1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4 TD {font-family: Verdana,Arial,Helvetica} 5 BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6 H1 {font-family: Verdana,Arial,Helvetica} 7 H2 {font-family: Verdana,Arial,Helvetica} 8 H3 {font-family: Verdana,Arial,Helvetica} 9 A:link, A:visited, A:active { text-decoration: underline } 10 </style><title>I/O Interfaces</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 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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>Table of Content:</p><ol> 11 <li><a href="#General1">General overview</a></li> 12 <li><a href="#basic">The basic buffer type</a></li> 13 <li><a href="#Input">Input I/O handlers</a></li> 14 <li><a href="#Output">Output I/O handlers</a></li> 15 <li><a href="#entities">The entities loader</a></li> 16 <li><a href="#Example2">Example of customized I/O</a></li> 17 </ol><h3><a name="General1" id="General1">General overview</a></h3><p>The module <code><a href="http://xmlsoft.org/html/libxml-xmlio.html">xmlIO.h</a></code> provides 18 the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul> 19 <li>Entities loader, this is a routine which tries to fetch the entities 20 (files) based on their PUBLIC and SYSTEM identifiers. The default loader 21 don't look at the public identifier since libxml2 do not maintain a 22 catalog. You can redefine you own entity loader by using 23 <code>xmlGetExternalEntityLoader()</code> and 24 <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the 25 example</a>.</li> 26 <li>Input I/O buffers which are a commodity structure used by the parser(s) 27 input layer to handle fetching the information to feed the parser. This 28 provides buffering and is also a placeholder where the encoding 29 converters to UTF8 are piggy-backed.</li> 30 <li>Output I/O buffers are similar to the Input ones and fulfill similar 31 task but when generating a serialization from a tree.</li> 32 <li>A mechanism to register sets of I/O callbacks and associate them with 33 specific naming schemes like the protocol part of the URIs. 34 <p>This affect the default I/O operations and allows to use specific I/O 35 handlers for certain names.</p> 36 </li> 37 </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for 38 example in the HTML parser is the following:</p><ol> 39 <li>The default entity loader calls <code>xmlNewInputFromFile()</code> with 40 the parsing context and the URI string.</li> 41 <li>the URI string is checked against the existing registered handlers 42 using their match() callback function, if the HTTP module was compiled 43 in, it is registered and its match() function will succeeds</li> 44 <li>the open() function of the handler is called and if successful will 45 return an I/O Input buffer</li> 46 <li>the parser will the start reading from this buffer and progressively 47 fetch information from the resource, calling the read() function of the 48 handler until the resource is exhausted</li> 49 <li>if an encoding change is detected it will be installed on the input 50 buffer, providing buffering and efficient use of the conversion 51 routines</li> 52 <li>once the parser has finished, the close() function of the handler is 53 called once and the Input buffer and associated resources are 54 deallocated.</li> 55 </ol><p>The user defined callbacks are checked first to allow overriding of the 56 default libxml2 I/O routines.</p><h3><a name="basic" id="basic">The basic buffer type</a></h3><p>All the buffer manipulation handling is done using the 57 <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a 58 resizable memory buffer. The buffer allocation strategy can be selected to be 59 either best-fit or use an exponential doubling one (CPU vs. memory use 60 trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and 61 <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a 62 system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number 63 of functions allows to manipulate buffers with names starting with the 64 <code>xmlBuffer...</code> prefix.</p><h3><a name="Input" id="Input">Input I/O handlers</a></h3><p>An Input I/O handler is a simple structure 65 <code>xmlParserInputBuffer</code> containing a context associated to the 66 resource (file descriptor, or pointer to a protocol handler), the read() and 67 close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset 68 encoding handler are also present to support charset conversion when 69 needed.</p><h3><a name="Output" id="Output">Output I/O handlers</a></h3><p>An Output handler <code>xmlOutputBuffer</code> is completely similar to an 70 Input one except the callbacks are write() and close().</p><h3><a name="entities" id="entities">The entities loader</a></h3><p>The entity loader resolves requests for new entities and create inputs for 71 the parser. Creating an input from a filename or an URI string is done 72 through the xmlNewInputFromFile() routine. The default entity loader do not 73 handle the PUBLIC identifier associated with an entity (if any). So it just 74 calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in 75 XML).</p><p>If you want to hook up a catalog mechanism then you simply need to 76 override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h> 77 78 xmlExternalEntityLoader defaultLoader = NULL; 79 80 xmlParserInputPtr 81 xmlMyExternalEntityLoader(const char *URL, const char *ID, 82 xmlParserCtxtPtr ctxt) { 83 xmlParserInputPtr ret; 84 const char *fileID = NULL; 85 /* lookup for the fileID depending on ID */ 86 87 ret = xmlNewInputFromFile(ctxt, fileID); 88 if (ret != NULL) 89 return(ret); 90 if (defaultLoader != NULL) 91 ret = defaultLoader(URL, ID, ctxt); 92 return(ret); 93 } 94 95 int main(..) { 96 ... 97 98 /* 99 * Install our own entity loader 100 */ 101 defaultLoader = xmlGetExternalEntityLoader(); 102 xmlSetExternalEntityLoader(xmlMyExternalEntityLoader); 103 104 ... 105 }</pre><h3><a name="Example2" id="Example2">Example of customized I/O</a></h3><p>This example come from <a href="http://xmlsoft.org/messages/0708.html">a 106 real use case</a>, xmlDocDump() closes the FILE * passed by the application 107 and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a 108 new output handler with the closing call deactivated:</p><ol> 109 <li>First define a new I/O output allocator where the output don't close 110 the file: 111 <pre>xmlOutputBufferPtr 112 xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) { 113 xmlOutputBufferPtr ret; 114 115 if (xmlOutputCallbackInitialized == 0) 116 xmlRegisterDefaultOutputCallbacks(); 117 118 if (file == NULL) return(NULL); 119 ret = xmlAllocOutputBuffer(encoder); 120 if (ret != NULL) { 121 ret->context = file; 122 ret->writecallback = xmlFileWrite; 123 ret->closecallback = NULL; /* No close callback */ 124 } 125 return(ret); 126 } </pre> 127 </li> 128 <li>And then use it to save the document: 129 <pre>FILE *f; 130 xmlOutputBufferPtr output; 131 xmlDocPtr doc; 132 int res; 133 134 f = ... 135 doc = .... 136 137 output = xmlOutputBufferCreateOwn(f, NULL); 138 res = xmlSaveFileTo(output, doc, NULL); 139 </pre> 140 </li> 141 </ol><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> 142
