SWI-Prolog -- Manual

tantiation error if the conversion fails.

12.4.16 Miscellaneous

12.4.16.1 Term Comparison

int PL_compare(term_t t1, term_t t2): Compares two terms using the standard order of terms and returns -1, 0 or 1. See also compare/3.
int PL_same_compound(term_t t1, term_t t2): Yields TRUE if t1 and t2 refer to physically the same compound term and FALSE otherwise.

12.4.16.2 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.
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.
int 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 to make it possible to store Prolog terms fast and compact 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)

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_external() is called on it.

It is allowed to copy the data and use PL_recorded_external() on the copy. The user is responsible for the memory management of the copy. After copying, the original may be discarded using PL_erase_external().

PL_recorded_external() is used to copy such recorded terms back to the Prolog stack.

int 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().

int PL_erase_external(char *record)

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

12.4.16.3 Database

int PL_assert(term_t t, module_t m, int flags)

Provides direct access to asserta/1 and assertz/1 by asserting t into the database in the module m. Defined flags are:

PL_ASSERTZ: Add the new clause as last. Calls assertz/1. This macros is defined as 0 and thus the default.
PL_ASSERTA: Add the new clause as first. Calls asserta/1.
PL_CREATE_THREAD_LOCAL: If the predicate is not defined, create it as thread-local. See thread_local/1.
PL_CREATE_INCREMENTAL: If the predicate is not defined, create it as incremental see table/1 and section 7.7.

On success this function returns TRUE. On failure FALSE is returned and an exception is left in the environment that describes the reason of failure. See PL_exception().

This predicate bypasses creating a Prolog callback environment and is faster than setting up a call to assertz/1. It may be used together with PL_chars_to_term(), but the typical use case will create a number of clauses for the same predicate. The fastest way to achieve this is by creating a term that represents the invariable structure of the desired clauses using variables for the variable sub terms. Now we can loop over the data, binding the variables, asserting the term and undoing the bindings. Below is an example loading words from a file that contains a word per line.

#include <SWI-Prolog.h>
#include <stdio.h>
#include <string.h>

#define MAXWLEN 256

static foreign_t
load_words(term_t name)
{ char *fn;

  if ( PL_get_file_name(name, &fn, PL_FILE_READ) )
  { FILE *fd = fopen(fn, "r");
    char word[MAXWLEN];
    module_t m = PL_new_module(PL_new_atom("words"));
    term_t cl = PL_new_term_ref();
    term_t w  = PL_new_term_ref();
    fid_t fid;

    if ( !PL_unify_term(cl, PL_FUNCTOR_CHARS, "word", 1, PL_TERM, w) )
      return FALSE;

    if ( (fid = PL_open_foreign_frame()) )
    { while(fgets(word, sizeof(word), fd))
      { size_t len;

        if ( (len=strlen(word)) )
        { word[len-1] = '\0';
          if ( !PL_unify_chars(w, PL_ATOM|REP_MB, (size_t)-1, word) ||
               !PL_assert(cl, m, 0) )
            return FALSE;
          PL_rewind_foreign_frame(fid);
        }
      }

      PL_close_foreign_frame(fid);
    }

    fclose(fd);
    return TRUE;
  }

  return FALSE;
}

install_t
install(void)
{ PL_register_foreign("load_words", 1, load_words, 0);
}

12.4.16.4 Getting file names

The function PL_get_file_name() provides access to Prolog filenames and its file-search mechanism described with absolute_file_name/3. Its existence is motivated to realise a uniform interface to deal with file properties, search, naming conventions, etc., from foreign code.

int PL_get_file_name(term_t spec, char **name, int flags)

Translate a Prolog term into a file name. The name is stored in the buffer stack described with the PL_get_chars() option BUF_STACK. Conversion from the internal UNICODE encoding is done using standard C library functions. flags is a bit-mask controlling the conversion process. Options are:

PL_FILE_ABSOLUTE: Return an absolute path to the requested file.
PL_FILE_OSPATH: Return the name using the hosting OS conventions. On MS-Windows, \ is used to separate directories rather than the canonical /.
PL_FILE_SEARCH: Invoke absolute_file_name/3. This implies rules from file_search_path/2 are used.
PL_FILE_EXIST: Demand the path to refer to an existing entity.
PL_FILE_READ: Demand read-access on the result.
PL_FILE_WRITE: Demand write-access on the result.
PL_FILE_EXECUTE: Demand execute-access on the result.
PL_FILE_NOERRORS: Do not raise any exceptions.

int PL_get_file_nameW(term_t spec, wchar_t **name, int flags)

Same as PL_get_file_name(), but returns the filen