Creating a parser in C

Before you start

Ensure that you have read and understood the following topics:

A loadable implementation library, or a LIL, is the implementation module for a C parser (or node). A LIL is a Linux or UNIX shared object or Windows dynamic link library (DLL), that does not have the file extension .dll but .lil.

The implementation functions that have to be written by the developer are listed in C parser implementation functions. The utility functions that are provided by WebSphere Message Broker to aid this process are listed in C parser utility functions.

WebSphere Message Broker provides the source for a sample user-defined parser called BipSampPluginParser.c. This is a simple pseudo-XML parser that you can use in its current state, or you can modify.

The task of writing a parser varies considerably according to the complexity of the bit stream to be parsed. Only the basic steps are described here. They are described in the following sections:
  1. Defining the parser during broker initialization
  2. Creating an instance of the parser
  3. Deleting an instance of the user-defined parser

Defining the parser during broker initialization

The user-defined parser initialization function is invoked automatically during broker initialization. The user-defined parser is responsible for:
  • Creating and naming the message parser factory that is implemented by the user-defined parser. The parser factory is a container for related parser implementations. Parser factory names must be unique within a broker.
  • Defining the supported message parser class names, and supplying a pointer to a virtual function table that contains pointers to the user-defined parser implementation functions. Parser class names must be unique within a broker.

Each LIL that implements a user-defined parser must export a function called bipGetParserFactory as its initialization function. The initialization function defines the name of the factory that the user-defined parser supports and the classes of objects, or shared object, supported by the factory.

The initialization function must also create the factory object and define the names of all parsers supported by the LIL. A factory can support any number of object classes (parsers). When a parser is defined, a list of pointers to the implementation functions for that parser is passed to the broker. If a parser of the same name already exists, the request is rejected.

For example, to define the parser:
  1. Export the bipGetParserFactory initialization function:
    void LilFactoryExportPrefix * LilFactoryExportSuffix bipGetParserFactory()
    {
  2. Declare the variables:
     CciFactory*     factoryObject;
      int             rc;
      static CPI_VFT  vftable = {CPI_VFT_DEFAULT};
  3. Initialize all the static constants:
      initParserConstants();
  4. Setup function table with pointers to parser implementation functions:
      vftable.iFpCreateContext            = cpiCreateContext;
      vftable.iFpParseBufferEncoded       = cpiParseBufferEncoded;
      vftable.iFpParseFirstChild          = cpiParseFirstChild;
      vftable.iFpParseLastChild           = cpiParseLastChild;
      vftable.iFpParsePreviousSibling     = cpiParsePreviousSibling;
      vftable.iFpParseNextSibling         = cpiParseNextSibling;
      vftable.iFpWriteBufferEncoded       = cpiWriteBufferEncoded;
      vftable.iFpDeleteContext            = cpiDeleteContext;
      vftable.iFpSetElementValue          = cpiSetElementValue;
      vftable.iFpElementValue             = cpiElementValue;
      vftable.iFpNextParserClassName      = cpiNextParserClassName;
      vftable.iFpSetNextParserClassName   = cpiSetNextParserClassName;
      vftable.iFpNextParserEncoding       = cpiNextParserEncoding;
      vftable.iFpNextParserCodedCharSetId = cpiNextParserCodedCharSetId;

The initialization function must then create a parser factory by invoking cpiCreateParserFactory. The parser classes supported by the factory are defined by calling cpiDefineParserClass. The address of the factory object (returned by cpiCreateParserFactory) must be returned to the broker as the return value from the initialization function.

For example:
  1. Create the parser factory using the cpiCreateParserFactory function:
      factoryObject = cpiCreateParserFactory(&rc, constParserFactory);
      
  2. Define the classes of message supported by the factory using the cpiDefineParserClass function:
    if (factoryObject) {
       cpiDefineParserClass(&rc, factoryObject, constPXML, &vftable);
      }
    else {
        /* Error: Unable to create parser factory */
      }
  3. Return the address of this factory object to the broker:
      return(factoryObject);
    }

Creating an instance of the parser

Whenever an instance of a user-defined parser object is created, the context creation implementation function cpiCreateContext is invoked by the message broker. This allows the user-defined parser to allocate instance data associated with the parser.

For example:
  1. Call cpiCreateContext:
    CciContext* cpiCreateContext(
      CciParser* parser
    ){
      PARSER_CONTEXT_ST *p;
    
  2. Allocate a pointer to the local context:
      p = (PARSER_CONTEXT_ST *)malloc(sizeof(PARSER_CONTEXT_ST));
    
  3. Clear the context area:
      if (p) {
         memset(p, 0, sizeof(PARSER_CONTEXT_ST));
        }
      else {
        /* Error: Unable to allocate memory */
      }
  4. Return the pointer to the local context:
      return((CciContext*)p);
    }

Deleting an instance of the user-defined parser

To delete an instance of a parser, you use the cpiDeleteContext function. For example:
void cpiDeleteContext(
  CciParser*  parser,
  CciContext* context
){
  PARSER_CONTEXT_ST* pc = (PARSER_CONTEXT_ST *)context ;
  int                rc = 0;

  return;
}