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14<title>Memory Management</title>
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22<h1>The XML C library for Gnome</h1>
23<h2>Memory Management</h2>
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Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]88<p>Table of Content:</p>
89<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]90<li><a href="#General3">General overview</a></li>
Daniel Veillard9c466822001-10-25 12:03:39 +0000[diff] [blame]91<li><a href="#setting">Setting libxml set of memory routines</a></li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]92<li><a href="#cleanup">Cleaning up after parsing</a></li>
93<li><a href="#Debugging">Debugging routines</a></li>
94<li><a href="#General4">General memory requirements</a></li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]95</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]96<h3><a name="General3">General overview</a></h3>
97<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]98provides the interfaces to the libxml memory system:</p>
99<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]100<li>libxml does not use the libc memory allocator directly but xmlFree(),
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]101 xmlMalloc() and xmlRealloc()</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]102<li>those routines can be reallocated to a specific set of routine, by
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]103 default the libc ones i.e. free(), malloc() and realloc()</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]104<li>the xmlmemory.c module includes a set of debugging routine</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]105</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]106<h3><a name="setting">Setting libxml set of memory routines</a></h3>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]107<p>It is sometimes useful to not use the default memory allocator, either for
108debugging, analysis or to implement a specific behaviour on memory management
109(like on embedded systems). Two function calls are available to do so:</p>
110<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]111<li>
Daniel Veillardaf43f632002-03-08 15:05:20 +0000[diff] [blame]112<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet
113 ()</a> which return the current set of functions in use by the parser</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]114<li>
115<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>
Daniel Veillardaf43f632002-03-08 15:05:20 +0000[diff] [blame]116 which allow to set up a new set of memory allocation functions</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]117</ul>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]118<p>Of course a call to xmlMemSetup() should probably be done before calling
119any other libxml routines (unless you are sure your allocations routines are
120compatibles).</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]121<h3><a name="cleanup">Cleaning up after parsing</a></h3>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]122<p>Libxml is not stateless, there is a few set of memory structures needing
123allocation before the parser is fully functionnal (some encoding structures
124for example). This also mean that once parsing is finished there is a tiny
125amount of memory (a few hundred bytes) which can be recollected if you don't
126reuse the parser immediately:</p>
127<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]128<li>
129<a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser
Daniel Veillardaf43f632002-03-08 15:05:20 +0000[diff] [blame]130 ()</a> is a centralized routine to free the parsing states. Note that it
131 won't deallocate any produced tree if any (use the xmlFreeDoc() and
132 related routines for this).</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]133<li>
134<a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser
Daniel Veillardaf43f632002-03-08 15:05:20 +0000[diff] [blame]135 ()</a> is the dual routine allowing to preallocate the parsing state
136 which can be useful for example to avoid initialization reentrancy
137 problems when using libxml in multithreaded applications</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]138</ul>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]139<p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild
140at the next invocation of parser routines, but be careful of the consequences
141in multithreaded applications.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]142<h3><a name="Debugging">Debugging routines</a></h3>
143<p>When configured using --with-mem-debug flag (off by default), libxml uses
144a set of memory allocation debugging routineskeeping track of all allocated
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]145blocks and the location in the code where the routine was called. A couple of
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]146other debugging routines allow to dump the memory allocated infos to a file
147or call a specific routine when a given block number is allocated:</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]148<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]149<li>
150<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMallocLoc()</a><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>
151 and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]152 are the memory debugging replacement allocation routines</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]153<li>
154<a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump
Daniel Veillardaf43f632002-03-08 15:05:20 +0000[diff] [blame]155 ()</a> dumps all the informations about the allocated memory block lefts
156 in the <code>.memdump</code> file</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]157</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]158<p>When developping libxml memory debug is enabled, the tests programs call
159xmlMemoryDump () and the &quot;make test&quot; regression tests will check for any
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]160memory leak during the full regression test sequence, this helps a lot
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]161ensuring that libxml does not leak memory and bullet proof memory
162allocations use (some libc implementations are known to be far too permissive
163resulting in major portability problems!).</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]164<p>If the .memdump reports a leak, it displays the allocation function and
165also tries to give some informations about the content and structure of the
166allocated blocks left. This is sufficient in most cases to find the culprit,
167but not always. Assuming the allocation problem is reproductible, it is
168possible to find more easilly:</p>
169<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]170<li>write down the block number xxxx not allocated</li>
Daniel Veillarda8a89fe2002-04-12 21:03:34 +0000[diff] [blame]171<li>export the environement variable XML_MEM_BREAKPOINT=xxxx , the easiest
172 when using GDB is to simply give the command
173 <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p>
174<p>before running the program.</p>
175</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]176<li>run the program under a debugger and set a breakpoint on
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]177 xmlMallocBreakpoint() a specific function called when this precise block
178 is allocated</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]179<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]180 allocation an step to see the condition resulting in the missing
181 deallocation.</li>
182</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]183<p>I used to use a commercial tool to debug libxml memory problems but after
184noticing that it was not detecting memory leaks that simple mechanism was
Daniel Veillarda8a89fe2002-04-12 21:03:34 +0000[diff] [blame]185used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some
186success, it is tied to the i386 architecture since it works by emulating the
187processor and instruction set, it is slow but extremely efficient, i.e. it
188spot memory usage errors in a very precise way.</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]189<h3><a name="General4">General memory requirements</a></h3>
190<p>How much libxml memory require ? It's hard to tell in average it depends
191of a number of things:</p>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]192<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]193<li>the parser itself should work in a fixed amout of memory, except for
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]194 information maintained about the stacks of names and entities locations.
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]195 The I/O and encoding handlers will probably account for a few KBytes.
196 This is true for both the XML and HTML parser (though the HTML parser
197 need more state).</li>
198<li>If you are generating the DOM tree then memory requirements will grow
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]199 nearly lineary with the size of the data. In general for a balanced
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]200 textual document the internal memory requirement is about 4 times the
201 size of the UTF8 serialization of this document (exmple the XML-1.0
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]202 recommendation is a bit more of 150KBytes and takes 650KBytes of main
203 memory when parsed). Validation will add a amount of memory required for
204 maintaining the external Dtd state which should be linear with the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]205 complexity of the content model defined by the Dtd</li>
206<li>If you don't care about the advanced features of libxml like
207 validation, DOM, XPath or XPointer, but really need to work fixed memory
208 requirements, then the SAX interface should be used.</li>
Daniel Veillard189446d2000-10-13 10:23:06 +0000[diff] [blame]209</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]210<p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]211<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]212</td></tr></table></td></tr></table></td></tr></table></td>
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