15 Using the XML Parser for C

Prerequisites

The Oracle XML parser for C reads an XML document and uses DOM or SAX APIs to provide programmatic access to its content and structure. You can use the parser in validating or nonvalidating mode.

This chapter assumes that you are familiar with the following technologies:

• Document Object Model (DOM). DOM is an in-memory tree representation of the structure of an XML document.

• Simple API for XML (SAX). SAX is a standard for event-based XML parsing.

• Document Type Definition (DTD). An XML DTD defines the legal structure of an XML document.

• XML Schema. Like a DTD, an XML schema defines the legal structure of an XML document.

• XML Namespaces. Namespaces are a mechanism for differentiating element and attribute names.

If you require a general introduction to the preceding technologies, consult the XML resources listed in "Related Documents" of the preface.

Standards and Specifications

XML 1.0 is a W3C Recommendation. The C XDK API provides full support for XML 1.0 (Second Edition). You can find the specification for the Second Edition at the following URL:

http://www.w3.org/TR/2000/REC-xml-20001006

The DOM Level 1, Level 2, and Level 3 specifications are W3C Recommendations. The C XDK API provides full suport for DOM Level 1 and 2, but no support for Level 3. You can find links to the specifications for all three levels at the following URL:

http://www.w3.org/DOM/DOMTR

SAX is available in version 1.0, which is deprecated, and 2.0. SAX is not a W3C specification. The C XDK API provides full support for both SAX 1.0 and 2.0. You can find the documentation for SAX at the following URL:

http://www.saxproject.org

XML Namespaces are a W3C Recommendation. You can find the specification at the following URL:

http://www.w3.org/TR/REC-xml-names

Overview of the Parser API for C

The core of the XML parsing API are the XML, DOM, and SAX APIs. Table 15-1 describes the interfaces for these APIs. Refer to Oracle XML API Reference for the complete API documentation.

Using the XML Parser for C: Basic Process

Figure 15-1 illustrates the calling sequence for the XML parser for C.

Figure 15-1 XML Parser for C Calling Sequence

Description of the illustration adxdk096.gif

Perform the following steps in your application:

1. Initialize the parsing process with the XmlCreate() function. The following sample code fragment is from DOMNamespace.c:

   xmlctx     *xctx;
   ...
   xctx = XmlCreate(&ecode, (oratext *) "namespace_xctx", NULL);
   
   

2. Parse the input item, which can be an XML document or string buffer

   If you are parsing with DOM, call the XmlLoadDom() function. The following sample code fragment is from DOMNamespace.c:

   xmldocnode *doc;
   ...
   doc = XmlLoadDom(xctx, &ecode, "file", DOCUMENT,
                    "validate", TRUE, "discard_whitespace", TRUE, NULL);
   
   

   If you are parsing with SAX, call the XmlLoadSax() function. The following sample code fragment is from SAXNamespace.c:

   xmlerr      ecode;
   ...
   ecode = XmlLoadSax(xctx, &sax_callback, &sc, "file", DOCUMENT,
                          "validate", TRUE, "discard_whitespace", TRUE, NULL);
   
   

3. If you are using the DOM interface, then include the following steps:

   • Use the XmlLoadDom() function to call XmlDomGetDocElem(). This step calls other DOM functions, which are typically node or print functions that output the DOM document, as required. The following sample code fragment is from DOMNamespace.c:

     printElements(xctx, XmlDomGetDocElem(xctx, doc));
     
     

   • Invoke the XmlFreeDocument() function to clean up any data structures created during the parse process. The following sample code fragment is from DOMNamespace.c:

     XmlFreeDocument(xctx, doc);
     
     

   If you are using the SAX interface, then include the following steps:

   • Process the results of the invocation of XmlLoadSax() with callback functions.

   • Register the callback functions. Note that you can set any of the SAX callback functions to NULL if not needed.

4. Use XmlFreeDocument() to clean up the memory and structures used during a parse. The program does not free memory allocated for parameters passed to the SAX callbacks or for nodes and data stored with the DOM parse tree until you call XMLFreeDocument() or XMLDestroy(). The following sample code fragment is from DOMNamespace.c:

   XmlFreeDocument(xctx, doc);
   
   

   Either return to Step 2 or proceed to the next step.

5. Terminate the parsing process with XmlDestroy(). The following sample code fragment is from DOMNamespace.c:

   (void) XmlDestroy(xctx);
   
   

   If threads fork off somewhere in the sequence of calls between initialization and termination, the application produces unpredictable behavior and results.

You can use the memory callback functions XML_ALLOC_F and XML_FREE_F for your own memory allocation. If you do, then specify both functions.

Running the XML Parser for C Demo Programs

The $ORACLE_HOME/xdk/demo/c/ (UNIX) and %ORACLE_HOME%\xdk\demo\c (Windows) directories include several XML applications that illustrate how to use the XML parser for C with the DOM and SAX interfaces. Table 15-3 describes the demos.

DOMNamespace.c
DOMSample.c
FullDom.c
FullDom.xml
NSExample.xml
Traverse.c
XPointer.c
class.xml
cleo.xml
pantry.xml

NSExample.xml
SAXNamespace.c
SAXSample.c
cleo.xml

You can find documentation that describes how to compile and run the sample programs in the README in the same directory. The basic steps are as follows:

1. Change into the $ORACLE_HOME/xdk/demo/c directory (UNIX) or %ORACLE_HOME%\xdk\demo\c directory (Windows).

2. Make sure that your environment variables are set as described in "Setting C XDK Environment Variables on UNIX" and "Setting C XDK Environment Variables on Windows".

3. Run make (UNIX) or Make.bat (Windows) at the system prompt. The make utility changes into each demo subdirectory and runs make to do the following:

   1. Compiles the C source files with the cc utility. For example, the Makefile in the $ORACLE_HOME/xdk/demo/c/dom directory includes the following line:

      $(CC) -o DOMSample $(INCLUDE) $@.c $(LIB)
      
      

   2. Runs each demo program and redirects the output to a file. For example, the Makefile in the $ORACLE_HOME/xdk/demo/c/dom directory includes the following line:

      ./DOMSample > DOMSample.out
      
      

4. Compare the *.std files to the *.out files for each program. The *.std file contains the expected output for each program. For example, DOMSample.std contains the expected output from running DOMSample.

Using the C XML Parser Command-Line Utility

The xml utility, which is located in $ORACLE_HOME/bin (UNIX) or %ORACLE_HOME%\bin (Windows), is a command-line interface that parses XML documents. It checks for both well-formedness and validity.

To use xml ensure that your environment is set up as described in "Setting C XDK Environment Variables on UNIX" and "Setting C XDK Environment Variables on Windows".

Use the following syntax on the command line to invoke xml (the Windows version is xml.exe):

xml [options] [document URI]
xml -f [options] [document filespec]

Table 15-4 describes the command-line options.

ELEMENT       1
 PCDATA       1
    DOC       1
  TOTAL       3 * 60 = 180

+---ELEMENT greeting     +---PCDATA "hello"

<greeting>hello</greeting>

StartDocumentXMLDECL version='1.0' encoding=FALSE<greeting>    "hello"</greeting>EndDocument

<!DOCTYPE doc [
<!ELEMENT doc (child*)>
<!ATTLIST doc xmlns:nsprefix CDATA #IMPLIED>
<!ATTLIST doc xmlns CDATA #IMPLIED>
<!ATTLIST doc nsprefix:a1 CDATA #IMPLIED>
<!ELEMENT child (#PCDATA)>
]>
<doc nsprefix:a1 = "v1" xmlns="http://www.w3c.org" 
     xmlns:nsprefix="http://www.oracle.com">
<child>
This element inherits the default Namespace of doc.
</child>
</doc>

xml -np class.xml > xml.out

ELEMENT       2
    PCDATA       1
       DOC       1
       DTD       1
  ELEMDECL       2
  ATTRDECL       3
     TOTAL      10 * 112 = 1120
+---ELEMENT doc [nsprefix:a1='v1'*, xmlns='http://www.w3c.org'*, xmlns:nsprefix=
'http://www.oracle.com'*]
    +---ELEMENT child
        +---PCDATA "
This element inherits the default Namespace of doc.
"

Creating the Top-Level Context

When using the DOM to process XML in a traditional file system or on the Web, you need to perform implementation-specific steps only for startup.

Your application requires a top-level xmlctx. This context contains encoding information, low-level memory callbacks, error message language, encoding, and so on. In short, this context contains those things that should remain consistent for all XDK components. The basic steps for initialization are as follows:

1. Allocate an xmlctx with XmlCreate(). For example:

   xmlctx *xctx;
   xmlerr  err;
   
   xctx = (xmlctx *) XmlCreate(&err, "xdk context", "data-encoding", 
                               "ascii", ..., NULL);
   
   

2. Load documents and generate DOM events. After the application obtains an xmlctx, it can load a serialized XML document with the XmlLoadDom() or XmlLoadSax() functions. Given the Document node, all API DOM functions are available. You can generate DOM as in the following example:

   xmldocnode *domctx;
   xmlerr err;
   
   domctx = XmlLoadDom(xctx, &err, "file", "some_file.xml", NULL);
   
   

   To generate SAX events, you need a SAX callback structure as shown in the following example:

   xmlsaxcb saxcb = {
     UserAttrDeclNotify,  /* user's own callback functions */
     UserCDATANotify,
     /* ... */
   };
   
   if (XmlLoadSax(xctx, &saxcb, NULL, "file", "some_file.xml", NULL) != 0)
     /* an error occured */
   
   

3. You can tear down the XML context with the XmlDestroy() function.

Controlling the Data Encoding of XML Documents for the C API

XML data occurs in many encodings. You can control the XML encoding in the following ways:

• Specify a default encoding to assume for files that are not self-describing

• Specify the presentation encoding for DOM or SAX

• Re-encode when a DOM is serialized

Input XML data is always encoded. Some encodings are entirely self-describing, such as UTF-16, which requires a specific BOM before the start of the actual data. The XMLDecl or MIME header of the document can also specify an encoding. If the application cannot determine the specific encoding, then it applies the default input encoding. If you do not provide a default, then the application assumes UTF-8 on ASCII platforms and UTF-E on EBCDIC platforms.

The API makes a provision for cases when the encoding data of the input document is corrupt. For example, suppose an ASCII document with an XMLDecl of encoding=ascii is blindly converted to EBCDIC. The new EBCDIC document contains (in EBCDIC) an XMLDecl that incorrectly claims the document is ASCII. The correct behavior for a program that is re-encoding XML data is to regenerate but not convert the XMLDecl. The XMLDecl is metadata, not data itself. This rule is often ignored, however, which results in corrupt documents. To work around this problem, the API provides an additional flag that enables you to forcibly set the input encoding, thereby overcoming an incorrect XMLDecl.

The precedence rules for determining input encoding are as follows:

1. Forced encoding as specified by the user

   
   

   Caution: Forced encoding can result in a fatal error if there is a conflict. For example, the input document is UTF-16 and starts with a UTF-16 BOM, but the user specifies a forced UTF-8 encoding. In this case, the parser will object about the conflict.

   
   

2. Protocol specification (HTTP header, and so on)

3. XMLDecl specification

4. User's default input encoding

5. The default, which is UTF-8 on ASCII platforms or UTF-E on EBCDIC platforms

After the application has determined the input encoding has been determined, it can parse the document and present the data. You are allowed to choose the presentation encoding; the data will be in that encoding regardless of the original input encoding.

When an application writes back a DOM in serialized form, it can choose at that time to re-encode the presentation data. Thus, the you can place the serialized document in any encoding.

Using NULL-Terminated and Length-Encoded C API Functions

The native string representation in C is NULL-terminated. Thus, the primary DOM interface takes and returns NULL-terminated strings. When stored in table form, however, Oracle XML DB data is not NULL-terminated but length-encoded. Consequently, the XDK provides an additional set of length-encoded APIs for the high-frequency cases to improve performance. In particular, the DOM functions in Table 15-5 have dual APIs.

Handling Errors with the C API

The C API functions typically either return a numeric error code (0 for success, nonzero on failure), or pass back an error code through a variable. In all cases, the API stores error codes. Your application can retrieve the most recent error by calling the XmlDomGetLastError() function.

By default, the API outputs error messages to stderr. However, you can register an error message callback at initialization time. When an error occurs, the application invokes the registered callback and does not print an error.

Using XMLType Functions and Descriptions

You can use the C API for XML for XMLType columns in the database. An Oracle Call Interface (OCI) program can access XML data stored in a table by initializing the values of OCI handles such as the following:

• Environment handle

• Service handle

• Error handle

• Optional parameters

The program can pass these input values to the function OCIXmlDbInitXmlCtx(), which returns an XML context. After the program makes calls to the C API, the function OCIXmlDbFreeXmlCtx() frees the context.

Table 15-6 describes a few of the functions for XML operations.

Initializing an XML Context

An XML context is a required parameter in all the C DOM API functions. This opaque context encapsulates information pertaining to data encoding, error message language, and so on. The contents of this XML context are different for XDK applications and for Oracle XML DB applications.

For Oracle XML DB, the two OCI functions that initialize and free an XML context have the following prototypes:

xmlctx *OCIXmlDbInitXmlCtx (OCIEnv *envhp, OCISvcCtx *svchp, OCIError *errhp,
       ocixmldbparam *params, ub4 num_params);

void OCIXmlDbFreeXmlCtx (xmlctx *xctx);

Creating XMLType Instances on the Client

You can construct new XMLType instances on the client by using the XmlLoadDom() calls. Follow these basic steps:

1. You first have to initialize the xmlctx, as illustrated in the example in "Using the DOM API for C".

2. You can construct the XML data itself from the following sources:

   • User buffer

   • Local file

   • URI

   The return value from these is an (xmldocnode *), which you can use in the rest of the common C API.

3. Finally, you can cast the (xmldocnode *) to a (void *) and directly provide it as the bind value if required.

You can construct empty XMLType instances by using the XmlCreateDocument() call. This function would be equivalent to an OCIObjectNew() for other types. You can operate on the (xmldocnode *) returned by the preceding call and finally cast it to a (void *) if it needs to be provided as a bind value.

Operating on XML Data in the Database Server

You can operate on XML data in Oracle Database by means of OCI statement calls. You can bind and define XMLType values using xmldocnode and use OCI statements to select XML data from the database. You can use this data directly in the C DOM functions. Similarly, you can bind the values directly to SQL statements.

Using OCI and the XDK C API: Examples

Example 15-3 illustrates how to construct a schema-based document with the DOM API and save it to the database. Note that you must include the header files xml.h and ocixmldb.h.

#include <xml.h>
#include <ocixmldb.h>
static oratext tlpxml_test_sch[] = "<TOP xmlns='example1.xsd'\n\
xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' \n\
xsi:schemaLocation='example1.xsd example1.xsd'/>";

void example1()
{
    OCIEnv *envhp;
    OCIError *errhp;
    OCISvcCtx *svchp;
    OCIStmt *stmthp;
    OCIDuration dur;
    OCIType *xmltdo;

    xmldocnode  *doc;
    ocixmldbparam params[1];
    xmlnode *quux, *foo, *foo_data;
    xmlerr       err;

    /* Initialize envhp, svchp, errhp, dur, stmthp */
    /* ........ */

    /* Get an xml context */
    params[0].name_ocixmldbparam = XCTXINIT_OCIDUR;
    params[0].value_ocixmldbparam = &dur;
    xctx = OCIXmlDbInitXmlCtx(envhp, svchp, errhp, params, 1);

    /* Start processing */ 
    printf("Supports XML 1.0: %s\n",
       XmlHasFeature(xctx, (oratext *) "xml", (oratext *) "1.0") ?
                     "YES" : "NO");

    /* Parsing a schema-based document */
    if (!(doc = XmlLoadDom(xctx, &err, "buffer", tlpxml_test_sch,
                          "buffer_length", sizeof(tlpxml_test_sch)-1,
                          "validate", TRUE, NULL)))
    {
       printf("Parse failed, code %d\n");
       return;
    }

    /* Create some elements and add them to the document */
    top = XmlDomGetDocElem(xctx, doc);
    quux = (xmlnode *) XmlDomCreateElem(xctx ,doc, (oratext *) "QUUX");
    foo = (xmlnode *) XmlDomCreateElem(xctx, doc, (oratext *) "FOO");
    foo_data = (xmlnode *) XmlDomCreateText(xctx, doc, (oratext *)"foo's data");
    foo_data = XmlDomAppendChild(xctx, (xmlnode *) foo, (xmlnode *) foo_data);
    foo = XmlDomAppendChild(xctx, quux, foo);
    quux = XmlDomAppendChild(xctx, top, quux);

    XmlSaveDom(xctx, &err, top, "stdio", stdout, NULL);
    XmlSaveDom(xctx, &err, doc, "stdio", stdout, NULL);

    /* Insert the document to my_table */
    ins_stmt = "insert into my_table values (:1)";

    status = OCITypeByName(envhp, errhp, svchp, (const text *) "SYS",
                   (ub4) strlen((char *)"SYS"), (const text *) "XMLTYPE",
                   (ub4) strlen((char *)"XMLTYPE"), (CONST text *) 0,
                   (ub4) 0, dur, OCI_TYPEGET_HEADER,
                   (OCIType **) &xmltdo)) ;

    if (status == OCI_SUCCESS)
    {
       exec_bind_xml(svchp, errhp, stmthp, (void *)doc, xmltdo, ins_stmt));
    }

   /* free xml ctx */
   OCIXmlDbFreeXmlCtx(xctx);
}

/*--------------------------------------------------------*/
/* execute a sql statement which binds xml data */
/*--------------------------------------------------------*/
sword exec_bind_xml(svchp, errhp, stmthp, xml, xmltdo, sqlstmt)
OCISvcCtx *svchp;
OCIError *errhp;
OCIStmt *stmthp;
void *xml;
OCIType *xmltdo;
OraText *sqlstmt;
{
   OCIBind *bndhp1 = (OCIBind *) 0;
   OCIBind *bndhp2 = (OCIBind *) 0;
   sword  status = 0;
   OCIInd ind = OCI_IND_NOTNULL;
   OCIInd *indp = &ind;

   if(status = OCIStmtPrepare(stmthp, errhp, (OraText *)sqlstmt,
                    (ub4)strlen((char *)sqlstmt),
                    (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT)) {
     return OCI_ERROR;
   }

   if(status = OCIBindByPos(stmthp, &bndhp1, errhp, (ub4) 1, (dvoid *) 0,
                   (sb4) 0, SQLT_NTY, (dvoid *) 0, (ub2 *)0,
                   (ub2 *)0, (ub4) 0, (ub4 *) 0, (ub4) OCI_DEFAULT)) {
     return OCI_ERROR;
   }

   if(status = OCIBindObject(bndhp1, errhp, (CONST OCIType *) xmltdo,
               (dvoid **) &xml, (ub4 *) 0, (dvoid **) &indp, (ub4 *) 0)) {
     return OCI_ERROR;
   }

   if(status = OCIStmtExecute(svchp, stmthp, errhp, (ub4) 1, (ub4) 0,
                (CONST OCISnapshot*) 0, (OCISnapshot*) 0, (ub4) OCI_DEFAULT)) {
     return OCI_ERROR;
  }

   return OCI_SUCCESS;
}

Example 15-4 illustrates how to get a document from the database and modify it with the DOM API.

#include <xml.h>
#include <ocixmldb.h>
sword example2()
{
    OCIEnv *envhp;
    OCIError *errhp;
    OCISvcCtx *svchp;
    OCIStmt *stmthp;
    OCIDuration dur;
    OCIType *xmltdo;
  
    xmldocnode  *doc;
    xmlnodelist *item_list; ub4 ilist_l;
    ocixmldbparam params[1];
    text *sel_xml_stmt = (text *)"SELECT xml_col FROM my_table";
    ub4    xmlsize = 0;
    sword  status = 0;
    OCIDefine *defnp = (OCIDefine *) 0;

    /* Initialize envhp, svchp, errhp, dur, stmthp */
    /* ........ */

    /* Get an xml context */
    params[0].name_ocixmldbparam = XCTXINIT_OCIDUR;
    params[0].value_ocixmldbparam = &dur;
    xctx = OCIXmlDbInitXmlCtx(envhp, svchp, errhp, params, 1);

    /* Start processing */
    if(status = OCITypeByName(envhp, errhp, svchp, (const text *) "SYS",
                   (ub4) strlen((char *)"SYS"), (const text *) "XMLTYPE",
                   (ub4) strlen((char *)"XMLTYPE"), (CONST text *) 0,
                   (ub4) 0, dur, OCI_TYPEGET_HEADER,
                   (OCIType **) xmltdo_p)) {
       return OCI_ERROR;
    }

    if(!(*xmltdo_p)) {
       printf("NULL tdo returned\n");
       return OCI_ERROR;
    }

    if(status = OCIStmtPrepare(stmthp, errhp, (OraText *)selstmt,
                    (ub4)strlen((char *)selstmt),
                    (ub4) OCI_NTV_SYNTAX, (ub4) OCI_DEFAULT)) {
      return OCI_ERROR;
    }

    if(status = OCIDefineByPos(stmthp, &defnp, errhp, (ub4) 1, (dvoid *) 0,
                   (sb4) 0, SQLT_NTY, (dvoid *) 0, (ub2 *)0,
                   (ub2 *)0, (ub4) OCI_DEFAULT)) {
       return OCI_ERROR;
    }

    if(status = OCIDefineObject(defnp, errhp, (OCIType *) *xmltdo_p,
                            (dvoid **) &doc,
                            &xmlsize, (dvoid **) 0, (ub4 *) 0)) {
      return OCI_ERROR;
    }

    if(status = OCIStmtExecute(svchp, stmthp, errhp, (ub4) 1, (ub4) 0,
                 (CONST OCISnapshot*) 0, (OCISnapshot*) 0, (ub4) OCI_DEFAULT)) {
      return OCI_ERROR;
    }

    /* We have the doc. Now we can operate on it */
    printf("Getting Item list...\n");

   item_list = XmlDomGetElemsByTag(xctx,(xmlelemnode *) elem,(oratext *)"Item"); 
    ilist_l   = XmlDomGetNodeListLength(xctx, item_list);
    printf(" Item list length = %d \n", ilist_l);

    for (i = 0; i < ilist_l; i++)
    {
      elem = XmlDomGetNodeListItem(xctx, item_list, i);
      printf("Elem Name:%s\n", XmlDomGetNodeName(xctx, fragelem));
      XmlDomRemoveChild(xctx, fragelem);
    }

    XmlSaveDom(xctx, &err, doc, "stdio", stdout, NULL);

   /* free xml ctx */
   OCIXmlDbFreeXmlCtx(xctx);

   return OCI_SUCCESS;
}