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13<title>Memory Management</title>
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21<h1>The XML C library for Gnome</h1>
22<h2>Memory Management</h2>
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Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]73<p>Table of Content:</p>
74<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]75<li><a href="#General3">General overview</a></li>
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame^]76<li><a href="#setting">Setting libxml set of memory routines</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]77<li><a href="#cleanup">Cleaning up after parsing</a></li>
78<li><a href="#Debugging">Debugging routines</a></li>
79<li><a href="#General4">General memory requirements</a></li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]80</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]81<h3><a name="General3">General overview</a></h3>
82<p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlmemory.h</a></code>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]83provides the interfaces to the libxml memory system:</p>
84<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]85<li>libxml does not use the libc memory allocator directly but xmlFree(),
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]86 xmlMalloc() and xmlRealloc()</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]87<li>those routines can be reallocated to a specific set of routine, by
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]88 default the libc ones i.e. free(), malloc() and realloc()</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]89<li>the xmlmemory.c module includes a set of debugging routine</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]90</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]91<h3><a name="setting">Setting libxml set of memory routines</a></h3>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]92<p>It is sometimes useful to not use the default memory allocator, either for
93debugging, analysis or to implement a specific behaviour on memory management
94(like on embedded systems). Two function calls are available to do so:</p>
95<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]96<li>
97<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet ()</a>
98 which return the current set of functions in use by the parser</li>
99<li>
100<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>
101 which allow to set up a new set of memory allocation functions</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]102</ul>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]103<p>Of course a call to xmlMemSetup() should probably be done before calling
104any other libxml routines (unless you are sure your allocations routines are
105compatibles).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]106<h3><a name="cleanup">Cleaning up after parsing</a></h3>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]107<p>Libxml is not stateless, there is a few set of memory structures needing
108allocation before the parser is fully functionnal (some encoding structures
109for example). This also mean that once parsing is finished there is a tiny
110amount of memory (a few hundred bytes) which can be recollected if you don't
111reuse the parser immediately:</p>
112<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]113<li>
114<a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
115 ()</a>
116 is a centralized routine to free the parsing states. Note that it won't
117 deallocate any produced tree if any (use the xmlFreeDoc() and related
118 routines for this).</li>
119<li>
120<a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
121 ()</a>
122 is the dual routine allowing to preallocate the parsing state which can
123 be useful for example to avoid initialization reentrancy problems when
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]124 using libxml in multithreaded applications</li>
125</ul>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]126<p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
127at the next invocation of parser routines, but be careful of the consequences
128in multithreaded applications.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]129<h3><a name="Debugging">Debugging routines</a></h3>
130<p>When configured using --with-mem-debug flag (off by default), libxml uses
131a set of memory allocation debugging routineskeeping track of all allocated
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]132blocks and the location in the code where the routine was called. A couple of
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]133other debugging routines allow to dump the memory allocated infos to a file
134or call a specific routine when a given block number is allocated:</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]135<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]136<li>
137<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
138 and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]139 are the memory debugging replacement allocation routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]140<li>
141<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
142 ()</a>
143 dumps all the informations about the allocated memory block lefts in the
144 <code>.memdump</code> file</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]145</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]146<p>When developping libxml memory debug is enabled, the tests programs call
147xmlMemoryDump () and the &quot;make test&quot; regression tests will check for any
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]148memory leak during the full regression test sequence, this helps a lot
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]149ensuring that libxml does not leak memory and bullet proof memory
150allocations use (some libc implementations are known to be far too permissive
151resulting in major portability problems!).</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]152<p>If the .memdump reports a leak, it displays the allocation function and
153also tries to give some informations about the content and structure of the
154allocated blocks left. This is sufficient in most cases to find the culprit,
155but not always. Assuming the allocation problem is reproductible, it is
156possible to find more easilly:</p>
157<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]158<li>write down the block number xxxx not allocated</li>
159<li>export the environement variable XML_MEM_BREAKPOINT=xxxx</li>
160<li>run the program under a debugger and set a breakpoint on
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]161 xmlMallocBreakpoint() a specific function called when this precise block
162 is allocated</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]163<li>when the breakpoint is reached you can then do a fine analysis of the
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]164 allocation an step to see the condition resulting in the missing
165 deallocation.</li>
166</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]167<p>I used to use a commercial tool to debug libxml memory problems but after
168noticing that it was not detecting memory leaks that simple mechanism was
169used and proved extremely efficient until now.</p>
170<h3><a name="General4">General memory requirements</a></h3>
171<p>How much libxml memory require ? It's hard to tell in average it depends
172of a number of things:</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]173<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]174<li>the parser itself should work in a fixed amout of memory, except for
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]175 information maintained about the stacks of names and entities locations.
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]176 The I/O and encoding handlers will probably account for a few KBytes.
177 This is true for both the XML and HTML parser (though the HTML parser
178 need more state).</li>
179<li>If you are generating the DOM tree then memory requirements will grow
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]180 nearly lineary with the size of the data. In general for a balanced
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]181 textual document the internal memory requirement is about 4 times the
182 size of the UTF8 serialization of this document (exmple the XML-1.0
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]183 recommendation is a bit more of 150KBytes and takes 650KBytes of main
184 memory when parsed). Validation will add a amount of memory required for
185 maintaining the external Dtd state which should be linear with the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]186 complexity of the content model defined by the Dtd</li>
187<li>If you don't care about the advanced features of libxml like
188 validation, DOM, XPath or XPointer, but really need to work fixed memory
189 requirements, then the SAX interface should be used.</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]190</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]191<p>
Daniel Veillardc5d64342001-06-24 12:13:24 +0000[diff] [blame]192<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]193</td></tr></table></td></tr></table></td></tr></table></td>
194</tr></table></td></tr></table>
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