STATIC mpz_t *
mpz_from_sv_nofail (SV *sv)
{
  MAGIC *mg;

  if (!sv_derived_from(sv, "Math::BigInt::GMP"))
    croak("not of type Math::BigInt::GMP");

  for (mg = SvMAGIC(SvRV(sv)); mg; mg = mg->mg_moremagic) {
    if (mg->mg_type == PERL_MAGIC_ext
#if GMP_HAS_MAGICEXT
        && mg->mg_virtual == &vtbl_gmp
#endif
        ) {
#if GMP_HAS_MAGICEXT
      return (mpz_t *)mg->mg_ptr;
#else
      return INT2PTR(mpz_t *, SvIV((SV *)mg->mg_ptr));
#endif
    }
  }

  return (mpz_t *)NULL;
}

STATIC mpz_t *
mpz_from_sv (SV *sv)
{
  mpz_t *mpz;

  if (!(mpz = mpz_from_sv_nofail(sv)))
    croak("failed to fetch mpz pointer");

  return mpz;
}