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| </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 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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> |
| <li><a href="#basic">The basic buffer type</a></li> |
| <li><a href="#Input">Input I/O handlers</a></li> |
| <li><a href="#Output">Output I/O handlers</a></li> |
| <li><a href="#entities">The entities loader</a></li> |
| <li><a href="#Example2">Example of customized I/O</a></li> |
| </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 |
| 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 |
| (files) based on their PUBLIC and SYSTEM identifiers. The default loader |
| don't look at the public identifier since libxml2 do not maintain a |
| catalog. You can redefine you own entity loader by using |
| <code>xmlGetExternalEntityLoader()</code> and |
| <code>xmlSetExternalEntityLoader()</code>. <a href="#entities">Check the |
| example</a>.</li> |
| <li>Input I/O buffers which are a commodity structure used by the parser(s) |
| input layer to handle fetching the informations to feed the parser. This |
| provides buffering and is also a placeholder where the encoding |
| converters to UTF8 are piggy-backed.</li> |
| <li>Output I/O buffers are similar to the Input ones and fulfill similar |
| task but when generating a serialization from a tree.</li> |
| <li>A mechanism to register sets of I/O callbacks and associate them with |
| specific naming schemes like the protocol part of the URIs. |
| <p>This affect the default I/O operations and allows to use specific I/O |
| handlers for certain names.</p> |
| </li> |
| </ul><p>The general mechanism used when loading http://rpmfind.net/xml.html for |
| example in the HTML parser is the following:</p><ol><li>The default entity loader calls <code>xmlNewInputFromFile()</code> with |
| the parsing context and the URI string.</li> |
| <li>the URI string is checked against the existing registered handlers |
| using their match() callback function, if the HTTP module was compiled |
| in, it is registered and its match() function will succeeds</li> |
| <li>the open() function of the handler is called and if successful will |
| return an I/O Input buffer</li> |
| <li>the parser will the start reading from this buffer and progressively |
| fetch information from the resource, calling the read() function of the |
| handler until the resource is exhausted</li> |
| <li>if an encoding change is detected it will be installed on the input |
| buffer, providing buffering and efficient use of the conversion |
| routines</li> |
| <li>once the parser has finished, the close() function of the handler is |
| called once and the Input buffer and associated resources are |
| deallocated.</li> |
| </ol><p>The user defined callbacks are checked first to allow overriding of the |
| 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 |
| <code>xmlBuffer</code> type define in <code><a href="http://xmlsoft.org/html/libxml-tree.html">tree.h</a> </code>which is a |
| resizable memory buffer. The buffer allocation strategy can be selected to be |
| either best-fit or use an exponential doubling one (CPU vs. memory use |
| trade-off). The values are <code>XML_BUFFER_ALLOC_EXACT</code> and |
| <code>XML_BUFFER_ALLOC_DOUBLEIT</code>, and can be set individually or on a |
| system wide basis using <code>xmlBufferSetAllocationScheme()</code>. A number |
| of functions allows to manipulate buffers with names starting with the |
| <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 |
| <code>xmlParserInputBuffer</code> containing a context associated to the |
| resource (file descriptor, or pointer to a protocol handler), the read() and |
| close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset |
| encoding handler are also present to support charset conversion when |
| 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 |
| 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 |
| the parser. Creating an input from a filename or an URI string is done |
| through the xmlNewInputFromFile() routine. The default entity loader do not |
| handle the PUBLIC identifier associated with an entity (if any). So it just |
| calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in |
| XML).</p><p>If you want to hook up a catalog mechanism then you simply need to |
| override the default entity loader, here is an example:</p><pre>#include <libxml/xmlIO.h> |
| |
| xmlExternalEntityLoader defaultLoader = NULL; |
| |
| xmlParserInputPtr |
| xmlMyExternalEntityLoader(const char *URL, const char *ID, |
| xmlParserCtxtPtr ctxt) { |
| xmlParserInputPtr ret; |
| const char *fileID = NULL; |
| /* lookup for the fileID depending on ID */ |
| |
| ret = xmlNewInputFromFile(ctxt, fileID); |
| if (ret != NULL) |
| return(ret); |
| if (defaultLoader != NULL) |
| ret = defaultLoader(URL, ID, ctxt); |
| return(ret); |
| } |
| |
| int main(..) { |
| ... |
| |
| /* |
| * Install our own entity loader |
| */ |
| defaultLoader = xmlGetExternalEntityLoader(); |
| xmlSetExternalEntityLoader(xmlMyExternalEntityLoader); |
| |
| ... |
| }</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 |
| real use case</a>, xmlDocDump() closes the FILE * passed by the application |
| and this was a problem. The <a href="http://xmlsoft.org/messages/0711.html">solution</a> was to redefine a |
| 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 |
| the file: |
| <pre>xmlOutputBufferPtr |
| xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) { |
| xmlOutputBufferPtr ret; |
| |
| if (xmlOutputCallbackInitialized == 0) |
| xmlRegisterDefaultOutputCallbacks(); |
| |
| if (file == NULL) return(NULL); |
| ret = xmlAllocOutputBuffer(encoder); |
| if (ret != NULL) { |
| ret->context = file; |
| ret->writecallback = xmlFileWrite; |
| ret->closecallback = NULL; /* No close callback */ |
| } |
| return(ret); |
| } </pre> |
| </li> |
| <li>And then use it to save the document: |
| <pre>FILE *f; |
| xmlOutputBufferPtr output; |
| xmlDocPtr doc; |
| int res; |
| |
| f = ... |
| doc = .... |
| |
| output = xmlOutputBufferCreateOwn(f, NULL); |
| res = xmlSaveFileTo(output, doc, NULL); |
| </pre> |
| </li> |
| </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> |