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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 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>I/O Interfaces</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 Manual</a></li><li><a href="examples/index.html" style="font-weight:bold">Code Examples</a></li><li><a href="guidelines.html">XML Guidelines</a></li><li><a href="tutorial/index.html">Tutorial</a></li><li><a href="xmlreader.html">The Reader Interface</a></li><li><a href="ChangeLog.html">ChangeLog</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="architecture.html">libxml2 architecture</a></li><li><a href="tree.html">The tree output</a></li><li><a href="interface.html">The SAX interface</a></li><li><a href="xmlmem.html">Memory Management</a></li><li><a href="xmlio.html">I/O Interfaces</a></li><li><a href="library.html">The parser interfaces</a></li><li><a href="entities.html">Entities or no entities</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="upgrade.html">Upgrading 1.x code</a></li><li><a href="threads.html">Thread safety</a></li><li><a href="DOM.html">DOM Principles</a></li><li><a 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://codespeak.net/lxml/">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>Table of Content:</p><ol><li><a href="#General1">General overview</a></li> 11: <li><a href="#basic">The basic buffer type</a></li> 12: <li><a href="#Input">Input I/O handlers</a></li> 13: <li><a href="#Output">Output I/O handlers</a></li> 14: <li><a href="#entities">The entities loader</a></li> 15: <li><a href="#Example2">Example of customized I/O</a></li> 16: </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 17: the interfaces to the libxml2 I/O system. This consists of 4 main parts:</p><ul><li>Entities loader, this is a routine which tries to fetch the entities 18: (files) based on their PUBLIC and SYSTEM identifiers. The default loader 19: don't look at the public identifier since libxml2 do not maintain a 20: catalog. You can redefine you own entity loader by using 21: <code>xmlGetExternalEntityLoader()</code> and 22: <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the 23: example</a>.</li> 24: <li>Input I/O buffers which are a commodity structure used by the parser(s) 25: input layer to handle fetching the information to feed the parser. This 26: provides buffering and is also a placeholder where the encoding 27: converters to UTF8 are piggy-backed.</li> 28: <li>Output I/O buffers are similar to the Input ones and fulfill similar 29: task but when generating a serialization from a tree.</li> 30: <li>A mechanism to register sets of I/O callbacks and associate them with 31: specific naming schemes like the protocol part of the URIs. 32: <p>This affect the default I/O operations and allows to use specific I/O 33: handlers for certain names.</p> 34: </li> 35: </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for 36: example in the HTML parser is the following:</p><ol><li>The default entity loader calls <code>xmlNewInputFromFile()</code> with 37: the parsing context and the URI string.</li> 38: <li>the URI string is checked against the existing registered handlers 39: using their match() callback function, if the HTTP module was compiled 40: in, it is registered and its match() function will succeeds</li> 41: <li>the open() function of the handler is called and if successful will 42: return an I/O Input buffer</li> 43: <li>the parser will the start reading from this buffer and progressively 44: fetch information from the resource, calling the read() function of the 45: handler until the resource is exhausted</li> 46: <li>if an encoding change is detected it will be installed on the input 47: buffer, providing buffering and efficient use of the conversion 48: routines</li> 49: <li>once the parser has finished, the close() function of the handler is 50: called once and the Input buffer and associated resources are 51: deallocated.</li> 52: </ol><p>The user defined callbacks are checked first to allow overriding of the 53: 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 54: <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a 55: resizable memory buffer. The buffer allocation strategy can be selected to be 56: either best-fit or use an exponential doubling one (CPU vs. memory use 57: trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and 58: <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a 59: system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number 60: of functions allows to manipulate buffers with names starting with the 61: <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 62: <code>xmlParserInputBuffer</code> containing a context associated to the 63: resource (file descriptor, or pointer to a protocol handler), the read() and 64: close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset 65: encoding handler are also present to support charset conversion when 66: 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 67: 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 68: the parser. Creating an input from a filename or an URI string is done 69: through the xmlNewInputFromFile() routine. The default entity loader do not 70: handle the PUBLIC identifier associated with an entity (if any). So it just 71: calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in 72: XML).</p><p>If you want to hook up a catalog mechanism then you simply need to 73: override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h> 74: 75: xmlExternalEntityLoader defaultLoader = NULL; 76: 77: xmlParserInputPtr 78: xmlMyExternalEntityLoader(const char *URL, const char *ID, 79: xmlParserCtxtPtr ctxt) { 80: xmlParserInputPtr ret; 81: const char *fileID = NULL; 82: /* lookup for the fileID depending on ID */ 83: 84: ret = xmlNewInputFromFile(ctxt, fileID); 85: if (ret != NULL) 86: return(ret); 87: if (defaultLoader != NULL) 88: ret = defaultLoader(URL, ID, ctxt); 89: return(ret); 90: } 91: 92: int main(..) { 93: ... 94: 95: /* 96: * Install our own entity loader 97: */ 98: defaultLoader = xmlGetExternalEntityLoader(); 99: xmlSetExternalEntityLoader(xmlMyExternalEntityLoader); 100: 101: ... 102: }</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 103: real use case</a>, xmlDocDump() closes the FILE * passed by the application 104: and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a 105: new output handler with the closing call deactivated:</p><ol><li>First define a new I/O output allocator where the output don't close 106: the file: 107: <pre>xmlOutputBufferPtr 108: xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) { 109: xmlOutputBufferPtr ret; 110: 111: if (xmlOutputCallbackInitialized == 0) 112: xmlRegisterDefaultOutputCallbacks(); 113: 114: if (file == NULL) return(NULL); 115: ret = xmlAllocOutputBuffer(encoder); 116: if (ret != NULL) { 117: ret->context = file; 118: ret->writecallback = xmlFileWrite; 119: ret->closecallback = NULL; /* No close callback */ 120: } 121: return(ret); 122: } </pre> 123: </li> 124: <li>And then use it to save the document: 125: <pre>FILE *f; 126: xmlOutputBufferPtr output; 127: xmlDocPtr doc; 128: int res; 129: 130: f = ... 131: doc = .... 132: 133: output = xmlOutputBufferCreateOwn(f, NULL); 134: res = xmlSaveFileTo(output, doc, NULL); 135: </pre> 136: </li> 137: </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>