# Parsing syntaxes to RDF Triples

## Introduction

The typical sequence of operations to parse is to create a parser object, set various callback and features, start the parsing, send some syntax content to the parser object, finish the parsing and destroy the parser object.

Several parts of this process are optional, including actually using the triple results, which is useful as a syntax checking process.

## Create the Parser object

The parser can be created directly from a known name such as `rdfxml` for the W3C Recommendation RDF/XML syntax:

      raptor_parser* rdf_parser;

      rdf_parser = raptor_new_parser("rdfxml");

or the name can be discovered from an enumeration as discussed in Querying Functionality

The parser can also be created by identifying the syntax by a URI, specifying the syntax by a MIME Type, providng an identifier for the content such as filename or URI string or giving some initial content bytes that can be used to guess. Using the `raptor_new_parser_for_content()` function, all of these can be given as optional parameters, using NULL or 0 for undefined parameters. The constructor will then use as much of this information as possible.

      raptor_parser* rdf_parser;

Create a parser that reads the MIME Type for RDF/XML `application/rdf+xml`

      rdf_parser = raptor_new_parser_for_content(NULL, "application/rdf+xml", NULL, 0, NULL);

Create a parser that can read a syntax identified by the URI for Turtle `http://www.dajobe.org/2004/01/turtle/`, which has no registered MIME Type at this date:

      syntax_uri = raptor_new_uri("http://www.dajobe.org/2004/01/turtle/");
      rdf_parser = raptor_new_parser_for_content(syntax_uri, NULL, NULL, 0, NULL);

Create a parser that recognises the identifier `foo.rss`:

      rdf_parser = raptor_new_parser_for_content(NULL, NULL, NULL, 0, "foo.rss");

Create a parser that recognises the content in buffer:

      rdf_parser = raptor_new_parser_for_content(NULL, NULL, buffer, len, NULL);

Any of the constructor calls can return NULL if no matching parser could be found, or the construction failed in another way.

## Parser features

There are several options that can be set on parsers, called features. The exact list of features can be found via Querying Functionality or in the API reference for `raptor_set_feature()`. (This should be properly called `raptor_parser_set_feature()` as it only applies to `raptor_parser` objects).

Features are integer enumerations of the raptor_feature enum and have values that are either integers (often acting as booleans) or strings. The two functions that set features are:

      /* Set an integer (or boolean) valued feature */
      raptor_set_feature(rdf_parser, feature, 1);

      /* Set a string valued feature */
      raptor_set_feature_string(rdf_parser, feature, "abc");

There are also two corresponding functions for reading the values of parser features: `raptor_get_feature()` and `raptor_get_feature_string()` taken the feature enumeration parameter and returning the integer or string value correspondingly.

## Set RDF triple callback handler

The main reason to parse a syntax is to get RDF triples returned and this is done by a callback function which is called with parameters of a user data pointer and the triple itself. The handler is set with `raptor_set_statement_handler()` as follows:

      void
      triples_handler(void* user_data, const raptor_statement* triple)
      {
        /* do something with the triple */
      }

      raptor_set_statement_handler(rdf_parser, user_data, triples_handler);

It is optional to set a handler function for triples, which does have some uses if just counting triples or validating a syntax.

## Set fatal error, error and warning handlers

There are several other callback handlers that can be set on parsers. These can be set any time before parsing is called. Errors and warnings from parsing can be returned with functions that all take a callback of type raptor_message_handler and signature:

    void
    message_handler(void *user_data, raptor_locator* locator,
                    const char *message)
    {
      /* do something with the message */
    }

returning the user data given, associated location information as a raptor_locator and the error/warning message itself. The locator structure contains full information on the details of where in the file or URI the message occurred.

The fatal error, error and warning handlers are all set with similar functions that take a handler as follows:

      raptor_set_fatal_error_handler(rdf_parser, user_data, fatal_handler);

      raptor_set_error_handler(rdf_parser, user_data, error_handler);

      raptor_set_warning_handler(rdf_parser, user_data, warning_handler);

The program will terminate with `abort()` if the fatal error handler returns.

## Set the identifier creator handler

Identifiers are created in some parsers by generating them automatically or via hints given a syntax. Raptor can customise this process using a user-supplied identifier handler function. For example, in RDF/XML generated blank node identifiers and those those specified `rdf:nodeID` are passed through this process. Setting a handler allows the identifier generation mechanism to be fully replaced. A lighter alternative is to use `raptor_set_default_generate_id_parameters()` to adjust the default algorithm for generated identifiers.

It is used as follows

      raptor_generate_id_handler id_handler;

      raptor_set_generate_id_handler(rdf_parser, user_data, id_handler);

The id_handler takes the following signature:

    unsigned char*
    generate_id_handler(void* user_data, raptor_genid_type type,
                        unsigned char* user_id) {
       /* return a new generated ID based on user_id (optional) */
    }

where the raptor_genid_type provides extra information on the identifier being created and user_id an optional user-supplied identifier, such as the value of a `rdf:nodeID` in RDF/XML.

## Set namespace declared handler

Raptor can report when namespace prefix/URIs are declared in during parsing a syntax such as those in XML, RDF/XML or Turtle. A handler function can be set to receive these declarations using the namespace handler method.

      raptor_namespace_handler namespaces_handler;

      raptor_set_namespace_handler(rdf_parser, user_data, namespaces_handler);

The namespaces_handler takes the following signature:

    void
    namespaces_handler(void* user_data, raptor_namespace *nspace) {
      /*  */
    }

This may be called multiple times with the same namespace, if the namespace is declared inside different XML sub-trees.

## Set the parsing strictness

`raptor_set_parser_strict()` allows setting of the parser strictness flag. The default is lax parsing, accepting older or deprecated syntax forms but may generate a warning. Setting to non-0 (true) will cause parser errors to be generated in these cases.

## Provide syntax content to parse

The operation of turning syntax into RDF triples has several alternatives from functions that do most of the work starting from a URI to functions that allow passing in data buffers.

Parsing and MIME Types

The mime type of the retrieved content is not used to choose a parser unless the parser is of type `guess`. The guess parser will send an `Accept:` header for all known parser syntax mime types (if a URI request is made) and based on the response, including the identifiers used, pick the appropriate parser to execute. See `raptor_guess_parser_name()` for a full discussion of the inputs to the guessing.

### Parse the content from a URI (`raptor_parse_uri()`)

The URI is resolved and the content read from it and passed to the parser:

      raptor_parse_uri(rdf_parser, uri, base_uri);

The base_uri is optional (can be `NULL`) and will default to the uri.

### Parse the content of a URI using an existing WWW connection (`raptor_parse_uri_with_connection()`)

The URI is resolved using an existing WWW connection (for example a libcurl CURL handle) to allow for any existing WWW configuration to be reused. See `raptor_www_new_with_connection` for full details of how this works. The content is then read from the result of resolving the URI:

      raptor_parse_uri_with_connection(rdf_parser, uri, base_uri, connection);

The base_uri is optional (can be `NULL`) and will default to the uri.

### Parse the content of a C `FILE*` (`raptor_parse_file_stream()`)

Parsing can read from a C STDIO file handle:

      stream=fopen(filename, "rb");
      raptor_parse_file_stream(rdf_parser, stream, filename, base_uri);
      fclose(stream);

This function can use take an optional filename which is used in locator error messages. The base_uri may be required by some parsers and if `NULL` will cause the parsing to fail.

### Parse the content of a file URI (`raptor_parse_file()`)

Parsing can read from a URI known to be a `file:` URI:

      raptor_parse_file(rdf_parser, file_uri, base_uri);

This function requires that the file_uri is a file URI, that is `raptor_uri_uri_string_is_file_uri( raptor_uri_as_string( file_uri) )` must be true. The base_uri may be required by some parsers and if `NULL` will cause the parsing to fail.

### Parse chunks of syntax content provided by the application (`raptor_start_parse()` and `raptor_parse_chunk()`)

      raptor_start_parse(rdf_parser, base_uri);
      while(/* not finished getting content */) {
        unsigned char *buffer;
        size_t buffer_len;
        /* obtain some syntax content in buffer of size buffer_len bytes */
        raptor_parse_chunk(rdf_parser, buffer, buffer_len, 0);
      }
      raptor_parse_chunk(rdf_parser, NULL, 0, 1); /* no data and is_end = 1 */

The base_uri argument to `raptor_start_parse()` may be required by some parsers and if `NULL` will cause the parsing to fail.

On the last `raptor_parse_chunk()` call, or after the loop is ended, the `is_end` parameter must be set to non-0. Content can be passed with the final call. If no content is present at the end (such as in some kind of end of file situation), then a 0-length buffer_len or NULL buffer can be used.

The minimal case is an entire parse in one chunk as follows:

      raptor_start_parse(rdf_parser, base_uri);
      raptor_parse_chunk(rdf_parser, buffer, buffer_len, 1); /* is_end = 1 */

## Restrict parser network access

Parsing can cause network requests to be performed, especially if a URI is given as an argument such as with `raptor_parse_uri()` however there may also be indirect requests such as with the GRDDL parser that retrieves URIs depending on the results of initial parse requests. The URIs requested may not be wanted to be fetched or need to be filtered, and this can be done in three ways.

### Filtering parser network requests with feature `RAPTOR_FEATURE_NO_NET`

The parser feature `RAPTOR_FEATURE_NO_NET` can be set with `raptor_set_feature()` and forbids all network requests. There is no customisation of this approach.

      rdf_parser = raptor_new_parser("rdfxml");
      raptor_set_feature(rdf_parser, RAPTOR_FEATURE_NO_NET);

### Filtering parser network requests with `raptor_www_set_uri_filter()`

The `raptor_www_set_uri_filter()` allows setting of a filtering function to operate on all URIs retrieved by a WWW connection. This connection can be used in parsing when operated by hand.

    void write_bytes_handler(raptor_www* www, void *user_data,
                             const void *ptr, size_t size, size_t nmemb) {
    {
      raptor_parser* rdf_parser=(raptor_parser*)user_data;
      raptor_parse_chunk(rdf_parser, (unsigned char*)ptr, size*nmemb, 0);
    }

    int uri_filter(void* filter_user_data, raptor_uri* uri) {
      /* return non-0 to forbid the request */
    }

    int main(int argc, char *argv[]) {
      ...

      rdf_parser = raptor_new_parser("rdfxml");
      www = raptor_new_www();

      /* filter all URI requests */
      raptor_www_set_uri_filter(www, uri_filter, filter_user_data);

      /* make WWW write bytes to parser */
      raptor_www_set_write_bytes_handler(www, write_bytes_handler, rdf_parser);

      raptor_start_parse(rdf_parser, uri);
      raptor_www_fetch(www, uri);
      /* tell the parser that we are done */
      raptor_parse_chunk(rdf_parser, NULL, 0, 1);

      raptor_www_free(www);
      raptor_free_parser(rdf_parser);

      ...
    }

### Filtering parser network requests with `raptor_parser_set_uri_filter()`

The `raptor_parser_set_uri_filter()` allows setting of a filtering function to operate on all URIs that the parser sees. This operates on the internal raptor_www object used inside parsing to retrieve URIs, similar to that described in the previous section.

      int uri_filter(void* filter_user_data, raptor_uri* uri) {
        /* return non-0 to forbid the request */
      }

      rdf_parser = raptor_new_parser("rdfxml");
      raptor_parser_set_uri_filter(rdf_parser, uri_filter, filter_user_data);

      /* parse content as normal */
      raptor_parse_uri(rdf_parser, uri, base_uri);

## Querying parser static information

These methods return information about the constructed parser implementation corresponding to the information available via `raptor_syntaxes_enumerate()` for all parsers.

`raptor_get_name()` return the parser syntax name, `raptor_get_label()` the long label for the parser and `raptor_get_mime_type()` the primary MIME Type for the parser (there may be others that the parser will accept but this is the main one).

`raptor_parser_get_accept_header()` returns a string that would be sent in an HTTP request `Accept:` header for the syntaxes accepted by this parser only.

## Querying parser run-time information

`raptor_get_locator()` returns the raptor_locator for the current position in the input stream. The locator structure contains full information on the details of where in the file or URI the current parser has reached.

## Aborting parsing

`raptor_parse_abort()` allows the current parsing to be aborted, at which point no further triples will be passed to callbacks and the parser will attempt to return control to the application. This is most useful when called inside a handler function which allows the application to decide to stop an active parsing.

## Destroy the parser

To tidy up, delete the parser object as follows:

      raptor_free_parser(rdf_parser);

## Parsing example code

`rdfprint.c`: Parse an RDF/XML file and print the triples

Compile it like this:

    $ gcc -o rdfprint rdfprint.c `raptor-config --cflags` `raptor-config --libs`

and run it on an RDF file as:

    $ ./rdfprint raptor.rdf
    _:genid1 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://usefulinc.com/ns/doap#Project> .
    _:genid1 <http://usefulinc.com/ns/doap#name> "Raptor" .
    _:genid1 <http://usefulinc.com/ns/doap#homepage> <http://librdf.org/raptor/> .
    ...