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| Daniel Veillard | 28fdf8b | 2011-03-07 08:12:39 +0800 | [diff] [blame] | 10 | </style><title>Entities or no entities</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>Entities or no entities</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Developer Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html" style="font-weight:bold">Main Menu</a></li><li><a href="html/index.html" style="font-weight:bold">Reference 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href="example.html">A real example</a></li><li><a href="xml.html">flat page</a>, <a href="site.xsl">stylesheet</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="http://libxmlplusplus.sourceforge.net/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table 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>Entities in principle are similar to simple C macros. An entity defines an |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 11 | abbreviation for a given string that you can reuse many times throughout the |
| 12 | content of your document. Entities are especially useful when a given string |
| 13 | may occur frequently within a document, or to confine the change needed to a |
| 14 | document to a restricted area in the internal subset of the document (at the |
| 15 | beginning). Example:</p><pre>1 <?xml version="1.0"?> |
| Daniel Veillard | 024f199 | 2003-12-10 16:43:49 +0000 | [diff] [blame] | 16 | 2 <!DOCTYPE EXAMPLE SYSTEM "example.dtd" [ |
| 17 | 3 <!ENTITY xml "Extensible Markup Language"> |
| Daniel Veillard | c948420 | 2001-10-24 12:35:52 +0000 | [diff] [blame] | 18 | 4 ]> |
| 19 | 5 <EXAMPLE> |
| 20 | 6 &xml; |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 21 | 7 </EXAMPLE></pre><p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing |
| 22 | its name with '&' and following it by ';' without any spaces added. There |
| 23 | are 5 predefined entities in libxml2 allowing you to escape characters with |
| 24 | predefined meaning in some parts of the xml document content: |
| 25 | <strong>&lt;</strong> for the character '<', <strong>&gt;</strong> |
| 26 | for the character '>', <strong>&apos;</strong> for the character ''', |
| 27 | <strong>&quot;</strong> for the character '"', and |
| 28 | <strong>&amp;</strong> for the character '&'.</p><p>One of the problems related to entities is that you may want the parser to |
| 29 | substitute an entity's content so that you can see the replacement text in |
| 30 | your application. Or you may prefer to keep entity references as such in the |
| 31 | content to be able to save the document back without losing this usually |
| 32 | precious information (if the user went through the pain of explicitly |
| 33 | defining entities, he may have a a rather negative attitude if you blindly |
| 34 | substitute them as saving time). The <a href="html/libxml-parser.html#xmlSubstituteEntitiesDefault">xmlSubstituteEntitiesDefault()</a> |
| 35 | function allows you to check and change the behaviour, which is to not |
| 36 | substitute entities by default.</p><p>Here is the DOM tree built by libxml2 for the previous document in the |
| 37 | default case:</p><pre>/gnome/src/gnome-xml -> ./xmllint --debug test/ent1 |
| Daniel Veillard | c948420 | 2001-10-24 12:35:52 +0000 | [diff] [blame] | 38 | DOCUMENT |
| 39 | version=1.0 |
| 40 | ELEMENT EXAMPLE |
| 41 | TEXT |
| 42 | content= |
| 43 | ENTITY_REF |
| 44 | INTERNAL_GENERAL_ENTITY xml |
| 45 | content=Extensible Markup Language |
| 46 | TEXT |
| Daniel Veillard | 1177ca4 | 2003-04-26 22:29:54 +0000 | [diff] [blame] | 47 | content=</pre><p>And here is the result when substituting entities:</p><pre>/gnome/src/gnome-xml -> ./tester --debug --noent test/ent1 |
| Daniel Veillard | c948420 | 2001-10-24 12:35:52 +0000 | [diff] [blame] | 48 | DOCUMENT |
| 49 | version=1.0 |
| 50 | ELEMENT EXAMPLE |
| 51 | TEXT |
| Daniel Veillard | f781dba | 2006-06-09 13:34:49 +0000 | [diff] [blame] | 52 | content= Extensible Markup Language</pre><p>So, entities or no entities? Basically, it depends on your use case. I |
| 53 | suggest that you keep the non-substituting default behaviour and avoid using |
| 54 | entities in your XML document or data if you are not willing to handle the |
| 55 | entity references elements in the DOM tree.</p><p>Note that at save time libxml2 enforces the conversion of the predefined |
| 56 | entities where necessary to prevent well-formedness problems, and will also |
| 57 | transparently replace those with chars (i.e. it will not generate entity |
| 58 | reference elements in the DOM tree or call the reference() SAX callback when |
| 59 | finding them in the input).</p><p><span style="background-color: #FF0000">WARNING</span>: handling entities |
| 60 | on top of the libxml2 SAX interface is difficult!!! If you plan to use |
| 61 | non-predefined entities in your documents, then the learning curve to handle |
| 62 | then using the SAX API may be long. If you plan to use complex documents, I |
| 63 | strongly suggest you consider using the DOM interface instead and let libxml |
| 64 | deal with the complexity rather than trying to do it yourself.</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> |