41:02 +0000[diff] [blame]53 ()</a> dumps all the information about the allocated memory block lefts
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]54 in the <code>.memdump</code> file</li>
55</ul><p>When developing libxml2 memory debug is enabled, the tests programs call
56xmlMemoryDump () and the "make test" regression tests will check for any
57memory leak during the full regression test sequence, this helps a lot
58ensuring that libxml2 does not leak memory and bullet proof memory
59allocations use (some libc implementations are known to be far too permissive
60resulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function and
William M. Brack43a87292007-02-15 20:41:02 +0000[diff] [blame]61also tries to give some information about the content and structure of the
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]62allocated blocks left. This is sufficient in most cases to find the culprit,
63but not always. Assuming the allocation problem is reproducible, it is
64possible to find more easily:</p><ol><li>write down the block number xxxx not allocated</li>
65 <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest
66 when using GDB is to simply give the command
Daniel Veillarda8a89fe2002-04-12 21:03:34 +0000[diff] [blame]67 <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]68 <p>before running the program.</p>
69 </li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]70 <li>run the program under a debugger and set a breakpoint on
71 xmlMallocBreakpoint() a specific function called when this precise block
72 is allocated</li>
73 <li>when the breakpoint is reached you can then do a fine analysis of the
74 allocation an step to see the condition resulting in the missing
75 deallocation.</li>
76</ol><p>I used to use a commercial tool to debug libxml2 memory problems but after
77noticing that it was not detecting memory leaks that simple mechanism was
78used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some
79success, it is tied to the i386 architecture since it works by emulating the
80processor and instruction set, it is slow but extremely efficient, i.e. it
81spot 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
82of a number of things:</p><ul><li>the parser itself should work in a fixed amount of memory, except for
83 information maintained about the stacks of names and entities locations.
84 The I/O and encoding handlers will probably account for a few KBytes.
85 This is true for both the XML and HTML parser (though the HTML parser
86 need more state).</li>
87 <li>If you are generating the DOM tree then memory requirements will grow
88 nearly linear with the size of the data. In general for a balanced
89 textual document the internal memory requirement is about 4 times the
90 size of the UTF8 serialization of this document (example the XML-1.0
91 recommendation is a bit more of 150KBytes and takes 650KBytes of main
92 memory when parsed). Validation will add a amount of memory required for
93 maintaining the external Dtd state which should be linear with the
94 complexity of the content model defined by the Dtd</li>
95 <li>If you need to work with fixed memory requirements or don't need the
96 full DOM tree then using the <a href="xmlreader.html">xmlReader
97 interface</a> is probably the best way to proceed, it still allows to
98 validate or operate on subset of the tree if needed.</li>
99 <li>If you don't care about the advanced features of libxml2 like
100 validation, DOM, XPath or XPointer, don't use entities, need to work with
101 fixed memory requirements, and try to get the fastest parsing possible
102 then the SAX interface should be used, but it has known restrictions.</li>
Daniel Veillardad87d5e2008-02-04 16:50:03 +0000[diff] [blame]103</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
104reduced memory usage although you freed the trees. This is because
105libxml2 allocates memory in a number of small chunks. When freeing one
106of those chunks, the OS may decide that giving this little memory back
107to the kernel will cause too much overhead and delay the operation. As
108all chunks are this small, they get actually freed but not returned to
109the kernel. On systems using glibc, there is a function call
110"malloc_trim" from malloc.h which does this missing operation (note that
111it is allowed to fail). Thus, after freeing your tree you may simply try
112"malloc_trim(0);" to really get the memory back. If your OS does not
113provide 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>