RDF Triple (librdf_statement)
3
REDLAND Library

# RDF Triple (librdf_statement)

RDF Triple.

## Synopsis

    typedef             librdf_statement;
    enum                librdf_statement_part;
    librdf_statement*   librdf_new_statement                (librdf_world *world);
    librdf_statement*   librdf_new_statement_from_statement (librdf_statement *statement);
    librdf_statement*   librdf_new_statement_from_nodes     (librdf_world *world,
                                                             librdf_node *subject,
                                                             librdf_node *predicate,
                                                             librdf_node *object);
    void                librdf_statement_init               (librdf_world *world,
                                                             librdf_statement *statement);
    void                librdf_statement_clear              (librdf_statement *statement);
    void                librdf_free_statement               (librdf_statement *statement);
    librdf_node*        librdf_statement_get_subject        (librdf_statement *statement);
    void                librdf_statement_set_subject        (librdf_statement *statement,
                                                             librdf_node *node);
    librdf_node*        librdf_statement_get_predicate      (librdf_statement *statement);
    void                librdf_statement_set_predicate      (librdf_statement *statement,
                                                             librdf_node *node);
    librdf_node*        librdf_statement_get_object         (librdf_statement *statement);
    void                librdf_statement_set_object         (librdf_statement *statement,
                                                             librdf_node *node);
    int                 librdf_statement_is_complete        (librdf_statement *statement);
    unsigned char*      librdf_statement_to_string          (librdf_statement *statement);
    void                librdf_statement_print              (librdf_statement *statement,
                                                             FILE *fh);
    int                 librdf_statement_equals             (librdf_statement *statement1,
                                                             librdf_statement *statement2);
    int                 librdf_statement_match              (librdf_statement *statement,
                                                             librdf_statement *partial_statement);
    size_t              librdf_statement_encode             (librdf_statement *statement,
                                                             unsigned char *buffer,
                                                             size_t length);
    size_t              librdf_statement_encode_parts       (librdf_statement *statement,
                                                             librdf_node *context_node,
                                                             unsigned char *buffer,
                                                             size_t length,
                                                             librdf_statement_part fields);
    size_t              librdf_statement_decode             (librdf_statement *statement,
                                                             unsigned char *buffer,
                                                             size_t length);
    size_t              librdf_statement_decode_parts       (librdf_statement *statement,
                                                             librdf_node **context_node,
                                                             unsigned char *buffer,
                                                             size_t length);

## Description

An object representing an RDF Triple of three RDF terms (librdf_node). Triples can be created, accessed, destroyed and en/decoded into a binary form for use in storage.

## Details

### librdf_statement

    typedef struct librdf_statement_s librdf_statement;

Redland statement class.

### enum librdf_statement_part

    typedef enum {
      LIBRDF_STATEMENT_SUBJECT   = 1 << 0,
      LIBRDF_STATEMENT_PREDICATE = 1 << 1,
      LIBRDF_STATEMENT_OBJECT    = 1 << 2,

      /* must be a combination of all of the above */
      LIBRDF_STATEMENT_ALL       = (LIBRDF_STATEMENT_SUBJECT|
                                    LIBRDF_STATEMENT_PREDICATE|
                                    LIBRDF_STATEMENT_OBJECT)
    } librdf_statement_part;

Flags that are or-ed to indicate statement parts.

Used in fields arguments to methods such as the public `librdf_statement_encode_parts()` `librdf_statement_decode_parts()` `librdf_new_stream_from_node_iterator()` and the internal `librdf_storage_node_stream_to_node_create()`

`LIBRDF_STATEMENT_SUBJECT`

  Subject of a statement.

`LIBRDF_STATEMENT_PREDICATE`

  Predicate of a statement.

`LIBRDF_STATEMENT_OBJECT`

  Object of a statement.

`LIBRDF_STATEMENT_ALL`

  All parts of a statement.

### librdf_new_statement ()

    librdf_statement*   librdf_new_statement                (librdf_world *world);

Constructor - create a new empty librdf_statement.

`world` :

  redland world object

Returns :

  a new librdf_statement or NULL on failure

### librdf_new_statement_from_statement ()

    librdf_statement*   librdf_new_statement_from_statement (librdf_statement *statement);

Copy constructor - create a new librdf_statement from an existing librdf_statement.

`statement` :

  librdf_statement to copy

Returns :

  a new librdf_statement with copy or NULL on failure

### librdf_new_statement_from_nodes ()

    librdf_statement*   librdf_new_statement_from_nodes     (librdf_world *world,
                                                             librdf_node *subject,
                                                             librdf_node *predicate,
                                                             librdf_node *object);

Constructor - create a new librdf_statement from existing librdf_node objects.

The node objects become owned by the new statement (or freed on error).

`world` :

  redland world object

`subject` :

  librdf_node

`predicate` :

  librdf_node

`object` :

  librdf_node

Returns :

  a new librdf_statement with copy or NULL on failure

### librdf_statement_init ()

    void                librdf_statement_init               (librdf_world *world,
                                                             librdf_statement *statement);

Initialise a statically declared librdf_statement.

This MUST be called on a statically declared librdf_statement to initialise it properly. It is the responsibility of the user of the statically allocated librdf_statement to deal with deallocation of any statement parts (subject, predicate, object).

`world` :

  redland world object

`statement` :

  librdf_statement object

### librdf_statement_clear ()

    void                librdf_statement_clear              (librdf_statement *statement);

Empty a librdf_statement of nodes.

`statement` :

  librdf_statement object

### librdf_free_statement ()

    void                librdf_free_statement               (librdf_statement *statement);

Destructor - destroy a librdf_statement.

`statement` :

  librdf_statement object

### librdf_statement_get_subject ()

    librdf_node*        librdf_statement_get_subject        (librdf_statement *statement);

Get the statement subject.

This method returns a SHARED pointer to the subject which must be copied by the caller if needed.

`statement` :

  librdf_statement object

Returns :

  a pointer to the librdf_node of the statement subject -

### librdf_statement_set_subject ()

    void                librdf_statement_set_subject        (librdf_statement *statement,
                                                             librdf_node *node);

Set the statement subject.

The subject passed in becomes owned by the statement object and must not be used by the caller after this call.

`statement` :

  librdf_statement object

`node` :

  librdf_node of subject

### librdf_statement_get_predicate ()

    librdf_node*        librdf_statement_get_predicate      (librdf_statement *statement);

Get the statement predicate.

This method returns a SHARED pointer to the predicate which must be copied by the caller if needed.

`statement` :

  librdf_statement object

Returns :

  a pointer to the librdf_node of the statement predicate -

### librdf_statement_set_predicate ()

    void                librdf_statement_set_predicate      (librdf_statement *statement,
                                                             librdf_node *node);

Set the statement predicate.

The predicate passed in becomes owned by the statement object and must not be used by the caller after this call.

`statement` :

  librdf_statement object

`node` :

  librdf_node of predicate

### librdf_statement_get_object ()

    librdf_node*        librdf_statement_get_object         (librdf_statement *statement);

Get the statement object.

This method returns a SHARED pointer to the object which must be copied by the caller if needed.

`statement` :

  librdf_statement object

Returns :

  a pointer to the librdf_node of the statement object -

### librdf_statement_set_object ()

    void                librdf_statement_set_object         (librdf_statement *statement,
                                                             librdf_node *node);

Set the statement object.

The object passed in becomes owned by the statement object and must not be used by the caller after this call.

`statement` :

  librdf_statement object

`node` :

  librdf_node of object

### librdf_statement_is_complete ()

    int                 librdf_statement_is_complete        (librdf_statement *statement);

Check if statement is a complete and legal RDF triple.

Checks that all subject, predicate, object fields are present and they have the allowed node types.

`statement` :

  librdf_statement object

Returns :

  non 0 if the statement is complete and legal

### librdf_statement_to_string ()

    unsigned char*      librdf_statement_to_string          (librdf_statement *statement);

Format the librdf_statement as a string.

Formats the statement as a newly allocate string that must be freed by the caller.

`statement` :

  the statement

Returns :

  the string or NULL on failure.

### librdf_statement_print ()

    void                librdf_statement_print              (librdf_statement *statement,
                                                             FILE *fh);

Pretty print the statement to a file descriptor.

This method is for debugging and the format of the output should not be relied on.

`statement` :

  the statement

`fh` :

  file handle

### librdf_statement_equals ()

    int                 librdf_statement_equals             (librdf_statement *statement1,
                                                             librdf_statement *statement2);

Check if two statements are equal.

`statement1` :

  first librdf_statement

`statement2` :

  second librdf_statement

Returns :

  non 0 if statements are equal

### librdf_statement_match ()

    int                 librdf_statement_match              (librdf_statement *statement,
                                                             librdf_statement *partial_statement);

Match a statement against a 'partial' statement.

A partial statement is where some parts of the statement - subject, predicate or object can be empty (NULL). Empty parts match against any value, parts with values must match exactly. Node matching is done via `librdf_node_equals()`

`statement` :

  statement

`partial_statement` :

  statement with possible empty parts

Returns :

  non 0 on match

### librdf_statement_encode ()

    size_t              librdf_statement_encode             (librdf_statement *statement,
                                                             unsigned char *buffer,
                                                             size_t length);

Serialise a statement into a buffer.

Encodes the given statement in the buffer, which must be of sufficient size. If buffer is NULL, no work is done but the size of buffer required is returned.

`statement` :

  the statement to serialise

`buffer` :

  the buffer to use

`length` :

  buffer size

Returns :

  the number of bytes written or 0 on failure.

### librdf_statement_encode_parts ()

    size_t              librdf_statement_encode_parts       (librdf_statement *statement,
                                                             librdf_node *context_node,
                                                             unsigned char *buffer,
                                                             size_t length,
                                                             librdf_statement_part fields);

Serialise parts of a statement into a buffer.

Encodes the given statement in the buffer, which must be of sufficient size. If buffer is NULL, no work is done but the size of buffer required is returned.

The fields values are or-ed combinations of: LIBRDF_STATEMENT_SUBJECT LIBRDF_STATEMENT_PREDICATE LIBRDF_STATEMENT_OBJECT or LIBRDF_STATEMENT_ALL for subject,prdicate,object fields

If context_node is given, it is encoded also

`statement` :

  statement to serialise

`context_node` :

  librdf_node context node (can be NULL)

`buffer` :

  the buffer to use

`length` :

  buffer size

`fields` :

  fields to encode

Returns :

  the number of bytes written or 0 on failure.

### librdf_statement_decode ()

    size_t              librdf_statement_decode             (librdf_statement *statement,
                                                             unsigned char *buffer,
                                                             size_t length);

Decodes a statement from a buffer.

Decodes the serialised statement (as created by `librdf_statement_encode()` ) from the given buffer.

`statement` :

  the statement to deserialise into

`buffer` :

  the buffer to use

`length` :

  buffer size

Returns :

  number of bytes used or 0 on failure (bad encoding, allocation failure)

### librdf_statement_decode_parts ()

    size_t              librdf_statement_decode_parts       (librdf_statement *statement,
                                                             librdf_node **context_node,
                                                             unsigned char *buffer,
                                                             size_t length);

Decodes a statement + context node from a buffer.

Decodes the serialised statement (as created by `librdf_statement_encode()` ) from the given buffer. If a context node is found and context_node is not NULL, a pointer to the new librdf_node is stored in *context_node.

`statement` :

  the statement to deserialise into

`context_node` :

  pointer to librdf_node context_node to deserialise into

`buffer` :

  the buffer to use

`length` :

  buffer size

Returns :

  number of bytes used or 0 on failure (bad encoding, allocation failure)