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141 lines
7.1 KiB
HTML
141 lines
7.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<title>Upgrading libxml client code from 1.x to 2.x</title>
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<meta name="GENERATOR" content="amaya V5.0">
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<meta http-equiv="Content-Type" content="text/html">
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</head>
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<body bgcolor="#ffffff">
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<h1 align="center">Upgrading libxml client code from 1.x to 2.x</h1>
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<h2>Incompatible changes:</h2>
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<p>Version 2 of libxml is the first version introducing serious backward
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incompatible changes. The main goals were:</p>
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<ul>
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<li>a general cleanup. A number of mistakes inherited from the very early
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versions couldn't be changed due to compatibility constraints. Example
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the "childs" element in the nodes.</li>
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<li>Uniformization of the various nodes, at least for their header and link
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parts (doc, parent, children, prev, next), the goal is a simpler
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programming model and simplifying the task of the DOM implementors.</li>
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<li>better conformances to the XML specification, for example version 1.x
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had an heuristic to try to detect ignorable white spaces. As a result the
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SAX event generated were ignorableWhitespace() while the spec requires
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character() in that case. This also mean that a number of DOM node
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containing blank text may populate the DOM tree which were not present
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before.</li>
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</ul>
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<h2>How to fix libxml-1.x code:</h2>
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<p>So client code of libxml designed to run with version 1.x may have to be
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changed to compile against version 2.x of libxml. Here is a list of changes
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that I have collected, they may not be sufficient, so in case you find other
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change which are required, <a href="mailto:Daniel.Ïeillardw3.org">drop me a
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mail</a>:</p>
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<ol>
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<li>The package name have changed from libxml to libxml2, the library name
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is now -lxml2 . There is a new xml2-config script which should be used to
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select the right parameters libxml2</li>
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<li>Node <strong>childs</strong> field has been renamed
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<strong>children</strong> so s/childs/children/g should be applied
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(probablility of having "childs" anywere else is close to 0+</li>
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<li>The document don't have anymore a <strong>root</strong> element it has
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been replaced by <strong>children</strong> and usually you will get a
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list of element here. For example a Dtd element for the internal subset
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and it's declaration may be found in that list, as well as processing
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instructions or comments found before or after the document root element.
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Use <strong>xmlDocGetRootElement(doc)</strong> to get the root element of
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a document. Alternatively if you are sure to not reference Dtds nor have
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PIs or comments before or after the root element
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s/->root/->children/g will probably do it.</li>
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<li>The white space issue, this one is more complex, unless special case of
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validating parsing, the line breaks and spaces usually used for indenting
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and formatting the document content becomes significant. So they are
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reported by SAX and if your using the DOM tree, corresponding nodes are
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generated. Too approach can be taken:
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<ol>
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<li>lazy one, use the compatibility call
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<strong>xmlKeepBlanksDefault(0)</strong> but be aware that you are
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relying on a special (and possibly broken) set of heuristics of
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libxml to detect ignorable blanks. Don't complain if it breaks or
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make your application not 100% clean w.r.t. to it's input.</li>
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<li>the Right Way: change you code to accept possibly unsignificant
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blanks characters, or have your tree populated with weird blank text
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nodes. You can spot them using the comodity function
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<strong>xmlIsBlankNode(node)</strong> returning 1 for such blank
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nodes.</li>
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</ol>
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<p>Note also that with the new default the output functions don't add any
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extra indentation when saving a tree in order to be able to round trip
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(read and save) without inflating the document with extra formatting
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chars.</p>
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</li>
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<li>The include path has changed to $prefix/libxml/ and the includes
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themselves uses this new prefix in includes instructions... If you are
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using (as expected) the
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<pre>xml2-config --cflags</pre>
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<p>output to generate you compile commands this will probably work out of
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the box</p>
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</li>
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<li>xmlDetectCharEncoding takes an extra argument indicating the lenght in
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byte of the head of the document available for character detection.</li>
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</ol>
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<h2>Ensuring both libxml-1.x and libxml-2.x compatibility</h2>
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<p>Two new version of libxml (1.8.11) and libxml2 (2.3.4) have been released
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to allow smoth upgrade of existing libxml v1code while retaining
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compatibility. They offers the following:</p>
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<ol>
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<li>similar include naming, one should use
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<strong>#include<libxml/...></strong> in both cases.</li>
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<li>similar identifiers defined via macros for the child and root fields:
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respectively <strong>xmlChildrenNode</strong> and
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<strong>xmlRootNode</strong></li>
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<li>a new macro <strong>LIBXML_TEST_VERSION</strong> which should be
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inserted once in the client code</li>
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</ol>
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<p>So the roadmap to upgrade your existing libxml applications is the
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following:</p>
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<ol>
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<li>install the libxml-1.8.8 (and libxml-devel-1.8.8) packages</li>
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<li>find all occurences where the xmlDoc <strong>root</strong> field is
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used and change it to <strong>xmlRootNode</strong></li>
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<li>similary find all occurences where the xmlNode <strong>childs</strong>
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field is used and change it to <strong>xmlChildrenNode</strong></li>
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<li>add a <strong>LIBXML_TEST_VERSION</strong> macro somewhere in your
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<strong>main()</strong> or in the library init entry point</li>
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<li>Recompile, check compatibility, it should still work</li>
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<li>Change your configure script to look first for xml2-config and fallback
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using xml-config . Use the --cflags and --libs ouptut of the command as
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the Include and Linking parameters needed to use libxml.</li>
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<li>install libxml2-2.3.x and libxml2-devel-2.3.x (libxml-1.8.y and
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libxml-devel-1.8.y can be kept simultaneously)</li>
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<li>remove your config.cache, relaunch your configuration mechanism, and
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recompile, if steps 2 and 3 were done right it should compile as-is</li>
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<li>Test that your application is still running correctly, if not this may
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be due to extra empty nodes due to formating spaces being kept in libxml2
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contrary to libxml1, in that case insert xmlKeepBlanksDefault(1) in your
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code before calling the parser (next to
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<strong>LIBXML_TEST_VERSION</strong> is a fine place).</li>
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</ol>
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<p>Following those steps should work. It worked for some of my own code.</p>
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<p>Let me put some emphasis on the fact that there is far more changes from
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libxml 1.x to 2.x than the ones you may have to patch for. The overall code
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has been considerably improved and the conformance to the XML specification
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has been drastically improve. Don't take those changes as an excuse to not
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upgrade, it may cost a lot on the long term ...</p>
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<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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<p>$Id: upgrade.html,v 1.9 2001/06/24 12:13:21 veillard Exp $</p>
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</body>
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</html>
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