xmlio.html revision f781dbaee854950082d863a73f5f16fe73d899cc
1<?xml version="1.0" encoding="ISO-8859-1"?> 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=ISO-8859-1" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4TD {font-family: Verdana,Arial,Helvetica} 5BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6H1 {font-family: Verdana,Arial,Helvetica} 7H2 {font-family: Verdana,Arial,Helvetica} 8H3 {font-family: Verdana,Arial,Helvetica} 9A: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://www.blastwave.org/packages.php/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</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 17the 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 informations 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 36example 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 53default 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 55resizable memory buffer. The buffer allocation strategy can be selected to be 56either best-fit or use an exponential doubling one (CPU vs. memory use 57trade-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 59system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number 60of 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 63resource (file descriptor, or pointer to a protocol handler), the read() and 64close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset 65encoding handler are also present to support charset conversion when 66needed.</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 67Input 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 68the parser. Creating an input from a filename or an URI string is done 69through the xmlNewInputFromFile() routine. The default entity loader do not 70handle the PUBLIC identifier associated with an entity (if any). So it just 71calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in 72XML).</p><p>If you want to hook up a catalog mechanism then you simply need to 73override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h> 74 75xmlExternalEntityLoader defaultLoader = NULL; 76 77xmlParserInputPtr 78xmlMyExternalEntityLoader(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 92int 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 103real use case</a>, xmlDocDump() closes the FILE * passed by the application 104and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a 105new 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 108xmlOutputBufferCreateOwn(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; 126xmlOutputBufferPtr output; 127xmlDocPtr doc; 128int res; 129 130f = ... 131doc = .... 132 133output = xmlOutputBufferCreateOwn(f, NULL); 134res = 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> 138
