12.4.18.2 Recorded database

In some applications it is useful to store and retrieve Prolog terms from C code. For example, the XPCE graphical environment does this for storing arbitrary Prolog data as slot-data of XPCE objects.

Please note that the returned handles have no meaning at the Prolog level and the recorded terms are not visible from Prolog. The functions PL_recorded() and PL_erase() are the only functions that can operate on the stored term.

Two groups of functions are provided. The first group (PL_record() and friends) store Prolog terms on the Prolog heap for retrieval during the same session. These functions are also used by recorda/3 and friends. The recorded database may be used to communicate Prolog terms between threads.

record_t PL_record(term_t +t)

Record the term t into the Prolog database as recorda/3 and return an opaque handle to the term. The returned handle remains valid until PL_erase() is called on it. PL_recorded() is used to copy recorded terms back to the Prolog stack. Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return (record_t)0.

record_t PL_duplicate_record(record_t record)

Return a duplicate of record. As records are read-only objects this function merely increments the records reference count. Returns (record_t)0 if the record is an external record (see PL_record_external()).

bool PL_recorded(record_t record, term_t -t)

Copy a recorded term back to the Prolog stack. The same record may be used to copy multiple instances at any time to the Prolog stack. Returns TRUE on success, and FALSE if there is not enough space on the stack to accommodate the term. See also PL_record() and PL_erase().

void PL_erase(record_t record)

Remove the recorded term from the Prolog database, reclaiming all associated memory resources.

The second group (headed by PL_record_external()) provides the same functionality, but the returned data has properties that enable storing the data on an external device. It has been designed for fast and compact storage of Prolog terms in an external database. Here are the main features:

• Independent of session
  Records can be communicated to another Prolog session and made visible using PL_recorded_external().
• Binary
  The representation is binary for maximum performance. The returned data may contain zero bytes.
• Byte-order independent
  The representation can be transferred between machines with different byte order.
• No alignment restrictions
  There are no memory alignment restrictions and copies of the record can thus be moved freely. For example, it is possible to use this representation to exchange terms using shared memory between different Prolog processes.
• Compact
  It is assumed that a smaller memory footprint will eventually outperform slightly faster representations.
• Stable
  The format is designed for future enhancements without breaking compatibility with older records.

char * PL_record_external(term_t +t, size_t *len)

Similar to PL_record(), but the term is serialized such that it can be reloaded in another Prolog session. This implies that atoms and functors are stored by their content rather than their handle. As a result, PL_record_external() fails (returning NULL if the term contains blobs that cannot be serialized, such as streams.

These functions are used to implement library library(fastrw) as well as for storing Prolog terms in external databases such as BerkeleyDB (library library(bdb)) or RocksDB. The representation is optimized for plain atoms and numbers.

Records that are used only in the same Prolog process should use PL_record() as this can represent any term, is more compact and faster.

The returned string may be copied. Note that the string may contain null bytes and is not null terminated. The length in bytes is returned in len. After copying, the returned string may be discarded using PL_erase_external().

PL_recorded_external() is used to copy the term represented in the data back to the Prolog stack. PL_recorded_external() can be used on the returned string as well as on a copy.

bool PL_recorded_external(const char *record, term_t -t)

Copy a recorded term back to the Prolog stack. The same record may be used to copy multiple instances at any time to the Prolog stack. See also PL_record_external() and PL_erase_external().

bool PL_erase_external(char *record)

Remove the recorded term from the Prolog database, reclaiming all associated memory resources.