cleanup()">PL_cleanup() the return value is guaranteed to be NULL. During normal execution it may return the content of a newly allocated blob that reused the released handle.

12.4.9 Exchanging GMP numbers

If SWI-Prolog is linked with the GNU Multiple Precision Arithmetic Library (GMP, used by default), the foreign interface provides functions for exchanging numeric values to GMP types. To access these functions the header <gmp.h> must be included before <SWI-Prolog.h>. Foreign code using GMP linked to SWI-Prolog asks for some considerations.

• SWI-Prolog normally rebinds the GMP allocation functions using mp_set_memory_functions(). This means SWI-Prolog must be initialised before the foreign code touches any GMP function. You can call PL_action(PL_GMP_SET_ALLOC_FUNCTIONS, TRUE) to force Prolog's GMP initialization without doing the rest of the Prolog initialization. If you do not want Prolog rebinding the GMP allocation, call PL_action(PL_GMP_SET_ALLOC_FUNCTIONS, FALSE) before initializing Prolog.

• On Windows, each DLL has its own memory pool. To make exchange of GMP numbers between Prolog and foreign code possible you must either let Prolog rebind the allocation functions (default) or you must recompile SWI-Prolog to link to a DLL version of the GMP library.

Here is an example exploiting the function mpz_nextprime():

#include <gmp.h>
#include <SWI-Prolog.h>

static foreign_t
next_prime(term_t n, term_t prime)
{ mpz_t mpz;
  int rc;

  mpz_init(mpz);
  if ( PL_get_mpz(n, mpz) )
  { mpz_nextprime(mpz, mpz);

    rc = PL_unify_mpz(prime, mpz);
  } else
    rc = FALSE;

  mpz_clear(mpz);
  return rc;
}

install_t
install()
{ PL_register_foreign("next_prime", 2, next_prime, 0);
}

int PL_get_mpz(term_t t, mpz_t mpz)

If t represents an integer, mpz is filled with the value and the function returns TRUE. Otherwise mpz is untouched and the function returns FALSE. Note that mpz must have been initialised before calling this function and must be cleared using mpz_clear() to reclaim any storage associated with it.

int PL_get_mpq(term_t t, mpq_t mpq)

If t is an integer or rational number (term rdiv/2), mpq is filled with the normalised rational number and the function returns TRUE. Otherwise mpq is untouched and the function returns FALSE. Note that mpq must have been initialised before calling this function and must be cleared using mpq_clear() to reclaim any storage associated with it.

int PL_unify_mpz(term_t t, mpz_t mpz)

Unify t with the integer value represented by mpz and return TRUE on success. The mpz argument is not changed.

int PL_unify_mpq(term_t t, mpq_t mpq)

Unify t with a rational numb