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| Daniel Veillard | 28fdf8b | 2011-03-07 08:12:39 +0800 | [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 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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 Veillard | 8a46917 | 2003-06-12 16:05:07 +0000 | [diff] [blame] | 11 | <li><a href="#setting">Setting libxml2 set of memory routines</a></li> |
| Daniel Veillard | 05b37c6 | 2008-03-31 08:27:07 +0000 | [diff] [blame] | 12 | <li><a href="#cleanup">Cleaning up after using the library</a></li> |
| Daniel Veillard | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 13 | <li><a href="#Debugging">Debugging routines</a></li> |
| 14 | <li><a href="#General4">General memory requirements</a></li> |
| Daniel Veillard | ad87d5e | 2008-02-04 16:50:03 +0000 | [diff] [blame] | 15 | <li><a href="#Compacting">Returning memory to the kernel</a></li> |
| Daniel Veillard | f781dba | 2006-06-09 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> |
| 17 | provides 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 Veillard | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 21 | <li>the xmlmemory.c module includes a set of debugging routine</li> |
| Daniel Veillard | f781dba | 2006-06-09 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 |
| 23 | debugging, 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 |
| 29 | any other libxml2 routines (unless you are sure your allocations routines are |
| Daniel Veillard | 05b37c6 | 2008-03-31 08:27:07 +0000 | [diff] [blame] | 30 | compatibles).</p><h3><a name="cleanup" id="cleanup">Cleaning up after using the library</a></h3><p>Libxml2 is not stateless, there is a few set of memory structures needing |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 31 | allocation before the parser is fully functional (some encoding structures |
| 32 | for example). This also mean that once parsing is finished there is a tiny |
| 33 | amount of memory (a few hundred bytes) which can be recollected if you don't |
| Daniel Veillard | 05b37c6 | 2008-03-31 08:27:07 +0000 | [diff] [blame] | 34 | reuse the library or any document built with it:</p><ul><li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser |
| 35 | ()</a> is a centralized routine to free the library state and data. Note |
| 36 | that it won't deallocate any produced tree if any (use the xmlFreeDoc() |
| 37 | and related routines for this). This should be called only when the library |
| 38 | is not used anymore.</li> |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 39 | <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser |
| 40 | ()</a> is the dual routine allowing to preallocate the parsing state |
| 41 | which can be useful for example to avoid initialization reentrancy |
| 42 | problems when using libxml2 in multithreaded applications</li> |
| Daniel Veillard | 05b37c6 | 2008-03-31 08:27:07 +0000 | [diff] [blame] | 43 | </ul><p>Generally xmlCleanupParser() is safe assuming no parsing is ongoing and |
| 44 | no document is still being used, if needed the state will be rebuild at the |
| 45 | next invocation of parser routines (or by xmlInitParser()), but be careful |
| 46 | of the consequences in 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 |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 47 | a set of memory allocation debugging routines keeping track of all allocated |
| 48 | blocks and the location in the code where the routine was called. A couple of |
| 49 | other debugging routines allow to dump the memory allocated infos to a file |
| 50 | 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> |
| 51 | <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlReallocLoc()</a> |
| 52 | and <a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemStrdupLoc()</a> |
| 53 | are the memory debugging replacement allocation routines</li> |
| 54 | <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump |
| William M. Brack | 43a8729 | 2007-02-15 20:41:02 +0000 | [diff] [blame] | 55 | ()</a> dumps all the information about the allocated memory block lefts |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 56 | in the <code>.memdump</code> file</li> |
| 57 | </ul><p>When developing libxml2 memory debug is enabled, the tests programs call |
| 58 | xmlMemoryDump () and the "make test" regression tests will check for any |
| 59 | memory leak during the full regression test sequence, this helps a lot |
| 60 | ensuring that libxml2 does not leak memory and bullet proof memory |
| 61 | allocations use (some libc implementations are known to be far too permissive |
| 62 | resulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation function and |
| William M. Brack | 43a8729 | 2007-02-15 20:41:02 +0000 | [diff] [blame] | 63 | also tries to give some information about the content and structure of the |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 64 | allocated blocks left. This is sufficient in most cases to find the culprit, |
| 65 | but not always. Assuming the allocation problem is reproducible, it is |
| 66 | possible to find more easily:</p><ol><li>write down the block number xxxx not allocated</li> |
| 67 | <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , the easiest |
| 68 | when using GDB is to simply give the command |
| Daniel Veillard | a8a89fe | 2002-04-12 21:03:34 +0000 | [diff] [blame] | 69 | <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p> |
| Daniel Veillard | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 70 | <p>before running the program.</p> |
| 71 | </li> |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 72 | <li>run the program under a debugger and set a breakpoint on |
| 73 | xmlMallocBreakpoint() a specific function called when this precise block |
| 74 | is allocated</li> |
| 75 | <li>when the breakpoint is reached you can then do a fine analysis of the |
| 76 | allocation an step to see the condition resulting in the missing |
| 77 | deallocation.</li> |
| 78 | </ol><p>I used to use a commercial tool to debug libxml2 memory problems but after |
| 79 | noticing that it was not detecting memory leaks that simple mechanism was |
| 80 | used and proved extremely efficient until now. Lately I have also used <a href="http://developer.kde.org/~sewardj/">valgrind</a> with quite some |
| 81 | success, it is tied to the i386 architecture since it works by emulating the |
| 82 | processor and instruction set, it is slow but extremely efficient, i.e. it |
| 83 | spot 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 |
| 84 | of a number of things:</p><ul><li>the parser itself should work in a fixed amount of memory, except for |
| 85 | information maintained about the stacks of names and entities locations. |
| 86 | The I/O and encoding handlers will probably account for a few KBytes. |
| 87 | This is true for both the XML and HTML parser (though the HTML parser |
| 88 | need more state).</li> |
| 89 | <li>If you are generating the DOM tree then memory requirements will grow |
| 90 | nearly linear with the size of the data. In general for a balanced |
| 91 | textual document the internal memory requirement is about 4 times the |
| 92 | size of the UTF8 serialization of this document (example the XML-1.0 |
| 93 | recommendation is a bit more of 150KBytes and takes 650KBytes of main |
| 94 | memory when parsed). Validation will add a amount of memory required for |
| 95 | maintaining the external Dtd state which should be linear with the |
| 96 | complexity of the content model defined by the Dtd</li> |
| 97 | <li>If you need to work with fixed memory requirements or don't need the |
| 98 | full DOM tree then using the <a href="xmlreader.html">xmlReader |
| 99 | interface</a> is probably the best way to proceed, it still allows to |
| 100 | validate or operate on subset of the tree if needed.</li> |
| 101 | <li>If you don't care about the advanced features of libxml2 like |
| 102 | validation, DOM, XPath or XPointer, don't use entities, need to work with |
| 103 | fixed memory requirements, and try to get the fastest parsing possible |
| 104 | then the SAX interface should be used, but it has known restrictions.</li> |
| Daniel Veillard | ad87d5e | 2008-02-04 16:50:03 +0000 | [diff] [blame] | 105 | </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 |
| 106 | reduced memory usage although you freed the trees. This is because |
| 107 | libxml2 allocates memory in a number of small chunks. When freeing one |
| 108 | of those chunks, the OS may decide that giving this little memory back |
| 109 | to the kernel will cause too much overhead and delay the operation. As |
| 110 | all chunks are this small, they get actually freed but not returned to |
| 111 | the kernel. On systems using glibc, there is a function call |
| 112 | "malloc_trim" from malloc.h which does this missing operation (note that |
| 113 | it is allowed to fail). Thus, after freeing your tree you may simply try |
| 114 | "malloc_trim(0);" to really get the memory back. If your OS does not |
| 115 | provide 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> |