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| </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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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> |
| <li><a href="#setting">Setting libxml2 set of memory routines</a></li> |
| <li><a href="#cleanup">Cleaning up after parsing</a></li> |
| <li><a href="#Debugging">Debugging routines</a></li> |
| <li><a href="#General4">General memory requirements</a></li> |
| </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>provides |
| the interfaces to the libxml2 memory system:</p><ul><li>libxml2 does not use the libc memory allocator directly but |
| xmlFree(),xmlMalloc() and xmlRealloc()</li> |
| <li>those routines can be reallocated to a specific set of routine, |
| bydefault the libc ones i.e. free(), malloc() and realloc()</li> |
| <li>the xmlmemory.c module includes a set of debugging routine</li> |
| </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 |
| fordebugging, analysis or to implement a specific behaviour on memory |
| management(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()</a>which |
| return the current set of functions in use by the parser</li> |
| <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>which |
| allow to set up a new set of memory allocation functions</li> |
| </ul><p>Of course a call to xmlMemSetup() should probably be done before |
| callingany other libxml2 routines (unless you are sure your allocations |
| routines arecompatibles).</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 |
| needingallocation before the parser is fully functional (some encoding |
| structuresfor example). This also mean that once parsing is finished there is |
| a tinyamount of memory (a few hundred bytes) which can be recollected if you |
| don'treuse the parser immediately:</p><ul><li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser()</a>is |
| a centralized routine to free the parsing states. Note that itwon't |
| deallocate any produced tree if any (use the xmlFreeDoc() andrelated |
| routines for this).</li> |
| <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser()</a>is |
| the dual routine allowing to preallocate the parsing statewhich can be |
| useful for example to avoid initialization reentrancyproblems when using |
| libxml2 in multithreaded applications</li> |
| </ul><p>Generally xmlCleanupParser() is safe, if needed the state will be |
| rebuildat the next invocation of parser routines, but be careful of the |
| consequencesin 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 |
| usesa set of memory allocation debugging routines keeping track of all |
| allocatedblocks and the location in the code where the routine was called. A |
| couple ofother debugging routines allow to dump the memory allocated infos to |
| a fileor 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><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a>and |
| <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a>are |
| the memory debugging replacement allocation routines</li> |
| <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump()</a>dumps |
| all the informations about the allocated memory block leftsin the |
| <code>.memdump</code>file</li> |
| </ul><p>When developing libxml2 memory debug is enabled, the tests programs |
| callxmlMemoryDump () and the "make test" regression tests will check for |
| anymemory leak during the full regression test sequence, this helps a |
| lotensuring that libxml2 does not leak memory and bullet proof |
| memoryallocations use (some libc implementations are known to be far too |
| permissiveresulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function |
| andalso tries to give some informations about the content and structure of |
| theallocated blocks left. This is sufficient in most cases to find the |
| culprit,but not always. Assuming the allocation problem is reproducible, it |
| ispossible to find more easily:</p><ol><li>write down the block number xxxx not allocated</li> |
| <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the |
| easiestwhen using GDB is to simply give the command |
| <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p> |
| <p>before running the program.</p> |
| </li> |
| <li>run the program under a debugger and set a breakpoint |
| onxmlMallocBreakpoint() a specific function called when this precise |
| blockis allocated</li> |
| <li>when the breakpoint is reached you can then do a fine analysis of |
| theallocation an step to see the condition resulting in the |
| missingdeallocation.</li> |
| </ol><p>I used to use a commercial tool to debug libxml2 memory problems but |
| afternoticing that it was not detecting memory leaks that simple mechanism |
| wasused and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a>with quite somesuccess, |
| it is tied to the i386 architecture since it works by emulating theprocessor |
| and instruction set, it is slow but extremely efficient, i.e. itspot 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 |
| dependsof a number of things:</p><ul><li>the parser itself should work in a fixed amount of memory, except |
| forinformation maintained about the stacks of names and entities |
| locations.The I/O and encoding handlers will probably account for a few |
| KBytes.This is true for both the XML and HTML parser (though the HTML |
| parserneed more state).</li> |
| <li>If you are generating the DOM tree then memory requirements will |
| grownearly linear with the size of the data. In general for a |
| balancedtextual document the internal memory requirement is about 4 times |
| thesize of the UTF8 serialization of this document (example the |
| XML-1.0recommendation is a bit more of 150KBytes and takes 650KBytes of |
| mainmemory when parsed). Validation will add a amount of memory required |
| formaintaining the external Dtd state which should be linear with |
| thecomplexity of the content model defined by the Dtd</li> |
| <li>If you need to work with fixed memory requirements or don't need |
| thefull DOM tree then using the <a href="xmlreader.html">xmlReaderinterface</a>is probably the best way to |
| proceed, it still allows tovalidate or operate on subset of the tree if |
| needed.</li> |
| <li>If you don't care about the advanced features of libxml2 |
| likevalidation, DOM, XPath or XPointer, don't use entities, need to work |
| withfixed memory requirements, and try to get the fastest parsing |
| possiblethen the SAX interface should be used, but it has known |
| restrictions.</li> |
| </ul><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> |
