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| <title>Memory Management</title> |
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| <h1>The XML C library for Gnome</h1> |
| <h2>Memory Management</h2> |
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| <p>Table of Content:</p> |
| <ol> |
| <li><a href="#General3">General overview</a></li> |
| <li><a href="#setting">Setting libxml 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">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 libxml memory system:</p> |
| <ul> |
| <li>libxml 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, by |
| default 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">Setting libxml set of memory routines</a></h3> |
| <p>It is sometimes useful to not use the default memory allocator, either for |
| debugging, 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 calling |
| any other libxml routines (unless you are sure your allocations routines are |
| compatibles).</p> |
| <h3><a name="cleanup">Cleaning up after parsing</a></h3> |
| <p>Libxml is not stateless, there is a few set of memory structures needing |
| allocation before the parser is fully functionnal (some encoding structures |
| for example). This also mean that once parsing is finished there is a tiny |
| amount of memory (a few hundred bytes) which can be recollected if you don't |
| reuse 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 it won't |
| deallocate any produced tree if any (use the xmlFreeDoc() and related |
| 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 state which can |
| be useful for example to avoid initialization reentrancy problems when |
| using libxml in multithreaded applications</li> |
| </ul> |
| <p>Generally xmlCleanupParser() is safe, if needed the state will be rebuild |
| at the next invocation of parser routines, but be careful of the consequences |
| in multithreaded applications.</p> |
| <h3><a name="Debugging">Debugging routines</a></h3> |
| <p>When configured using --with-mem-debug flag (off by default), libxml uses |
| a set of memory allocation debugging routineskeeping track of all allocated |
| blocks and the location in the code where the routine was called. A couple of |
| other debugging routines allow to dump the memory allocated infos to a file |
| 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><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 lefts in the |
| <code>.memdump</code> file</li> |
| </ul> |
| <p>When developping libxml memory debug is enabled, the tests programs call |
| xmlMemoryDump () and the "make test" regression tests will check for any |
| memory leak during the full regression test sequence, this helps a lot |
| ensuring that libxml does not leak memory and bullet proof memory |
| allocations use (some libc implementations are known to be far too permissive |
| resulting in major portability problems!).</p> |
| <p>If the .memdump reports a leak, it displays the allocation function and |
| also tries to give some informations about the content and structure of the |
| allocated blocks left. This is sufficient in most cases to find the culprit, |
| but not always. Assuming the allocation problem is reproductible, it is |
| possible to find more easilly:</p> |
| <ol> |
| <li>write down the block number xxxx not allocated</li> |
| <li>export the environement variable XML_MEM_BREAKPOINT=xxxx</li> |
| <li>run the program under a debugger and set a breakpoint on |
| xmlMallocBreakpoint() a specific function called when this precise block |
| is allocated</li> |
| <li>when the breakpoint is reached you can then do a fine analysis of the |
| allocation an step to see the condition resulting in the missing |
| deallocation.</li> |
| </ol> |
| <p>I used to use a commercial tool to debug libxml memory problems but after |
| noticing that it was not detecting memory leaks that simple mechanism was |
| used and proved extremely efficient until now.</p> |
| <h3><a name="General4">General memory requirements</a></h3> |
| <p>How much libxml memory require ? It's hard to tell in average it depends |
| of a number of things:</p> |
| <ul> |
| <li>the parser itself should work in a fixed amout of memory, except for |
| information 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 parser |
| need more state).</li> |
| <li>If you are generating the DOM tree then memory requirements will grow |
| nearly lineary with the size of the data. In general for a balanced |
| textual document the internal memory requirement is about 4 times the |
| size of the UTF8 serialization of this document (exmple the XML-1.0 |
| recommendation is a bit more of 150KBytes and takes 650KBytes of main |
| memory when parsed). Validation will add a amount of memory required for |
| maintaining the external Dtd state which should be linear with the |
| complexity of the content model defined by the Dtd</li> |
| <li>If you don't care about the advanced features of libxml like |
| validation, DOM, XPath or XPointer, but really need to work fixed memory |
| requirements, then the SAX interface should be used.</li> |
| </ul> |
| <p> |
| <p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p> |
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