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| Daniel Veillard | d463c99 | 2006-02-23 22:07:59 +0000 | [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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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 | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 12 | <li><a href="#cleanup">Cleaning up after parsing</a></li> |
| 13 | <li><a href="#Debugging">Debugging routines</a></li> |
| 14 | <li><a href="#General4">General memory requirements</a></li> |
| Daniel Veillard | fabafd5 | 2006-06-08 08:16:33 +0000 | [diff] [blame^] | 15 | </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>providesthe |
| 16 | interfaces to the libxml2 memory system:</p><ul><li>libxml2 does not use the libc memory allocator directly |
| 17 | butxmlFree(),xmlMalloc() and xmlRealloc()</li> |
| 18 | <li>those routines can be reallocated to a specific set of |
| 19 | routine,bydefault the libc ones i.e. free(), malloc() and realloc()</li> |
| Daniel Veillard | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 20 | <li>the xmlmemory.c module includes a set of debugging routine</li> |
| Daniel Veillard | fabafd5 | 2006-06-08 08:16:33 +0000 | [diff] [blame^] | 21 | </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, |
| 22 | eitherfordebugging, analysis or to implement a specific behaviour on |
| 23 | memorymanagement(like on embedded systems). Two function calls are available |
| 24 | to doso:</p><ul><li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemGet()</a>whichreturn |
| 25 | the current set of functions in use by the parser</li> |
| 26 | <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemSetup()</a>whichallow |
| 27 | to set up a new set of memory allocation functions</li> |
| 28 | </ul><p>Of course a call to xmlMemSetup() should probably be done beforecallingany |
| 29 | other libxml2 routines (unless you are sure your allocationsroutines |
| 30 | 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 |
| 31 | structuresneedingallocation before the parser is fully functional (some |
| 32 | encodingstructuresfor example). This also mean that once parsing is finished |
| 33 | there isa tinyamount of memory (a few hundred bytes) which can be recollected |
| 34 | if youdon'treuse the parser immediately:</p><ul><li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlCleanupParser()</a>isa |
| 35 | centralized routine to free the parsing states. Note that |
| 36 | itwon'tdeallocate any produced tree if any (use the xmlFreeDoc() |
| 37 | andrelatedroutines for this).</li> |
| 38 | <li><a href="http://xmlsoft.org/html/libxml-parser.html">xmlInitParser()</a>isthe |
| 39 | dual routine allowing to preallocate the parsing statewhich can beuseful |
| 40 | for example to avoid initialization reentrancyproblems when usinglibxml2 |
| 41 | in multithreaded applications</li> |
| 42 | </ul><p>Generally xmlCleanupParser() is safe, if needed the state will berebuildat |
| 43 | the next invocation of parser routines, but be careful of theconsequencesin |
| 44 | multithreaded applications.</p><h3><a name="Debugging" id="Debugging">Debugging routines</a></h3><p>When configured using --with-mem-debug flag (off by default), libxml2usesa |
| 45 | set of memory allocation debugging routines keeping track of |
| 46 | allallocatedblocks and the location in the code where the routine was called. |
| 47 | Acouple ofother debugging routines allow to dump the memory allocated infos |
| 48 | toa 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>arethe |
| 49 | memory debugging replacement allocation routines</li> |
| 50 | <li><a href="http://xmlsoft.org/html/libxml-xmlmemory.html">xmlMemoryDump()</a>dumpsall |
| 51 | the informations about the allocated memory block leftsin |
| 52 | the<code>.memdump</code>file</li> |
| 53 | </ul><p>When developing libxml2 memory debug is enabled, the tests |
| 54 | programscallxmlMemoryDump () and the "make test" regression tests will check |
| 55 | foranymemory leak during the full regression test sequence, this helps |
| 56 | alotensuring that libxml2 does not leak memory and bullet |
| 57 | proofmemoryallocations use (some libc implementations are known to be far |
| 58 | toopermissiveresulting in major portability problems!).</p><p>If the .memdump reports a leak, it displays the allocation functionandalso |
| 59 | tries to give some informations about the content and structure |
| 60 | oftheallocated blocks left. This is sufficient in most cases to find |
| 61 | theculprit,but not always. Assuming the allocation problem is reproducible, |
| 62 | itispossible to find more easily:</p><ol><li>write down the block number xxxx not allocated</li> |
| 63 | <li>export the environment variable XML_MEM_BREAKPOINT=xxxx , |
| 64 | theeasiestwhen using GDB is to simply give the command |
| Daniel Veillard | a8a89fe | 2002-04-12 21:03:34 +0000 | [diff] [blame] | 65 | <p><code>set environment XML_MEM_BREAKPOINT xxxx</code></p> |
| Daniel Veillard | 0b28e88 | 2002-07-24 23:47:05 +0000 | [diff] [blame] | 66 | <p>before running the program.</p> |
| 67 | </li> |
| Daniel Veillard | fabafd5 | 2006-06-08 08:16:33 +0000 | [diff] [blame^] | 68 | <li>run the program under a debugger and set a |
| 69 | breakpointonxmlMallocBreakpoint() a specific function called when this |
| 70 | preciseblockis allocated</li> |
| 71 | <li>when the breakpoint is reached you can then do a fine analysis |
| 72 | oftheallocation an step to see the condition resulting in |
| 73 | themissingdeallocation.</li> |
| 74 | </ol><p>I used to use a commercial tool to debug libxml2 memory problems |
| 75 | butafternoticing that it was not detecting memory leaks that simple |
| 76 | mechanismwasused and proved extremely efficient until now. Lately I have also |
| 77 | used <a href="http://developer.kde.org/~sewardj/">valgrind</a>with quite |
| 78 | somesuccess,it is tied to the i386 architecture since it works by emulating |
| 79 | theprocessorand instruction set, it is slow but extremely efficient, i.e. |
| 80 | itspot memoryusage 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 itdependsof |
| 81 | a number of things:</p><ul><li>the parser itself should work in a fixed amount of memory, |
| 82 | exceptforinformation maintained about the stacks of names and |
| 83 | entitieslocations.The I/O and encoding handlers will probably account for |
| 84 | a fewKBytes.This is true for both the XML and HTML parser (though the |
| 85 | HTMLparserneed more state).</li> |
| 86 | <li>If you are generating the DOM tree then memory requirements |
| 87 | willgrownearly linear with the size of the data. In general for |
| 88 | abalancedtextual document the internal memory requirement is about 4 |
| 89 | timesthesize of the UTF8 serialization of this document (example |
| 90 | theXML-1.0recommendation is a bit more of 150KBytes and takes 650KBytes |
| 91 | ofmainmemory when parsed). Validation will add a amount of memory |
| 92 | requiredformaintaining the external Dtd state which should be linear |
| 93 | withthecomplexity of the content model defined by the Dtd</li> |
| 94 | <li>If you need to work with fixed memory requirements or don't needthefull |
| 95 | DOM tree then using the <a href="xmlreader.html">xmlReaderinterface</a>is |
| 96 | probably the best way toproceed, it still allows tovalidate or operate on |
| 97 | subset of the tree ifneeded.</li> |
| 98 | <li>If you don't care about the advanced features of libxml2likevalidation, |
| 99 | DOM, XPath or XPointer, don't use entities, need to workwithfixed memory |
| 100 | requirements, and try to get the fastest parsingpossiblethen the SAX |
| 101 | interface should be used, but it has knownrestrictions.</li> |
| Daniel Veillard | 1177ca4 | 2003-04-26 22:29:54 +0000 | [diff] [blame] | 102 | </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> |