12.2 Decoder as Recursive Descent Parser

The entry point to the decoder is

  struct zx_root_s* zx_DEC_root(struct zx_dec_ctx* c,
                                struct zx_ns_s* dummy,
                                int n_decode);

The decoding context holds pointer to the raw data and must be initialized prior to calling the decoder. The third argument specifies how many recognized elements are decoded before returning. Usually you would specify 1 to consume one top level element from the stream.

The returned data structure, struct zx_root_s, contains one pointer for each type of top level element that can be recognized. The tok field of the returned value identifies the last top level element recognized and can be used to dispatch to correct request handler:

  zx_prepare_dec_ctx(c, TPF_ns_tab, start_ptr, end_ptr);
  struct TPF_root_s* x = TPF_DEC_root(c, 0, 1);
  switch (x->gg.g.tok) {
  case TPF_NS_EEE_ELEM: return process_EEE_req(x->NN_EEE);
  }

When processing responses, it is generally already known which type of response you are expecting, so you can simply check for NULLness of the respective pointer in the returned data structure.

Internally zx_DEC_root() works much the same way: it scans a beginning of an element from the stream, looks up the token number corresponding to the element name, and switches on that, calling element specific decoder functions (see next section) to do the detailed processing.

In the above code fragment, you should note the call to zx_prepare_dec_ctx() which initializes the decoder machinery. It takes ns_tab argument, which specifies which namespaces will be recognized. This table MUST match the TPF_DEC_root() function you call (i.e. both must have been generated as part of the same xsd2sg.pl invocation). The other arguments are the start of the buffer to decode and pointer one past the end of the buffer to decode.