## 12.4 The Foreign Include File

### 12.4.1 Argument Passing and Control

If Prolog encounters a foreign predicate at run time it will call a function specified in the predicate definition of the foreign predicate. The arguments 1, ... , <arity> pass the Prolog arguments to the goal as Prolog terms. Foreign functions should be declared of type `foreign_t`.

All the arguments to a foreign predicate must be of type `term_t`. The only operation that is allowed with an argument to a foreign predicate is unification; for anything that might over-write the term, you must use a copy created by PL_copy_term_ref(). For an example, see PL_unify_list().

Deterministic foreign functions return with either `TRUE` (success) or `FALSE` (failure).213`SWI-Prolog.h` defines the macros `PL_succeed` and `PL_fail` to return with success or failure. These macros should be considered deprecated. The foreign function may raise an exception using PL_raise_exception() or one of the shorthands for commonly used exceptions such as PL_type_error(). Note that the C language does not provide exception handling and therefore the functions that raise an exception return (with the value `FALSE`). Functions that raise an exception must return `FALSE`.

#### 12.4.1.1 Non-deterministic Foreign Predicates

By default foreign predicates are deterministic. Using the `PL_FA_NONDETERMINISTIC` attribute (see PL_register_foreign()) it is possible to register a predicate as a non-deterministic predicate. Writing non-deterministic foreign predicates is slightly more complicated as the foreign function needs context information for generating the next solution. Note that the same foreign function should be prepared to be simultaneously active in more than one goal. Suppose the natural_number_below_n/2 is a non-deterministic foreign predicate, backtracking over all natural numbers lower than the first argument. Now consider the following predicate:

    quotient_below_n(Q, N) :-
            natural_number_below_n(N, N1),
            natural_number_below_n(N, N2),
            Q =:= N1 / N2, !.

In this predicate the function natural_number_below_n/2 simultaneously generates solutions for both its invocations.

Non-deterministic foreign functions should be prepared to handle three different calls from Prolog:

- Initial call (`PL_FIRST_CALL`)
  Prolog has just created a frame for the foreign function and asks it to produce the first answer.
- Redo call (`PL_REDO`)
  The previous invocation of the foreign function associated with the current goal indicated it was possible to backtrack. The foreign function should produce the next solution.
- Terminate call (`PL_PRUNED`)
  The choice point left by the foreign function has been destroyed by a cut or exception. The foreign function is given the opportunity to clean the environment. The context handle is the only meaningful argument -- the term arguments to the call are `(term_t)0`.

Both the context information and the type of call is provided by an argument of type `control_t` appended to the argument list for deterministic foreign functions. The macro PL_foreign_control() extracts the type of call from the control argument. The foreign function can pass a context handle using the `PL_retry*()` macros and extract the handle from the extra argument using the `PL_foreign_context*()` macro.

(return) foreign_t PL_retry(intptr_t value)
  The foreign function succeeds while leaving a choice point. On backtracking over this goal the foreign function will be called again, but the control argument now indicates it is a‘Redo' call and the macro PL_foreign_context() returns the handle passed via PL_retry(). This handle is a signed value two bits smaller than a pointer, i.e., 30 or 62 bits (two bits are used for status indication). Defined as `return _PL_retry(n)`. See also PL_succeed().
(return) foreign_t PL_retry_address(void *)
  As PL_retry(), but ensures an address as returned by malloc() is correctly recovered by PL_foreign_context_address(). Defined as `return _PL_retry_address(n)`. See also PL_succeed().
int PL_foreign_control(control_t)
  Extracts the type of call from the control argument. The return values are described above. Note that the function should be prepared to handle the `PL_PRUNED` case and should be aware that the other arguments are not valid in this case.
intptr_t PL_foreign_context(control_t)
  Extracts the context from the context argument. If the call type is `PL_FIRST_CALL` the context value is 0L. Otherwise it is the value returned by the last PL_retry() associated with this goal (both if the call type is `PL_REDO` or `PL_PRUNED`).
void * PL_foreign_context_address(control_t)
  Extracts an address as passed in by PL_retry_address().
predicate_t PL_foreign_context_predicate(control_t)

  Fetch the Prolog predicate that is executing this function. Note that if the predicate is imported, the returned predicate refers to the final definition rather than the imported predicate; i.e., the module reported by PL_predicate_info() is the module in which the predicate is defined rather than the module where it was called. See also PL_predicate_info().

Note: If a non-deterministic foreign function returns using PL_succeed() or PL_fail(), Prolog assumes the foreign function has cleaned its environment. No call with control argument `PL_PRUNED` will follow.

The code of figure 5 shows a skeleton for a non-deterministic foreign predicate definition.

    typedef struct                  /* define a context structure */
    { ...
    } context;

    foreign_t
    my_function(term_t a0, term_t a1, control_t handle)
    { struct context * ctxt;

      switch( PL_foreign_control(handle) )
      { case PL_FIRST_CALL:
            if ( !(ctxt = malloc(sizeof *ctxt)) )
              return PL_resource_error("memory");
            <initialize ctxt>
            break;
        case PL_REDO:
            ctxt = PL_foreign_context_address(handle);
            break;
        case PL_PRUNED:
            ctxt = PL_foreign_context_address(handle);
            ...
            free(ctxt);
            return TRUE;
      }

      <find first/next solution from ctxt>
      ...
      // We fail */
      if ( <no_solution> )
      { free(ctx);
        return FALSE;
      }
      // We succeed without a choice point */
      if ( <last_solution> )
      { free(ctx);
        return TRUE;
      }
      // We succeed with a choice point */
      PL_retry_address(ctxt);
    }

Figure 5 : Skeleton for non-deterministic foreign functions

#### 12.4.1.2 Yielding from foreign predicates

Starting with SWI-Prolog 8.5.5 we provide an experimental interface that allows using a SWI-Prolog engine for asynchronous processing. The idea is that an engine that calls a foreign predicate which would need to block may be suspended and later resumed. For example, consider an application that listens to a large number of network connections (sockets). SWI-Prolog offers three scenarios to deal with this:

1. Using a thread per connection. This model fits Prolog well as it allows to keep state in e.g. a DCG using phrase_from_stream/2. Maintaining an operating system thread per connection uses a significant amount of resources though.

2. Using wait_for_input/3 a single thread can wait for many connections. Each time input arrives we must associate this with a state engine and advance this engine using a chunk of input of unknown size. Programming a state engine in Prolog is typically a tedious job. Although we can use delimited continuations (see section 4.9) in some scenarios this is not a universal solution.

3. Using the primitives from this section we can create an engine (see PL_engine_create()) to handle a connection with the same benefits as using threads. When the engine calls a foreign predicate that would need to block it calls PL_yield_address() to suspend the engine. An overall scheduler watches for ready connections and calls PL_next_solution() to resume the suspended engine. This approach allows processing many connections on the same operating system thread.

As is, these features can only used through the foreign language interface. It was added after discussion with with Mattijs van Otterdijk aiming for using SWI-Prolog together with Rust's asynchronous programming support. Note that this feature is related to the engine API as described in section 11. It uis different though. Where the Prolog engine API allows for communicating with a Prolog engine, the facilities of this section merely allow an engine to suspend, to be resumed later.

To prepare a query for asynchronous usage we first create an engine using PL_create_engine(). Next, we create a query in the engine using PL_open_query() with the flags `PL_Q_ALLOW_YIELD` and `PL_Q_EXT_STATUS`. A foreign predicate that needs to be capable of suspending must be registered using PL_register_foreign() and the flags `PL_FA_VARARGS` and `PL_FA_NONDETERMINISTIC`; i.e., only non-det predicates can yield. This is no restriction as non-det predicate can always return `TRUE` to indicate deterministic success. Finally, PL_yield_address() allows the predicate to yield control, preparing to resume similar to PL_retry_address() does for non-deterministic results. PL_next_solution() returns `PL_S_YIELD` if a predicate calls PL_yield_address() and may be resumed by calling PL_next_solution() using the same query id (qid). We illustrate the above using some example fragments.

First, let us create a predicate that can read the available input from a Prolog stream and yield if it would block. Note that our predicate must the `PL_FA_VARARGS` interface, which implies the first argument is in a0, the second in `a0+1`, etc.214the other foreign interfaces do not support the yield API.

    /** read_or_block(+Stream, -String) is det.
    */

    #define BUFSIZE 4096

    static foreign_t
    read_or_block(term_t a0, int arity, void *context)
    { IOSTREAM *s;

      switch(PL_foreign_control(context))
      { case PL_FIRST_CALL:
          if ( PL_get_stream(a0, &s, SIO_INPUT) )
          { Sset_timeout(s, 0);
            break;
          }
          return FALSE;
        case PL_RESUME:
          s = PL_foreign_context_address(context);
          break;
        case PL_PRUNED:
          return PL_release_stream(s);
        default:
          assert(0);
          return FALSE;
      }

      char buf[BUFSIZE];

      size_t n = Sfread(buf, sizeof buf[0], sizeof buf / sizeof buf[0], s);
      if ( n == 0 )                     // timeout or error
      { if ( (s->flags&SIO_TIMEOUT) )
          PL_yield_address(s);        // timeout: yield
        else
          return PL_release_stream(s);  // raise error
      } else
      { return ( PL_release_stream(s) &&
                 PL_unify_chars(a0+1, PL_STRING|REP_ISO_LATIN_1, n, buf) );
      }
    }

This function must be registered using PL_register_foreign():

      PL_register_foreign("read_or_block", 2, read_or_block,
                          PL_FA_VARARGS|PL_FA_NONDETERMINISTIC);

Next, create an engine to run handle_connection/1 on a Prolog stream. Note that we omitted most of the error checking for readability. Also note that we must make our engine current using PL_set_engine() before we can interact with it.

    qid_t
    start_connection(IOSTREAM *c)
    { predicate_t p = PL_predicate("handle_connection", 1, "user");

      PL_engine_t e = PL_create_engine(NULL);
      PL_engine_t old;
      if ( PL_set_engine(e, &old) )
      { term_t av = PL_new_term_refs(1);
        PL_unify_stream(av+0, c);
        qid_t q = PL_open_query(e, NULL,
                                PL_Q_CATCH_EXCEPTION|
                                    PL_Q_ALLOW_YIELD|
                                     PL_Q_EXT_STATUS,
                                p, av);
        PL_set_engine(old, NULL);
        return q;
      } /* else error */
    }

Finally, our foreign code must manage this engine. Normally it will do so together with many other engines. First, we write a function that runs a query in the engine to which it belongs.215Possibly, future versions of PL_next_solution() may do that although the value is in general limited because interacting with the arguments of the query requires the query's engine to be current anyway.

    int
    PL_engine_next_solution(qid_t qid)
    { PL_engine_t old;
      int rc;

      if ( PL_set_engine(PL_query_engine(qid), &old) == PL_ENGINE_SET )
      { rc = PL_next_solution(qid);

        PL_set_engine(old, NULL);
      } else
        rc = FALSE;

      return rc;
    }

Now we can simply handle a connection using the loop below which restarts the query as long as it yields. Realistic code manages multiple queries and will (in this case) use the POSIX poll() or select() interfaces to activate the next query that can continue without blocking.

      int rc;
      do
      { rc = PL_engine_next_solution(qid);
      } while( rc == PL_S_YIELD );

After the query completes it must be closed using PL_close_query() or PL_cut_query(). The engine may be destroyed using PL_engine_destroy() or reused for a new query.

(return) foreign_t PL_yield_address(void *)
  Cause PL_next_solution() of the active query to return with `PL_S_YIELD`. A subsequent call to PL_next_solution() on the same query calls the foreign predicate again with the control status set to `PL_RESUME`, after which PL_foreign_context_address() retrieves the address passed to this function. The state of the Prolog engine is maintained, including `term_t` handles. If the passed address needs to be invalidated the predicate must do so when returning either `TRUE` or `FALSE`. If the engine terminates the predicate the predicate is called with status `PL_PRUNED`, in which case the predicate must cleanup.
int PL_can_yield(void)
  Returns `TRUE` when called from inside a foreign predicate if the query that (indirectly) calls this foreign predicate can yield using PL_yield_address(). Returns `FALSE` when either there is no current query or the query cannot yield.

Discussion

Asynchronous processing has become popular with modern programming languages, especially those aiming at network communication. Asynchronous processing uses fewer resources than threads while avoiding most of the complications associated with thread synchronization if only a single thread is used to manage the various states. The lack of good support for destructive state updates in Prolog makes it attractive to use threads for dealing with multiple inputs. The fact that Prolog discourages using shared global data such as dynamic predicates typically makes multithreaded code easy to manage.

It is not clear how much scalability we gain using Prolog engines instead of Prolog threads. The only difference between the two is the operating system task. Prolog engines are still rather memory intensive, mainly depending on the stack sizes. Global garbage collection (atoms and clauses) need to process all the stacks of all the engines and thus limit scalability.

One possible future direction is to allow all (possibly) blocking Prolog predicates to use the yield facility and provide a Prolog API to manage sets of engines that use this type of yielding. As is, these features are designed to allow SWI-Prolog for cooperating with languages that provide asynchronous functions.

### 12.4.2 Atoms and functors

The following functions provide for communication using atoms and functors.

atom_t PL_new_atom(const char *)
  Return an atom handle for the given C-string. This function always succeeds. The returned handle is valid as long as the atom is referenced (see section 12.4.2.1). Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return `(atom_t)0`.

  The following atoms are provided as macros, giving access to the empty list symbol and the name of the list constructor. Prior to version 7, `ATOM_nil` is the same as `PL_new_atom("[]")` and `ATOM_dot` is the same as `PL_new_atom(".")`. This is no longer the case in SWI-Prolog version 7.

  atom_t ATOM_nil(A)
    tomic constant that represents the empty list. It is advised to use PL_get_nil(), PL_put_nil() or PL_unify_nil() where applicable.
  atom_t ATOM_dot(A)
    tomic constant that represents the name of the list constructor. The list constructor itself is created using `PL_new_functor(ATOM_dot,2)`. It is advised to use PL_get_list(), PL_put_list() or PL_unify_list() where applicable.
atom_t PL_new_atom_mbchars(int rep, size_t len, const char *s)
  This function generalizes PL_new_atom() and PL_new_atom_nchars() while allowing for multiple encodings. The rep argument is one of `REP_ISO_LATIN_1`, `REP_UTF8` or `REP_MB`. If len is `(size_t)-1`, it is computed from s using strlen(). Raises an exception if s violates rep and returns `(atom_t)0`. For other error conditions, see PL_new_atom().
int PL_atom_mbchars(atom_t atom, size_t len, char *s, unsigned int flags)
  This function generalizes fetching the text associated with an atom. The encoding depends on the flags `REP_UTF8`, `REP_MB` or `REP_ISO_LATIN_1`. Storage is defined by the `BUF_*` flags as described with PL_get_chars(). The flag `CVT_EXCEPTION` defines whether or not the function fails silently or raises a Prolog exception. This function may fail because atom is not a text atom but a blob (see section 12.4.10), conversion to the requested encoding is not possible or a resource error occurs.
const char* PL_atom_chars(atom_t atom)
  Deprecated. This function returns a pointer to the content represented by the atom or blob regardless of its type. New code that uses blobs should use the blob functions such as PL_blob_data() to get a pointer to the content, the size of the content, and the type of the content. Most applications that need to get text from a `term_t` handle should use PL_atom_nchars(), PL_atom_wchars(), or PL_atom_mbchars(). If it is known that atom is a classical Prolog text atom, one can use PL_atom_nchars() to obtain the C string and its length (for ISO-Latin-1 atoms) or PL_atom_wchars() to obtain a C wide string (`wchar_t`).
functor_t PL_new_functor(atom_t name, int arity)
  Returns a functor identifier, a handle for the name/arity pair. The returned handle is valid for the entire Prolog session. Future versions may garbage collect functors as part of atom garbage collection. Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return `(atom_t)0`.
atom_t PL_functor_name(functor_t f)
  Return an atom representing the name of the given functor.
size_t PL_functor_arity(functor_t f)
  Return the arity of the given functor.

#### 12.4.2.1 Atoms and atom garbage collection

With the introduction of atom garbage collection in version 3.3.0, atoms no longer live as long as the process. Instead, their lifetime is guaranteed only as long as they are referenced. In the single-threaded version, atom garbage collections are only invoked at the call-port. In the multithreaded version (see chapter 10), they appear asynchronously, except for the invoking thread.

For dealing with atom garbage collection, two additional functions are provided:

void PL_register_atom(atom_t atom)
  Increment the reference count of the atom by one. PL_new_atom() performs this automatically, returning an atom with a reference count of at least one.216Otherwise asynchronous atom garbage collection might destroy the atom before it is used.
void PL_unregister_atom(atom_t atom)
  Decrement the reference count of the atom. If the reference count drops below zero, an assertion error is raised.

Please note that the following two calls are different with respect to atom garbage collection:

    PL_unify_atom_chars(t, "text");
    PL_unify_atom(t, PL_new_atom("text"));

The latter increments the reference count of the atom `text`, which effectively ensures the atom will never be collected. It is advised to use the *_chars() or *_nchars() functions whenever applicable.

### 12.4.3 Input and output

For input and output, `SWI-Stream.h` defines a set of functions that are similar to the C library functions, except prefixed by S, e.g. Sfprintf(). They differ from the C functions in following ways:

- Instead of returning the number of bytes written and a negative value for error, they return the number of characters written and a negative value for error.
- Instead of a `FILE`, they access the Prolog streams, using `IOSTREAM*`. In particular, `Scurrent_output` accesses the current output stream and works well with with_output_to/2. Similarly, there are `Scurrent_intput`, `Suser_output`, `Suser_error`, and `Suser_input`.
- If you wish to directly use the operating system's `stdin`, `stdout`, `stderr`, you can use `Sinput`, `Soutput`, `Serror`. These are not affected by predicates such as with_output_to/2.

In general, if a stream is acquired via PL_acquire_stream(), an error is raised when PL_release_stream() is called, so in that situation, there's no need to check the return codes from the IO functions. Blob write callbacks are also called in the context of an acquired stream, so there is no need to check the return codes from its IO function calls. However, if you use one of the standard streams such as `Scurrent_output`, you should check the return code and return `FALSE` from the foreign predicate, at which point an error will be raised. Not all IO functions follow this, because they need to return other information, so you should check the details with each one (e.g., Sputcode() returns -1 on error).

For more details, including formatting extensions for printing terms, see section 12.9.

### 12.4.4 Analysing Terms via the Foreign Interface

Each argument of a foreign function (except for the control argument) is of type `term_t`, an opaque handle to a Prolog term. Three groups of functions are available for the analysis of terms. The first just validates the type, like the Prolog predicates var/1, atom/1, etc., and are called `PL_is_*()`. The second group attempts to translate the argument into a C primitive type. These predicates take a `term_t` and a pointer to the appropriate C type and return `TRUE` or `FALSE` depending on successful or unsuccessful translation. If the translation fails, the pointed-to data is never modified.

#### 12.4.4.1 Testing the type of a term

int PL_term_type(term_t)
  Obtain the type of a term, which should be a term returned by one of the other interface predicates or passed as an argument. The function returns the type of the Prolog term. The type identifiers are listed below. Note that the extraction functions `PL_get_*()` also validate the type and thus the two sections below are equivalent.

              if ( PL_is_atom(t) )
              { char *s;

                PL_get_atom_chars(t, &s);
                ...;
              }

      or

              char *s;
              if ( PL_get_atom_chars(t, &s) )
              { ...;
              }

  Version 7 added `PL_NIL`, `PL_BLOB`, `PL_LIST_PAIR` and `PL_DICT`. Older versions classify `PL_NIL` and `PL_BLOB` as `PL_ATOM`, `PL_LIST_PAIR` as `PL_TERM` and do not have dicts.

  `PL_VARIABLE` | A variable or attributed variable
  `PL_ATOM` | A Prolog atom
  `PL_NIL` | The constant `[]`
  `PL_BLOB` | A blob (see section 12.4.10.2)
  `PL_STRING` | A string (see section 5.2)
  `PL_INTEGER` | A integer
  `PL_RATIONAL` | A rational number
  `PL_FLOAT` | A floating point number
  `PL_TERM` | A compound term
  `PL_LIST_PAIR` | A list cell (`[H|T]`)
  `PL_DICT` | A dict (see section 5.4))

The functions PL_is_<type> are an alternative to PL_term_type(). The test `PL_is_variable(term)` is equivalent to `PL_term_type(term) == PL_VARIABLE`, but the first is considerably faster. On the other hand, using a switch over PL_term_type() is faster and more readable then using an if-then-else using the functions below. All these functions return either `TRUE` or `FALSE`.

int PL_is_variable(term_t)
  Returns non-zero if term is a variable.
int PL_is_ground(term_t)
  Returns non-zero if term is a ground term. See also ground/1. This function is cycle-safe.
int PL_is_atom(term_t)
  Returns non-zero if term is an atom.
int PL_is_string(term_t)
  Returns non-zero if term is a string.
int PL_is_integer(term_t)
  Returns non-zero if term is an integer.
int PL_is_rational(term_t)
  Returns non-zero if term is a rational number (P/Q). Note that all integers are considered rational and this test thus succeeds for any term for which PL_is_integer() succeeds. See also PL_get_mpq() and PL_unify_mpq().
int PL_is_float(term_t)
  Returns non-zero if term is a float. Note that the corresponding PL_get_float() converts rationals (and thus integers).
int PL_is_callable(term_t)
  Returns non-zero if term is a callable term. See callable/1 for details.
int PL_is_compound(term_t)
  Returns non-zero if term is a compound term.
int PL_is_functor(term_t, functor_t)
  Returns non-zero if term is compound and its functor is functor. This test is equivalent to PL_get_functor(), followed by testing the functor, but easier to write and faster.
int PL_is_list(term_t)
  Returns non-zero if term is a compound term using the list constructor or the list terminator. See also PL_is_pair() and PL_skip_list().
int PL_is_pair(term_t)
  Returns non-zero if term is a compound term using the list constructor. See also PL_is_list() and PL_skip_list().
int PL_is_dict(term_t)
  Returns non-zero if term is a dict. See also PL_put_dict() and PL_get_dict_key().
int PL_is_atomic(term_t)
  Returns non-zero if term is atomic (not a variable or compound).
int PL_is_number(term_t)
  Returns non-zero if term is an rational (including integers) or float.
int PL_is_acyclic(term_t)
  Returns non-zero if term is acyclic (i.e. a finite tree).

#### 12.4.4.2 Reading data from a term

The functions `PL_get_*()` read information from a Prolog term. Most of them take two arguments. The first is the input term and the second is a pointer to the output value or a term reference. The return value is `TRUE` or `FALSE`, indicating the success of the "get" operation. Most functions have a related "_ex" function that raises an error if the argument is the operation cannot be completed. If the Prolog term is not suitable, this is a type, domain or instantiation error. If the receiving C type cannot represent the value this is a representation error.

For integers an alternative interface exists, which helps deal with the various integer types in C and C++. They are convenient for use with `_Generic` selection or C++ overloading.

int PL_get_atom(term_t +t, atom_t *a)
  If t is an atom, store the unique atom identifier over a. See also PL_atom_chars() and PL_new_atom(). If there is no need to access the data (characters) of an atom, it is advised to manipulate atoms using their handle. As the atom is referenced by t, it will live at least as long as t does. If longer lifetime is required, the atom should be locked using PL_register_atom().
int PL_get_atom_chars(term_t +t, char **s)
  If t is an atom, store a pointer to a 0-terminated C-string in s. It is explicitly not allowed to modify the contents of this string. Some built-in atoms may have the string allocated in read-only memory, so‘temporary manipulation' can cause an error.
int PL_get_string_chars(term_t +t, char **s, size_t *len)
  If t is a string object, store a pointer to a 0-terminated C-string in s and the length of the string in len. Note that this pointer is invalidated by backtracking, garbage collection and stack-shifts, so generally the only safe operations are to pass it immediately to a C function that doesn't involve Prolog.
int PL_get_chars(term_t +t, char **s, unsigned flags)
  Convert the argument term t to a 0-terminated C-string. flags is a bitwise disjunction from two groups of constants. The first specifies which term types should be converted and the second how the argument is stored. Below is a specification of these constants. `BUF_STACK` implies, if the data is not static (as from an atom), that the data is pushed on a stack. If BUF_MALLOC is used, the data must be freed using PL_free() when no longer needed.

  With the introduction of wide characters (see section 2.19.1), not all atoms can be converted into a `char*`. This function fails if t is of the wrong type, but also if the text cannot be represented. See the `REP_*` flags below for details. See also PL_get_wchars() and PL_get_nchars().

  CVT_ATOM
    Convert if term is an atom.
  CVT_STRING
    Convert if term is a string.
  CVT_LIST
    Convert if term is a list of of character codes.
  CVT_INTEGER
    Convert if term is an integer.
  CVT_RATIONAL
    Convert if term is an rational number. The number is written as <num>r<den>.
  CVT_XINTEGER
    Convert if term is an integer to hexadecimal notation. May be combined with `CVT_RATIONAL` to represent rational numbers using hexadecimal notation. Hexadecimal notation is notably useful for transferring big integers to other programming environments if the target system can read hexadecimal notation because the result is both more compact and faster to write and read.
  CVT_FLOAT
    Convert if term is a float. The characters returned are the same as write/1 would write for the floating point number.
  CVT_NUMBER
    Convert if term is an integer, rational number or or float. Equivalent to `CVT_INTEGER``|``CVT_RATIONAL``|``CVT_FLOAT`
  CVT_ATOMIC
    Convert if term is atomic.
  CVT_VARIABLE
    Convert variable to print-name
  CVT_WRITE
    Convert any term that is not converted by any of the other flags using write/1. If no `BUF_*` is provided, `BUF_STACK` is implied.
  CVT_WRITE_CANONICAL
    As `CVT_WRITE`, but using write_canonical/2.
  CVT_WRITEQ
    As `CVT_WRITE`, but using writeq/2.
  CVT_ALL
    Convert if term is any of the above, except for `CVT_VARIABLE` and `CVT_WRITE*`.
  CVT_EXCEPTION
    If conversion fails due to a type error, raise a Prolog type error exception in addition to failure
  BUF_DISCARDABLE
    Data must copied immediately.
  BUF_STACK
    Data is stored on a stack. The older `BUF_RING` is an alias for `BUF_STACK`. See section 12.4.14.
  BUF_MALLOC
    Data is copied to a new buffer returned by PL_malloc(3). When no longer needed the user must call PL_free() on the data.
  REP_ISO_LATIN_1
    Text is in ISO Latin-1 encoding and the call fails if text cannot be represented. This flag has the value 0 and is thus the default.
  REP_UTF8
    Convert the text to a UTF-8 string. This works for all text.
  REP_MB
    Convert to default locale-defined 8-bit string. Success depends on the locale. Conversion is done using the wcrtomb() C library function.
int PL_get_list_chars(+term_t l, char **s, unsigned flags)
  Same as `PL_get_chars(l, s, CVT_LIST|flags)`, provided flags contains none of the `CVT_*` flags.
int PL_get_integer(+term_t t, int *i)
  If t is a Prolog integer, assign its value over i. On 32-bit machines, this is the same as PL_get_long(), but avoids a warning from the compiler. See also PL_get_long() and PL_get_integer_ex().
int PL_get_long(term_t +t, long *i)
  If t is a Prolog integer that can be represented as a long, assign its value over i. If t is an integer that cannot be represented by a C long, this function returns `FALSE`. If t is a floating point number that can be represented as a long, this function succeeds as well. See also PL_get_int64() and PL_get_long_ex().
int PL_get_int64(term_t +t, int64_t *i)
  If t is a Prolog integer or float that can be represented as a `int64_t`, assign its value over i. See also PL_get_int64_ex().
int PL_get_uint64(term_t +t, uint64_t *i)
  If t is a Prolog integer that can be represented as a `uint64_t`, assign its value over i. Note that this requires GMP support for representing `uint64_t` values with the high bit set. See also PL_get_uint64_ex().
int PL_get_intptr(term_t +t, intptr_t *i)
  Get an integer that is at least as wide as a pointer. On most platforms this is the same as PL_get_long(), but on Win64 pointers are 8 bytes and longs only 4. Unlike PL_get_pointer(), the value is not modified.
int PL_get_bool(term_t +t, int *val)
  If t has the value `true`, `false`, set val to the C constant `TRUE` or `FALSE` and return success, otherwise return failure. The values `on`, `1`, `off`, const0 and are also accepted.
int PL_get_pointer(term_t +t, void **ptr)
  Together with PL_put_pointer() and PL_unify_pointer(), these functions allow representing a C pointer as a Prolog integer. The integer value is derived from the pointer, but not equivalent. The translation aims at producing smaller integers that fit more often in the tagged integer range. Representing C pointers as integers is unsafe. The blob API described in section 12.4.10 provides a safe way for handling foreign resources that cooperates with Prolog garbage collection.
int PL_get_float(term_t +t, double *f)
  If t is a float, integer or rational number, its value is assigned over f. Note that if t is an integer or rational conversion may fail because the number cannot be represented as a float.
int PL_get_functor(term_t +t, functor_t *f)
  If t is compound or an atom, the Prolog representation of the name-arity pair will be assigned over f. See also PL_get_name_arity() and PL_is_functor().
int PL_get_name_arity(term_t +t, atom_t *name, size_t *arity)
  If t is compound or an atom, the functor name will be assigned over name and the arity over arity (either or both may be NULL). See also PL_get_functor() and PL_is_functor().
int PL_get_compound_name_arity(term_t +t, atom_t *name, size_t *arity)
  If t is compound term, the functor name will be assigned over name and the arity over arity. This is the same as PL_get_name_arity(), but this function fails if t is an atom.
int PL_get_module(term_t +t, module_t *module)
  If t is an atom, the system will look up or create the corresponding module and assign an opaque pointer to it over module.
int PL_get_arg(size_t index, term_t +t, term_t -a)
  If t is compound and index is between 1 and arity (inclusive), assign a with a term reference to the argument.
int _PL_get_arg(size_t index, term_t +t, term_t -a)
  Same as PL_get_arg(), but no checking is performed, neither whether t is actually a term nor whether index is a valid argument index.
int PL_get_dict_key(atom_t key, term_t +dict, term_t -value)
  If dict is a dict, get the associated value in value. Fails silently if key does not appear in dict or if if dict is not a dict.

#### 12.4.4.3 Exchanging text using length and string

All internal text representation in SWI-Prolog is represented using `char *` plus length and allow for 0-bytes in them. The foreign library supports this by implementing a *_nchars() function for each applicable *_chars() function. Below we briefly present the signatures of these functions. For full documentation consult the *_chars() function.

int PL_get_atom_nchars(term_t t, size_t *len, char **s)
  See PL_get_atom_chars().
int PL_get_list_nchars(term_t t, size_t *len, char **s)
  See PL_get_list_chars().
int PL_get_nchars(term_t t, size_t *len, char **s, unsigned int flags)
  See PL_get_chars(). The len pointer may be `NULL`.
int PL_put_atom_nchars(term_t t, size_t len, const char *s)
  See PL_put_atom_chars().
int PL_put_string_nchars(term_t t, size_t len, const char *s)
  See PL_put_string_chars().
int PL_put_list_ncodes(term_t t, size_t len, const char *s)
  See PL_put_list_codes().
int PL_put_list_nchars(term_t t, size_t len, const char *s)
  See PL_put_list_chars().
int PL_unify_atom_nchars(term_t t, size_t len, const char *s)
  See PL_unify_atom_chars().
int PL_unify_string_nchars(term_t t, size_t len, const char *s)
  See PL_unify_string_chars().
int PL_unify_list_ncodes(term_t t, size_t len, const char *s)
  See PL_unify_codes().
int PL_unify_list_nchars(term_t t, size_t len, const char *s)
  See PL_unify_list_chars().

In addition, the following functions are available for creating and inspecting atoms:

atom_t PL_new_atom_nchars(size_t len, const char *s)
  Create a new atom as PL_new_atom(), but using the given length and characters. If len is `(size_t)-1`, it is computed from s using strlen(). See PL_new_atom() for error handling.
const char * PL_atom_nchars(atom_t a, size_t *len)
  Extract the text and length of an atom. If you do not need the length, pass NULL as the value of len. If PL_atom_nchars() is called for a blob, NULL is returned.

#### 12.4.4.4 Wide-character versions

Support for exchange of wide-character strings is still under consideration. The functions dealing with 8-bit character strings return failure when operating on a wide-character atom or Prolog string object. The functions below can extract and unify both 8-bit and wide atoms and string objects. Wide character strings are represented as C arrays of objects of the type `pl_wchar_t`, which is guaranteed to be the same as `wchar_t` on platforms supporting this type. For example, on MS-Windows, this represents a 16-bit UTF-16 string, while using the GNU C library (glibc) this represents 32-bit UCS4 characters.

atom_t PL_new_atom_wchars(size_t len, const pl_wchar_t *s)
  Create atom from wide-character string as PL_new_atom_nchars() does for ISO-Latin-1 strings. If s only contains ISO-Latin-1 characters a normal byte-array atom is created. If len is `(size_t)-1`, it is computed from s using wcslen(). See PL_new_atom() for error handling.
const pl_wchar_t* PL_atom_wchars(atom_t atom, size_t *len)
  Extract characters from a wide-character atom. Succeeds on any atom marked as‘text'. If the underlying atom is a wide-character atom, the returned pointer is a pointer into the atom structure. If the atom is represented as an ISO-Latin-1 string, the returned pointer comes from Prolog's‘buffer stack' (see section 12.4.14).
int PL_get_wchars(term_t t, size_t *len, pl_wchar_t **s, unsigned flags)
  Wide-character version of PL_get_chars(). The flags argument is the same as for PL_get_chars(). Note that this operation may return a pointer into Prolog's‘buffer stack' (see section 12.4.14).
int PL_put_wchars(term_t -t, int type, size_t len, const pl_wchar_t *s)
  Put text from a wide character array in t. Arguments are the same as PL_unify_wchars().217The current implemention uses PL_put_variable() followed by PL_unify_wchars().
int PL_unify_wchars(term_t +t, int type, size_t len, const pl_wchar_t *s)
  Unify t with a textual representation of the C wide-character array s. The type argument defines the Prolog representation and is one of `PL_ATOM`, `PL_STRING`, `PL_CODE_LIST` or `PL_CHAR_LIST`.
int PL_unify_wchars_diff(term_t +t, term_t -tail, int type, size_t len, const pl_wchar_t *s)
  Difference list version of PL_unify_wchars(), only supporting the types `PL_CODE_LIST` and `PL_CHAR_LIST`. It serves two purposes. It allows for returning very long lists from data read from a stream without the need for a resizing buffer in C. Also, the use of difference lists is often practical for further processing in Prolog. Examples can be found in `packages/clib/readutil.c` from the source distribution.

#### 12.4.4.5 Reading a list

The functions from this section are intended to read a Prolog list from C. Suppose we expect a list of atoms; the code below will print the atoms, each on a line. Please note the following:

- We need a `term_t` term reference for the elements (head). This reference is reused for each element.
- We walk over the list using PL_get_list_ex() which overwrites the list `term_t`. As it is not allowed to overwrite the `term_t` passed in as arguments to a predicate, we must copy the argument `term_t`.
- SWI-Prolog atoms are Unicode objects. The PL_get_chars() returns a `char*`. We want it to convert atoms, return the result as a multibyte string (`REP_UTF8` may also be used) and finally we want an exception on type, instantiation or representation errors (if the system's default encoding cannot represent some characters of the Unicode atom). This may create temporary copies of the atom text - PL_STRINGS_MARK() ... PL_STRINGS_RELEASE() handles that.
- The *_ex() API functions are functionally the same as the ones without the `_ex` suffix, but they raise type, domain, or instantiation errors when the input is invalid; whereas the plain version may only raise resource exceptions if the request cannot be fullfilled due to resource exhaustion.
- PL_get_nil_ex() is designed to propagate an already raised exception.

    foreign_t
    pl_write_atoms(term_t l)
    { term_t head = PL_new_term_ref();   /* the elements */
      term_t tail = PL_copy_term_ref(l); /* copy (we modify tail) */
      int rc = TRUE;

      while( rc && PL_get_list_ex(tail, head, tail) )
      { PL_STRINGS_MARK();
          char *s;
          if (rc=PL_get_chars(head, &s, CVT_ATOM|REP_MB|CVT_EXCEPTION)) )
            rc = Sfprintf(Scurrent_output, "%s\n", s);
        PL_STRINGS_RELEASE();
      }

      return rc && PL_get_nil_ex(tail); /* test end for [] */
    }

Note that as of version 7, lists have a new representation unless the option --traditional is used. see section 5.1.

int PL_get_list(term_t +l, term_t -h, term_t -t)
  If l is a list and not the empty list, assign a term reference to the head to h and to the tail to t.
int PL_get_head(term_t +l, term_t -h)
  If l is a list and not the empty list, assign a term reference to the head to h.
int PL_get_tail(term_t +l, term_t -t)
  If l is a list and not the empty list, assign a term reference to the tail to t.
int PL_get_nil(term_t +l)
  Succeeds if l represents the list termination constant.
int PL_skip_list(term_t +list, term_t -tail, size_t *len)
  This is a multi-purpose function to deal with lists. It allows for finding the length of a list, checking whether something is a list, etc. The reference tail is set to point to the end of the list, len is filled with the number of list-cells skipped, and the return value indicates the status of the list:
  PL_LIST
    The list is a‘proper' list: one that ends in the list terminator constant and tail is filled with the terminator constant.
  PL_PARTIAL_LIST
    The list is a‘partial' list: one that ends in a variable and tail is a reference to this variable.
  PL_CYCLIC_TERM
    The list is cyclic (e.g. X = [a|X]). tail points to an arbitrary cell of the list and len is at most twice the cycle length of the list.
  PL_NOT_A_LIST
    The term list is not a list at all. tail is bound to the non-list term and len is set to the number of list-cells skipped.

  It is allowed to pass 0 for tail and `NULL` for len.

#### 12.4.4.6 Processing option lists and dicts

int PL_scan_options(term_t options, int flags, const char* opttype, PL_option_t specs[], ...)
  Process an option list as we find with, e.g., write_term/2 and many other builtin predicates. This function takes an option list (or dict) and in the variadic argument list pointers that receive the option values. PL_scan_options() takes care of validating the list, ensuring the list is not cyclic, validating the option type and storing the converted values using the supplied pointers.

  Below is an example. While `PL_option_t` is a struct, its members are initialised using the PL_OPTION() macro. The data structure is not constant because PL_scan_options() adds the option names as atoms to speed up option processing. The macro PL_OPTIONS_END terminates the option list.

      static PL_option_t mypred_options[] =
      { PL_OPTION("quoted",   OPT_BOOL),
        PL_OPTION("length",   OPT_SIZE),
        PL_OPTION("callback", OPT_TERM),
        PL_OPTIONS_END
      };

      static foreign_t
      mypred(term_t a1, term_t options)
      { int    quoted   = FALSE;
        size_t length   = 10;
        term_t callback = 0;

        if ( !PL_scan_options(options, 0, "mypred_options", mypred_options,
                              &quoted, &length, &callback) )
          return FALSE;

        <implement mypred>
      }

  The only defined value for flags is currently `OPT_ALL`, which causes this function to raise a domain error if an option is passed that is not in specs. Default in SWI-Prolog is to silently ignore unknown options, unless the Prolog flag iso is `true`. The opttype argument defines the type (group) of the options, e.g., `"write_option"`. Option types are defined by the ISO standard. SWI-Prolog only uses this if `OPT_ALL` is specified, to raise a `domain_error` of the indicated type if some option is unused. The type name is normally the name of the predicate followed by `_option` or the name of a representative of a group of predicates to which the options apply.

  Defined option types and their corresponding pointer type are described below.

  `OPT_BOOL` `int`
    Convert the option value to a bool. This converts the values described by PL_get_bool(). In addition, an option without a value (i.e., a plain atom that denotes the option name) can act as a boolean `TRUE`.
  `OPT_INT` `int`
  `OPT_INT64` `int64_t`
  `OPT_UINT64` `uint64_t`
  `OPT_SIZE` `size_t`
  `OPT_DOUBLE` `double`
    Numeric values of various types. Raises an error if the Prolog value cannot be represented by the C type.
  `OPT_STRING` `char*`
    Uses PL_get_chars() using the flags `CVT_ALL|REP_UTF8|BUF_STACK|CVT_EXCEPTION`. The buffered string must be guarded using PL_STRINGS_MARK() and PL_STRINGS_RELEASE().
  `OPT_ATOM` `atom_t`
    Accepts an atom. Note that if the C function that implements the predicate wishes to keep hold of the atom after it returns it must use PL_register_atom().
  `OPT_TERM` `term_t`
    Accepts an arbitrary Prolog term. The term handle is scoped by the foreign predicate invocation. Terms can be preserved using PL_record().

  The ISO standard demands that if an option is repeated the last occurance holds. This implies that PL_scan_options() must scan the option list to the end.

#### 12.4.4.7 An example: defining write/1 in C

Figure 6 shows a simplified definition of write/1 to illustrate the described functions. This simplified version does not deal with operators. It is called display/1, because it mimics closely the behaviour of this Edinburgh predicate.

    foreign_t
    pl_display(term_t t)
    { functor_t functor;
      int arity, len, n;
      char *s;

      switch( PL_term_type(t) )
      { case PL_VARIABLE:
        case PL_ATOM:
        case PL_INTEGER:
        case PL_FLOAT:
          PL_get_chars(t, &s, CVT_ALL);
          if (! Sfprintf(Scurrent_output, "%s", s) )
            PL_fail;
          break;
        case PL_STRING:
          if ( !PL_get_string_chars(t, &s, &len) &&
               !Sfprintf(Scurrent_output, "\"%s\"", s) )
            PL_fail;
          break;
        case PL_TERM:
        { term_t a = PL_new_term_ref();

          if ( !PL_get_name_arity(t, &name, &arity) &&
               !Sfprintf(Scurrent_output, "%s(", PL_atom_chars(name)) )
            PL_fail
          for(n=1; n<=arity; n++)
          { if ( ! PL_get_arg(n, t, a) )
              PL_fail;
            if ( n > 1 )
              if ( ! Sfprintf(Scurrent_output, ", ") )
                PL_fail;
            if ( !pl_display(a) )
              PL_fail;
          }
          if ( !Sfprintf(Scurrent_output, ")") )
            PL_fail;
          break;
        default:
          PL_fail;                          /* should not happen */
        }
      }

      PL_succeed;
    }

Figure 6 : A Foreign definition of display/1

### 12.4.5 Constructing Terms

Terms can be constructed using functions from the `PL_put_*()` and `PL_cons_*()` families. This approach builds the term‘inside-out', starting at the leaves and subsequently creating compound terms. Alternatively, terms may be created‘top-down', first creating a compound holding only variables and subsequently unifying the arguments. This section discusses functions for the first approach. This approach is generally used for creating arguments for PL_call() and PL_open_query().

int PL_put_variable(term_t -t)
  Put a fresh variable in the term, resetting the term reference to its initial state.218Older versions created a variable on the global stack.
int PL_put_atom(term_t -t, atom_t a)
  Put an atom in the term reference from a handle. See also PL_new_atom() and PL_atom_chars().
int PL_put_bool(term_t -t, int val)
  Put one of the atoms `true` or `false` in the term reference See also PL_put_atom(), PL_unify_bool() and PL_get_bool().
int PL_put_chars(term_t -t, int flags, size_t len, const char *chars)
  New function to deal with setting a term from a `char*` with various encodings. The flags argument is a bitwise or specifying the Prolog target type and the encoding of chars. A Prolog type is one of `PL_ATOM`, `PL_STRING`, `PL_CODE_LIST` or `PL_CHAR_LIST`. A representation is one of `REP_ISO_LATIN_1`, `REP_UTF8` or `REP_MB`. See PL_get_chars() for a definition of the representation types. If len is `-1` chars must be zero-terminated and the length is computed from chars using strlen().
int PL_put_atom_chars(term_t -t, const char *chars)
  Put an atom in the term reference constructed from the zero-terminated string. The string itself will never be referenced by Prolog after this function.
int PL_put_string_chars(term_t -t, const char *chars)
  Put a zero-terminated string in the term reference. The data will be copied. See also PL_put_string_nchars().
int PL_put_string_nchars(term_t -t, size_t len, const char *chars)

  Put a string, represented by a length/start pointer pair in the term reference. The data will be copied. This interface can deal with 0-bytes in the string. See also section 12.4.24.

int PL_put_list_chars(term_t -t, const char *chars)
  Put a list of ASCII values in the term reference.
int PL_put_integer(term_t -t, long i)
  Put a Prolog integer in the term reference.
int PL_put_int64(term_t -t, int64_t i)
  Put a Prolog integer in the term reference.
int PL_put_uint64(term_t -t, uint64_t i)
  Put a Prolog integer in the term reference. Note that unbounded integer support is required for `uint64_t` values with the highest bit set to 1. Without unbounded integer support, too large values raise a `representation_error` exception.
int PL_put_pointer(term_t -t, void *ptr)
  Put a Prolog integer in the term reference. Provided ptr is in the‘malloc()-area', PL_get_pointer() will get the pointer back.
int PL_put_float(term_t -t, double f)
  Put a floating-point value in the term reference.
int PL_put_functor(term_t -t, functor_t functor)
  Create a new compound term from functor and bind t to this term. All arguments of the term will be variables. To create a term with instantiated arguments, either instantiate the arguments using the `PL_unify_*()` functions or use PL_cons_functor().
int PL_put_list(term_t -l)
  As PL_put_functor(), using the list-cell functor. Note that on classical Prolog systems or in SWI-Prolog using the option --traditional, this is ``.`/2`, while on SWI-Prolog version 7 this is ``[|]`/2`.
int PL_put_nil(term_t -l)
  Put the list terminator constant in l. Always returns `TRUE`. Note that in classical Prolog systems or in SWI-Prolog using the option --traditional, this is the same as `PL_put_atom_chars("[]")`. See section 5.1.
int PL_put_term(term_t -t1, term_t +t2)
  Make t1 point to the same term as t2. Under the unusual condition that t2 is a fresh term reference this function requires a global stack cell and may thus return FALSE and leave a resource exception in the environment.
int PL_cons_functor(term_t -h, functor_t f, ...)
  Create a term whose arguments are filled from a variable argument list holding the same number of `term_t` objects as the arity of the functor. To create the term `animal(gnu, 50)`, use:

      { term_t a1 = PL_new_term_ref();
        term_t a2 = PL_new_term_ref();
        term_t t  = PL_new_term_ref();
        functor_t animal2;

        /* animal2 is a constant that may be bound to a global
           variable and re-used
        */
        animal2 = PL_new_functor(PL_new_atom("animal"), 2);

        PL_put_atom_chars(a1, "gnu");
        PL_put_integer(a2, 50);
        PL_cons_functor(t, animal2, a1, a2);
      }

  After this sequence, the term references a1 and a2 may be used for other purposes.

int PL_cons_functor_v(term_t -h, functor_t f, term_t a0)
  Create a compound term like PL_cons_functor(), but a0 is an array of term references as returned by PL_new_term_refs(). The length of this array should match the number of arguments required by the functor.
int PL_cons_list(term_t -l, term_t +h, term_t +t)
  Create a list (cons-) cell in l from the head h and tail t. As with PL_cons_functor(), the term references h and t may be used for other purposes after the call to PL_cons_list(). The code below creates a list of atoms from a `char **`. The list is built tail-to-head. The `PL_unify_*()` functions can be used instead to build a list head-to-tail.

      void
      put_list(term_t l, int n, char **words)
      { term_t a = PL_new_term_ref();

        PL_put_nil(l);
        while( --n >= 0 )
        { PL_put_atom_chars(a, words[n]);
          PL_cons_list(l, a, l);
        }
      }

int PL_put_dict(term_t -h, atom_t tag, size_t len, const atom_t *keys, term_t values)
  Create a dict from a tag and vector of atom-value pairs and put the result in h. The dict's key is set by tag, which may be `0` to leave the tag unbound. The keys vector is a vector of atoms of at least len long. The values is a term vector allocated using PL_new_term_refs() of at least len long. This function returns `TRUE` on success, `FALSE` on a resource error (leaving a resource error exception in the environment), `-1` if some key or the tag is invalid and `-2` if there are duplicate keys.

### 12.4.6 Unifying data

The functions of this section unify terms with other terms or translated C data structures. Except for PL_unify(), these functions are specific to SWI-Prolog. They have been introduced because they shorten the code for returning data to Prolog and at the same time make this more efficient by avoiding the need to allocate temporary term references and reduce the number of calls to the Prolog API. Consider the case where we want a foreign function to return the host name of the machine Prolog is running on. Using the `PL_get_*()` and `PL_put_*()` functions, the code becomes:

    foreign_t
    pl_hostname(term_t name)
    { char buf[100];

      if ( gethostname(buf, sizeof buf) )
      { term_t tmp = PL_new_term_ref();

        PL_put_atom_chars(tmp, buf);
        return PL_unify(name, tmp);
      }

      PL_fail;
    }

Using PL_unify_atom_chars(), this becomes:

    foreign_t
    pl_hostname(term_t name)
    { char buf[100];

      if ( gethostname(buf, sizeof buf) )
        return PL_unify_atom_chars(name, buf);

      PL_fail;
    }

Note that unification functions that perform multiple bindings may leave part of the bindings in case of failure. See PL_unify() for details.

int PL_unify(term_t ?t1, term_t ?t2)
  Unify two Prolog terms and return `TRUE` on success. PL_unify() does not evaluate attributed variables (see section 8.1), it merely schedules the goals associated with the attributes to be executed after the foreign predicate succeeds.219Goal associated with attributes may be non-deterministic, which we cannot handle from a callback. A callback could also result in deeply nested mutual recursion between C and Prolog and eventually trigger a C stack overflow.

  Care is needed if PL_unify() returns `FALSE` and the foreign function does not immediately return to Prolog with `FALSE`. Unification may perform multiple changes to either t1 or t2. A failing unification may have created bindings before failure is detected. Already created bindings are not undone. For example, calling PL_unify() on `a(X, a)` and `a(c,b)` binds X to `c` and fails when trying to unify `a` to `b`. If control remains in C or even if we want to return success to Prolog, we must undo such bindings. In addition, PL_unify() may have failed on an exception, typically a resource (stack) overflow. This can be tested using PL_exception(), passing 0 (zero) for the query-id argument. Foreign functions that encounter an exception must return `FALSE` to Prolog as soon as possible or call PL_clear_exception() if they wish to ignore the exception. Note that there can only be an exception if PL_unify() returned `FALSE`. This is achieved using PL_open_foreign_frame() and PL_rewind_foreign_frame(), as shown in the snippet below.

          { fid_t fid = PL_open_foreign_frame();

            ...
            if ( !PL_unify(t1, t2) )
            { if ( PL_exception(0) )
              { PL_close_foreign_frame(fid);
                return FALSE;
              }
              PL_rewind_foreign_frame(fid);
            }
            ...

            PL_close_foreign_frame(fid);
          }

  This code is only needed if the foreign predicate does not return immediately to Prolog when PL_unify() fails - there is an implicit frame around the entire predicate, and returning `FALSE` undoes all bindings when that frame is closed.

int PL_unify_atom(term_t ?t, atom_t a)
  Unify t with the atom a and return non-zero on success.
int PL_unify_bool(term_t ?t, int a)
  Unify t with either `false` or `true`, according to whether a is zero or non-zero. If t is instantiated, `off` and `on` are also accepted.
int PL_unify_chars(term_t ?t, int flags, size_t len, const char *chars)
  New function to deal with unification of `char*` with various encodings to a Prolog representation. The flags argument is a bitwise or specifying the Prolog target type and the encoding of chars. A Prolog type is one of `PL_ATOM`, `PL_STRING`, `PL_CODE_LIST` or `PL_CHAR_LIST`. A representation is one of `REP_ISO_LATIN_1`, `REP_UTF8` or `REP_MB`. See PL_get_chars() for a definition of the representation types. If len is `-1` chars must be zero-terminated and the length is computed from chars using strlen().

  If flags includes `PL_DIFF_LIST` and type is one of `PL_CODE_LIST` or `PL_CHAR_LIST`, the text is converted to a difference list. The tail of the difference list is t+1.

int PL_unify_atom_chars(term_t ?t, const char *chars)
  Unify t with an atom created from chars and return non-zero on success.
int PL_unify_list_chars(term_t ?t, const char *chars)
  Unify t with a list of ASCII characters constructed from chars.
void PL_unify_string_chars(term_t ?t, const char *chars)
  Unify t with a Prolog string object created from the zero-terminated string chars. The data will be copied. See also PL_unify_string_nchars().
int PL_unify_integer(term_t ?t, intptr_t n)
  Unify t with a Prolog integer from n.
int PL_unify_int64(term_t ?t, int64_t n)
  Unify t with a Prolog integer from n.
int PL_unify_uint64(term_t ?t, uint64_t n)
  Unify t with a Prolog integer from n. Note that unbounded integer support is required if n does not fit in a signed `int64_t`. If unbounded integers are not supported a `representation_error` is raised.
int PL_unify_float(term_t ?t, double f)
  Unify t with a Prolog float from f.
int PL_unify_pointer(term_t ?t, void *ptr)
  Unify t with a Prolog integer describing the pointer. See also PL_put_pointer() and PL_get_pointer().
int PL_unify_functor(term_t ?t, functor_t f)
  If t is a compound term with the given functor, just succeed. If it is unbound, create a term and bind the variable, else fail. Note that this function does not create a term if the argument is already instantiated. If f is a functor with arity 0, t is unified with an atom. See also PL_unify_compound().
int PL_unify_compound(term_t ?t, functor_t f)
  If t is a compound term with the given functor, just succeed. If it is unbound, create a term and bind the variable, else fail. Note that this function does not create a term if the argument is already instantiated. If f is a functor with arity 0, t is unified with compound without arguments. See also PL_unify_functor().
int PL_unify_list(term_t ?l, term_t -h, term_t -t)
  Unify l with a list-cell (`./2`). If successful, write a reference to the head of the list into h and a reference to the tail of the list into t. This reference to h may be used for subsequent calls to this function. Suppose we want to return a list of atoms from a `char **`. We could use the example described by PL_cons_list(), followed by a call to PL_unify(), or we can use the code below. If the predicate argument is unbound, the difference is minimal (the code based on PL_cons_list() is probably slightly faster). If the argument is bound, the code below may fail before reaching the end of the word list, but even if the unification succeeds, this code avoids a duplicate (garbage) list and a deep unification.

  Note that PL_unify_list() is not used with env but with `tail`, which is a copy of env. PL_copy_term_ref() creates a copy `term_t` holding the same Prolog term, i.e., not a copy of the Prolog term. The only thing that is allowed to be done with an argument to a foreign predicate (such as env) is unification; for anything that might over-write the term, you must use a copy created by PL_copy_term_ref(). The name PL_unify_list() is slightly misleading - it unifies the first argumment (l but overwrites the second (h) and third (t) arguments.

      foreign_t
      pl_get_environ(term_t env)
      { term_t tail = PL_copy_term_ref(env);
        term_t item = PL_new_term_ref();
        extern char **environ;

        for(const char **e = environ; *e; e++)
        { if ( !PL_unify_list(tail, item, tail) ||
               !PL_unify_atom_chars(item, *e) )
            PL_fail;
        }

        return PL_unify_nil(tail);
      }

  In this example, `item` is initialized outside the loop. This allocates a single new reference to a term, which is used as a temporary inside the loop - there is no need to allocate a new reference each time around the loop because the `item` term reference can be reused and the call to PL_unify_list() copies a reference to the new list cell's head into the the term referenced by `item`.

int PL_unify_nil(term_t ?l)
  Unify l with the atom ``[]``.
int PL_unify_arg(int index, term_t ?t, term_t ?a)
  Unifies the index-th argument (1-based) of t with a.
int PL_unify_term(term_t ?t, ...)
  Unify t with a (normally) compound term. The remaining arguments are a sequence of a type identifier followed by the required arguments. This predicate is an extension to the Quintus and SICStus foreign interface from which the SWI-Prolog foreign interface has been derived, but has proved to be a powerful and comfortable way to create compound terms from C. Due to the vararg packing/unpacking and the required type-switching this interface is slightly slower than using the primitives. Please note that some bad C compilers have fairly low limits on the number of arguments that may be passed to a function.

  Special attention is required when passing numbers. C‘promotes' any integral smaller than `int` to `int`. That is, the types `char`, `short` and `int` are all passed as `int`. In addition, on most 32-bit platforms `int` and `long` are the same. Up to version 4.0.5, only `PL_INTEGER` could be specified, which was taken from the stack as `long`. Such code fails when passing small integral types on machines where `int` is smaller than `long`. It is advised to use `PL_SHORT`, `PL_INT` or `PL_LONG` as appropriate. Similarly, C compilers promote `float` to `double` and therefore `PL_FLOAT` and `PL_DOUBLE` are synonyms.

  The type identifiers are:

  `PL_VARIABLE` none
    No op. Used in arguments of `PL_FUNCTOR`.
  `PL_BOOL` int
    Unify the argument with `true` or `false`.
  `PL_ATOM` atom_t
    Unify the argument with an atom, as in PL_unify_atom().
  `PL_CHARS` const char *
    Unify the argument with an atom constructed from the C `char *`, as in PL_unify_atom_chars().
  `PL_NCHARS` size_t, const char *
    Unify the argument with an atom constructed from length and `char*` as in PL_unify_atom_nchars().
  `PL_UTF8_CHARS` const char *
    Create an atom from a UTF-8 string.
  `PL_UTF8_STRING` const char *
    Create a packed string object from a UTF-8 string.
  `PL_MBCHARS` const char *
    Create an atom from a multi-byte string in the current locale.
  `PL_MBCODES` const char *
    Create a list of character codes from a multi-byte string in the current locale.
  `PL_MBSTRING` const char *
    Create a packed string object from a multi-byte string in the current locale.
  `PL_NWCHARS` size_t, const wchar_t *
    Create an atom from a length and a wide character pointer.
  `PL_NWCODES` size_t, const wchar_t *
    Create a list of character codes from a length and a wide character pointer.
  `PL_NWSTRING` size_t, const wchar_t *
    Create a packed string object from a length and a wide character pointer.
  `PL_SHORT` short
    Unify the argument with an integer, as in PL_unify_integer(). As `short` is promoted to `int`, `PL_SHORT` is a synonym for `PL_INT`.
  `PL_INTEGER` long
    Unify the argument with an integer, as in PL_unify_integer().
  `PL_INT` int
    Unify the argument with an integer, as in PL_unify_integer().
  `PL_LONG` long
    Unify the argument with an integer, as in PL_unify_integer().
  `PL_INT64` int64_t
    Unify the argument with a 64-bit integer, as in PL_unify_int64().
  `PL_INTPTR` intptr_t
    Unify the argument with an integer with the same width as a pointer. On most machines this is the same as `PL_LONG`. but on 64-bit MS-Windows pointers are 64 bits while longs are only 32 bits.
  `PL_DOUBLE` double
    Unify the argument with a float, as in PL_unify_float(). Note that, as the argument is passed using the C vararg conventions, a float must be casted to a double explicitly.
  `PL_FLOAT` double
    Unify the argument with a float, as in PL_unify_float().
  `PL_POINTER` void *
    Unify the argument with a pointer, as in PL_unify_pointer().
  `PL_STRING` const char *
    Unify the argument with a string object, as in PL_unify_string_chars().
  `PL_TERM` term_t
    Unify a subterm. Note this may be the return value of a PL_new_term_ref() call to get access to a variable.
  `PL_FUNCTOR` functor_t, ...
    Unify the argument with a compound term. This specification should be followed by exactly as many specifications as the number of arguments of the compound term.
  `PL_FUNCTOR_CHARS` const char *name, int arity, ...
    Create a functor from the given name and arity and then behave as `PL_FUNCTOR`.
  `PL_LIST` int length, ...
    Create a list of the indicated length. The remaining arguments contain the elements of the list.

  For example, to unify an argument with the term `language(dutch)`, the following skeleton may be used:

      static functor_t FUNCTOR_language1;

      static void
      init_constants()
      { FUNCTOR_language1 = PL_new_functor(PL_new_atom("language"),1);
      }

      foreign_t
      pl_get_lang(term_t r)
      { return PL_unify_term(r,
                             PL_FUNCTOR, FUNCTOR_language1,
                                 PL_CHARS, "dutch");
      }

      install_t
      install()
      { PL_register_foreign("get_lang", 1, pl_get_lang, 0);
        init_constants();
      }

int PL_chars_to_term(const char *chars, term_t -t)
  Parse the string chars and put the resulting Prolog term into t. chars may or may not be closed using a Prolog full-stop (i.e., a dot followed by a blank). Returns `FALSE` if a syntax error was encountered and `TRUE` after successful completion. In addition to returning `FALSE`, the exception-term is returned in t on a syntax error. See also term_to_atom/2.

  The following example builds a goal term from a string and calls it.

      int
      call_chars(const char *goal)
      { fid_t fid = PL_open_foreign_frame();
        term_t g = PL_new_term_ref();
        BOOL rval;

        if ( PL_chars_to_term(goal, g) )
          rval = PL_call(goal, NULL);
        else
          rval = FALSE;

        PL_discard_foreign_frame(fid);
        return rval;
      }
        ...
        call_chars("consult(load)");
        ...

  PL_chars_to_term() is defined using PL_put_term_from_chars() which can deal with not null-terminated strings as well as strings using different encodings:

      int
      PL_chars_to_term(const char *s, term_t t)
      { return PL_put_term_from_chars(t, REP_ISO_LATIN_1, (size_t)-1, s);
      }

int PL_wchars_to_term(const pl_wchar_t *chars, term_t -t)
  Wide character version of PL_chars_to_term().
char * PL_quote(int chr, const char *string)
  Return a quoted version of string. If chr is `'\''`, the result is a quoted atom. If chr is `'"'`, the result is a string. The result string is stored in the same ring of buffers as described with the `BUF_STACK` argument of PL_get_chars();

  In the current implementation, the string is surrounded by chr and any occurrence of chr is doubled. In the future the behaviour will depend on the character_escapes Prolog flag.

### 12.4.7 Convenient functions to generate Prolog exceptions

The typical implementation of a foreign predicate first uses the PL_get_*() functions to extract C data types from the Prolog terms. Failure of any of these functions is normally because the Prolog term is of the wrong type. The *_ex() family of functions are wrappers around (mostly) the PL_get_*() functions, such that we can write code in the style below and get proper exceptions if an argument is uninstantiated or of the wrong type. Section 12.4.8 documents an alternative API to fetch values for the C basic types.

    /** set_size(+Name:atom, +Width:int, +Height:int) is det.

    static foreign_t
    set_size(term_t name, term_t width, term_t height)
    { char *n;
      int w, h;

      if ( !PL_get_chars(name, &n, CVT_ATOM|CVT_EXCEPTION) ||
           !PL_get_integer_ex(with, &w) ||
           !PL_get_integer_ex(height, &h) )
        return FALSE;

      ...

    }

int PL_get_atom_ex(term_t t, atom_t *a)
  As PL_get_atom(), but raises a type or instantiation error if t is not an atom.
int PL_get_integer_ex(term_t t, int *i)
  As PL_get_integer(), but raises a type or instantiation error if t is not an integer, or a representation error if the Prolog integer does not fit in a C `int`.
int PL_get_long_ex(term_t t, long *i)
  As PL_get_long(), but raises a type or instantiation error if t is not an atom, or a representation error if the Prolog integer does not fit in a C `long`.
int PL_get_int64_ex(term_t t, int64_t *i)
  As PL_get_int64(), but raises a type or instantiation error if t is not an integer, or a representation error if the Prolog integer does not fit in a C `int64_t`.
int PL_get_uint64_ex(term_t t, uint64_t *i)
  As PL_get_uint64(), but raises a type, domain or instantiation error if t is not an integer or t is less than zero, or a representation error if the Prolog integer does not fit in a C `int64_t`.
int PL_get_intptr_ex(term_t t, intptr_t *i)
  As PL_get_intptr(), but raises a type or instantiation error if t is not an atom, or a representation error if the Prolog integer does not fit in a C `intptr_t`.
int PL_get_size_ex(term_t t, size_t *i)
  As PL_get_size(), but raises a type or instantiation error if t is not an atom, or a representation error if the Prolog integer does not fit in a C `size_t`.
int PL_get_bool_ex(term_t t, int *i)
  As PL_get_bool(), but raises a type or instantiation error if t is not a valid boolean value (`true`, `false`, `on`, constoff, `1` or `0`). Note that the pointer is to an `int` because C has no `bool` type.
int PL_get_float_ex(term_t t, double *f)
  As PL_get_float(), but raises a type or instantiation error if t is not a float.
int PL_get_char_ex(term_t t, int *p, int eof)
  Get a character code from t, where t is either an integer or an atom with length one. If eof is `TRUE` and t is -1, p is filled with -1. Raises an appropriate error if the conversion is not possible.
int PL_get_pointer_ex(term_t t, void **addrp)
  As PL_get_pointer(), but raises a type or instantiation error if t is not a pointer.
int PL_get_list_ex(term_t l, term_t h, term_t t)
  As PL_get_list(), but raises a type or instantiation error if t is not a list.
int PL_get_nil_ex(term_t l)
  As PL_get_nil(), but raises a type or instantiation error if t is not the empty list. Because PL_get_nil_ex() is commonly used after a `while` loop over PL_get_list_ex(), it fails immediately if there is an exception pending (from PL_get_list_ex()).
int PL_unify_list_ex(term_t l, term_t h, term_t t)
  As PL_unify_list(), but raises a type error if t is not a variable, list-cell or the empty list.
int PL_unify_nil_ex(term_t l)
  As PL_unify_nil(), but raises a type error if t is not a variable, list-cell or the empty list.
int PL_unify_bool_ex(term_t t, int val)
  As PL_unify_bool(), but raises a type error if t is not a variable or a boolean.

The second family of functions in this section simplifies the generation of ISO compatible error terms. Any foreign function that calls this function must return to Prolog with the return code of the error function or the constant `FALSE`. If available, these error functions add the name of the calling predicate to the error context. See also PL_raise_exception().

int PL_instantiation_error(term_t culprit)
  Raise `instantiation_error`. Culprit is ignored, but should be bound to the term that is insufficiently instantiated. See instantiation_error/1.
int PL_uninstantiation_error(term_t culprit)
  Raise `uninstantiation_error(culprit)`. This should be called if an argument that must be unbound at entry is bound to culprit. This error is typically raised for a pure output arguments such as a newly created stream handle (e.g., the third argument of open/3).
int PL_representation_error(const char *resource)
  Raise `representation_error(resource)`. See representation_error/1.
int PL_type_error(const char *expected, term_t culprit)
  Raise `type_error(expected, culprit)`. See type_error/2.
int PL_domain_error(const char *expected, term_t culprit)
  Raise `domain_error(expected, culprit)`. See domain_error/2.
int PL_existence_error(const char *type, term_t culprit)
  Raise `existence_error(type, culprit)`. See existence_error/2.
int PL_permission_error(const char *operation, const char *type, term_t culprit)
  Raise `permission_error(operation, type, culprit)`. See permission_error/3.
int PL_resource_error(const char *resource)
  Raise `resource_error(resource)`. See resource_error/1.
int PL_syntax_error(const char *message, IOSTREAM *in)
  Raise `syntax_error(message)`. If arg is not `NULL`, add information about the current position of the input stream.

### 12.4.8 Foreign language wrapper support functions

In addition to the functions described in section 12.4.4.2, there is a family of functions that is used for automatic generation of wrapper functions, for example using the Prolog library `library(qpforeign)` that provides a Quintus/SICStus compatible foreign language interface.

The PL_cvt_i_*() family of functions is suitable for use with a `_Generic` selector or C++ overloading.220`_Generic` needs to take into account that there's no `bool` type in C but there is in C++. An overloaded integer() method is provided in the C++ interface.

Note that the documentation on this API is incomplete. Also note that many of these functions are equivalent to the PL_get_*_ex() functions described in section 12.4.7.

int PL_cvt_i_bool(term_t p, int *c)
  Equivalent to PL_get_bool_ex(). Note that the pointer is to an `int` because C has no `bool` type. The return value is either `0` or `1`.
int PL_cvt_i_char(term_t p, char *c)
int PL_cvt_i_schar(term_t p, signed char *c)
int PL_cvt_i_uchar(term_t p, unsigned char *c)
int PL_cvt_i_short(term_t p, short *s)
int PL_cvt_i_ushort(term_t p, unsigned short *s)
int PL_cvt_i_int(term_t p, int *c)
int PL_cvt_i_uint(term_t p, unsigned int *c)
int PL_cvt_i_long(term_t p, long *c)
int PL_cvt_i_ulong(term_t p, unsigned long *c)
int PL_cvt_i_llong(term_t p, long long *c)
int PL_cvt_i_ullong(term_t p, unsigned long long *c)
int PL_cvt_i_int32(term_t p, int32_t *c)
int PL_cvt_i_uint32(term_t p, uint32_t *c)
int PL_cvt_i_int64(term_t p, int64_t *c)
int PL_cvt_i_uint64(term_t p, uint64_t *c)
int PL_cvt_i_size_t(term_t p, size_t *c)
  Convert a Prolog integer into a C integer of the specified size. Generate an exception and return `FALSE` if the conversion is impossible because the Prolog term is not an integer or the C type cannot represent the value of the Prolog integer.

### 12.4.9 Serializing and deserializing Prolog terms

int PL_put_term_from_chars(term_t t, int flags, size_t len, const char *s)
  Parse the text from the C-string s holding len bytes and put the resulting term in t. len can be `(size_t)-1`, assuming a 0-terminated string. The flags argument controls the encoding and is currently one of `REP_UTF8` (string is UTF8 encoded), `REP_MB` (string is encoded in the current locale) or 0 (string is encoded in ISO latin 1). The string may, but is not required, to be closed by a full stop (.).

  If parsing produces an exception; the behaviour depends on the `CVT_EXCEPTION` flag. If present, the exception is propagated into the environment. Otherwise, the exception is placed in t and the return value is `FALSE`.221The `CVT_EXCEPTION` was added in version 8.3.12.

### 12.4.10 BLOBS: Using atoms to store arbitrary binary data

SWI-Prolog atoms as well as strings can represent arbitrary binary data of arbitrary length. This facility is attractive for storing foreign data such as images in an atom. An atom is a unique handle to this data and the atom garbage collector is able to destroy atoms that are no longer referenced by the Prolog engine. This property of atoms makes them attractive as a handle to foreign resources, such as Java atoms, Microsoft's COM objects, etc., providing safe combined garbage collection.

To exploit these features safely and in an organised manner, the SWI-Prolog foreign interface allows creating‘atoms' with additional type information. The type is represented by a structure holding C function pointers that tell Prolog how to handle releasing the atom, writing it, sorting it, etc. Two atoms created with different types can represent the same sequence of bytes. Atoms are first ordered on the rank number of the type and then on the result of the compare() function. Rank numbers are assigned when the type is registered. This implies that the results of inequality comparisons between blobs of different types is undefined and can change if the program is run twice (the ordering within a blob type will not change, of course).

While the blob is alive, neither its handle nor the location of the contents (see PL_blob_data()) change. If the blob's type has the `PL_BLOB_UNIQUE` feature, the content of the blob must remain unmodified. If the blob's type does not have the `PL_BLOB_UNIQUE` feature, then none of the instances will be equal. Blobs are only reclaimed by the atom garbage collector. In this case the atom garbage collector calls the release() function associated with the blob type and reclaims the memory allocated for the content unless this is owned by the creator of the blob indicated by the `PL_BLOB_NOCOPY` flag. After an `atom_t` value is reclaimed by the atom garbage collector, the value may be reused for allocating a new blob or atom.

If foreign code stores the `atom_t` handle in some permanent location it must make sure the handle is registered to prevent it from being garbage collected. If the handle is obtained from a `term_t` object it is not registered because it is protected by the `term_t` object. This applies to e.g., PL_get_atom(). Functions that create a handle from data, such as PL_new_atom(), return a registered handle to prevent the asynchronous atom garbage collector from reclaiming it immediately. Note that many of the API functions create an atom or blob handle and use this to fill a `term_t` object, e.g., PL_unify_blob(), PL_unify_chars(), etc. In this scenario the handle is protected by the `term_t` object. Registering and unregistering `atom_t` handles is done by PL_register_atom() and PL_unregister_atom().

Note that during program shutdown using PL_cleanup(), all atoms and blobs are reclaimed as described above. These objects are reclaimed regardless of their registration count. The order in which the atoms or blobs are reclaimed under PL_cleanup() is undefined. However, when these objects are reclaimed using garbage_collect_atoms/0, registration counts are taken into account.

#### 12.4.10.1 Defining a BLOB type

The type `PL_blob_t` represents a structure with the layout displayed below. The structure contains additional fields at the ... for internal bookkeeping as well as future extensions.

    typedef struct PL_blob_t
    { uintptr_t     magic;          /* PL_BLOB_MAGIC */
      uintptr_t     flags;          /* Bitwise or of PL_BLOB_* */
      const char *  name;           /* name of the type */
      int           (*release)(atom_t a);
      int           (*compare)(atom_t a, atom_t b);
      int           (*write)(IOSTREAM *s, atom_t a, int flags);
      void          (*acquire)(atom_t a);
      int           (*save)(atom_t a, IOSTREAM *s);
      atom_t        (*load)(IOSTREAM *s);
      ...
    } PL_blob_t;

For each type, exactly one such structure should be allocated and must not be moved because the address of the structure determines the blob's "type". Its first field must be initialised to `PL_BLOB_MAGIC`. If a blob type is registered from a loadable object (shared object or DLL) the blob type must be deregistered using PL_unregister_blob_type() before the object may be released.

The flags is a bitwise or of the following constants:

PL_BLOB_TEXT
  If specified, the blob is assumed to contain text and is considered a normal Prolog atom. The (currently) two predefined blob types that represent atoms have this flag set. User-defined blobs may not specify this, even if they contain only text. Applications should not use the blob API to create normal text atoms or get access to the text represented by normal text atoms. Most applications should use PL_get_nchars() and PL_unify_chars() to get text from Prolog terms or create Prolog terms that represent text.
PL_BLOB_UNIQUE
  If specified the system ensures that the blob-handle is a unique reference for a blob with the given type, length and content. If this flag is not specified, each lookup creates a new blob. Uniqueness is determined by comparing the bytes in the blobs unless `PL_BLOB_NOCOPY` is also specified, in which case the pointers are compared. Note that the lookup does not use the blob's compare function when testing for equality, but only tests the bytes; this means that terms from the recorded database or C++-style strings will typically not compare as equal when doing blob lookup.
PL_BLOB_NOCOPY
  By default the content of the blob is copied. Using this flag the blob references the external data directly. The user must ensure the provided pointer is valid as long as the atom lives. If `PL_BLOB_UNIQUE` is also specified, uniqueness is determined by comparing the pointer rather than the data pointed at. Using `PL_BLOB_UNIQUE``|``PL_BLOB_NOCOPY` can be used to make a blob reference an arbitrary pointer where the pointer data may be reclaimed in the release() handler.
PL_BLOB_WCHAR
  If `PL_BLOB_TEXT` is also set, then the text is made up of `pl_wchar_t` items and the blob's lenght is the number of bytes (that is, the number of characters times `sizeof(pl_wchar_t)`). As `PL_BLOB_TEXT`, this flag should not be set in user-defined blobs.

The name field represents the type name as available to Prolog. See also current_blob/2. The other fields are function pointers that must be initialised to proper functions or `NULL` to get the default behaviour of built-in atoms. Below are the defined member functions:

void acquire(atom_t a)
  Called if a new blob of this type is created through PL_put_blob(), PL_unify_blob(), or PL_new_blob(). Note this this call is done as part of creating the blob. The call to PL_unify_blob() may fail if the unification fails or cannot be completed due to a resource error. PL_put_blob() has no such error conditions. This callback is typically used to store the `atom_t` handle into the content of the blob. Given a pointer to the content, we can now use PL_unify_atom() to bind a Prolog term with a reference to the pointed to object. If the content of the blob can be modified (`PL_BLOB_UNIQUE` is not present) this is the only way to get access to the `atom_t` handle that belongs to this blob. If `PL_BLOB_UNIQUE` is provided and respected, PL_unify_blob() given the same pointer and length will produce the same `atom_t` handle.
int release(atom_t a)
  The blob (atom) a is about to be released. This function can retrieve the data of the blob using PL_blob_data(). If it returns `FALSE`, the atom garbage collector will not reclaim the atom. The release() function is called when the atom is reclaimed by the atom garbage collector. For critical resources such as file handles or significant memory resources it may be desirable to have an explicit call to dispose (most of) the resources. For example, close/1 reclaims the file handle and most of the resources associated with a stream, leaving only a tiny bit of content to the garbage collector. See also setup_call_cleanup/3.

  The release() callback is called in the context of the thread executing the atom garbage collect. Normally the thread `gc` runs all atom and clause garbage collections. The release() function may not call any of the PL_*() functions except for PL_unregister_atom() to unregister other atoms that are part data associated to the blob. Calling any of the other PL_* functions may return in deadlocks or crashes. The release() function should not call any potentially slow or blocking functions as this may cause serious slowdowns in the rest of the system.

  Blobs that require cleanup that is slow, blocking or requires calling Prolog must pass the data to be cleaned to another thread. Be aware that if the blob uses `PL_BLOB_NOCOPY` the user is responsible for discarding the data, otherwise the atom garbage collector will free the data.

  As SWI-Prolog atom garbage collector is conservative, there is no guarantee that the release() function will ever be called. If it is important to clean up some resource, there should be an explicit predicate for doing that, and calling that predicate should be guaranteed by using setup_call_cleanup/3 or some a process finalization hook such as at_halt/1.

  Normally, Prolog does not clean memory during shutdown. It does so on an explicit call to PL_cleanup().222Or if the system is compiled with the cmake build type `Debug`. In such a situation, there is no guarantee of the order in which atoms are released; if a blob contains an atom (or another blob), those atoms (or blobs) may have already been released. See also PL_blob_data().

int compare(atom_t a, atom_t b)
  Compare the blobs a and b, both of which are of the type associated to this blob type. Return values are as memcmp(): < 0 if a is less than b, = 0 if both are equal, and > 0 otherwise. The default implementation is a bitwise comparison of the blobs' contents. This default implementation suffices if `PL_BLOB_UNIQUE` is set and the blob follows the requirement that its contents do not change, although it might give an unexpected ordering, and the ordering may change if the blob is saved and restored using save_program/2.

  If the compare() function is defined, the sort/2 predicate uses that to determine if two blobs are equal and only keeps one of them. This can cause unexpected results with blobs that are actually different; if you cannot guarantee that the blobs all have unique contents, then you should incorporate the blob address (the system guarantees that blobs are not shifted in memory after they are allocated). This function should not call any PL_*() functions other than PL_blob_data().

  The following minimal compare function gives a stable total ordering:

      static int
      compare_my_blob(atom_t a, atom_t b)
      { const struct my_blob_data *blob_a = PL_blob_data(a, NULL, NULL);
        const struct my_blob_data *blob_b = PL_blob_data(b, NULL, NULL);
        return (blob_a < blob_b) ? -1 : (blob_a > blob_b) ? 1 : 0;
      }

int write(IOSTREAM *s, atom_t a, int flags)
  Write the content of the blob a to the stream s respecting the flags. The return value is `TRUE` or `FALSE` and does not follow the Unix convention of the number of bytes (where zero is possible) and negative for errors. Any I/O operations to s are in the context of a PL_acquire_stream(); upon return, the PL_release_stream() handles any errors, so it is safe to not check return codes from Sfprintf(), etc.

  In general, the output from the write() callback should be minimal. If you wish to output more debug information, it is suggested that you either add a debug option to your "open" predicate to output more information, or provide a "properties" predicate. A typical implementation is:

      static int write_my_blob(IOSTREAM *s, atom_t symbol, int flags)
      { (void)flags; /* unused */
        Sfprintf(s, "<my_blob>(%p)", PL_blob_data(symbol, NULL, NULL));
        return TRUE;
      }

  The flags are a bitwise or of zero or more of the `PL_WRT_*` flags that were passed in to the calling PL_write_term() that called write(), and are defined in `SWI-Prolog.h`. The flags do not have the `PL_WRT_NEWLINE` bit set, so it is safe to call PL_write_term() and there is no need for writing a trailing newline. This prototype is available if the `SWI-Stream.h` is included before `SWI-Prolog.h`. This function can retrieve the data of the blob using PL_blob_data().

  Most blobs reference some external data identified by a pointer and the write() function writes `<`type`>(`address`)`. If this function is not provided, write/1 emits the content of the blob for blobs of type `PL_BLOB_TEXT` or a string of the format `<#`hex data`>` for binary blobs.

int save(atom_t a, IOSTREAM *s)
  Write the blob to stream s, in an opaque form that is known only to the blob. If a “save'' function is not provided (that is, the field is `NULL`), the default implementation saves and restores the blob as if it is an array of bytes which may contain null (`’
  0'`) bytes.

  `SWI-Stream.h` defines a number of PL_qlf_put_*() functions that write data in a machine-independent form that can be read by the corresponding PL_qlf_get_*() functions.

  If the “save'' function encounters an error, it should call PL_warning(), raise an exception (see PL_raise_exception()), and return `FALSE`.223Details are subject to change. Note that failure to save/restore a blob makes it impossible to compile a file that contains such a blob using qcompile/2 as well as creating a saved state from a program that contains such a blob impossible. Here, contains means that the blob appears in a clause or directive.

atom_t load(IOSTREAM *s)
  Read the blob from its saved form as written by the “save'' function of the same blob type. If this cannot be done (e.g., a stream read failure or a corrupted external form), the “load'' function should call PL_warning(), then PL_fatal_error(), and return constFALSE.224Details are subject to change; see the “save'' function. If a “load'' function is not provided (that is, the field is `NULL`, the default implementation assumes that the blob was written by the default “save'' - that is, as an array of bytes

  `SWI-Stream.h` defines a number of PL_qlf_get_*() functions that read data in a machine-independent form, as written by the by the corresponding PL_qlf_put_*() functions.

  The atom that the “load'' function returns can be created using PL_new_blob().

int PL_unregister_blob_type(PL_blob_t *type)
  Unlink the blob type from the registered type and transform the type of possible living blobs to `unregistered`, avoiding further reference to the type structure, functions referred by it, as well as the data. This function returns `TRUE` if no blobs of this type existed and `FALSE` otherwise. PL_unregister_blob_type() is intended for the uninstall() hook of foreign modules, avoiding further references to the module.
int PL_register_blob_type(PL_blob_t *type)
  This function does not need to be called explicitly. It is called if needed when a blob is created by PL_unify_blob(), PL_put_blob(), or PL_new_blob().

#### 12.4.10.2 Accessing blobs

The blob access functions are similar to the atom accessing functions. Blobs being atoms, the atom functions operate on blobs and vice versa. For clarity and possible future compatibility issues, however, it is not advised to rely on this.

int PL_is_blob(term_t t, PL_blob_t **type)
  Succeeds if t refers to a blob, in which case type is filled with the type of the blob.
int PL_unify_blob(term_t t, void *blob, size_t len, PL_blob_t *type)
  Unify t to a blob constructed from the given data and associated with the given type. This performs the following steps:

  1. If the type has `PL_BLOB_UNIQUE` set, search the blob database for a blob of the same type with the same content. If found, unify t with the existing handle.
  2. If not found or `PL_BLOB_UNIQUE` is not set, create a new blob handle. If `PL_BLOB_NOCOPY` is set, associate it to the given memory; else, copy the memory to a new area owned by the blob. Call the acquire() function of the type.
  3. Unify t with the existing or new handle. This succeeds if t is already bound to the existing blob handle. If t is a variable, it succeeds if sufficient resources are available to perform the unification; if t is bound to something else, this fails.

  It is possible that a blob referencing critial resources is created after which the unification fails. Typically these resources are eventually reclaimed because the new blob is not referenced and reclaimed by the atom garbage collector. As described with the release() function, it can be desirable to reclaim the critical resources after the failing PL_unify_blob() call.

int PL_put_blob(term_t t, void *blob, size_t len, PL_blob_t *type)
  Store the described blob in t. The return value indicates whether a new blob was allocated (`FALSE`) or the blob is a reference to an existing blob (`TRUE`). Reporting new/existing can be used to deal with external objects having their own reference counts. If the return is `TRUE` this reference count must be incremented, and it must be decremented on blob destruction callback. See also PL_put_atom_nchars().
atom_t PL_new_blob(void *blob, size_t len, PL_blob_t *type)
  Create a blob from its internal opaque form. This function is intended for the “load'' function of a blob.
int PL_get_blob(term_t t, void **blob, size_t *len, PL_blob_t **type)
  If t holds a blob or atom, get the data and type and return `TRUE`. Otherwise return `FALSE`. Each result pointer may be `NULL`, in which case the requested information is ignored.
void * PL_blob_data(atom_t a, size_t *len, PL_blob_t **type)

  Get the data and type associated to a blob. This function is mainly used from the callback functions described in section 12.4.10.1. Note that if the release() hook is called from PL_cleanup(), blobs are released regardless of whether or not they are referenced and the order in which blobs are released is undefined (the order depends on the ordering in the atom hash table). PL_blob_data() may be called safely on a blob that has already been released. If this happens during PL_cleanup() the return value is guaranteed to be `NULL`. During normal execution it may return the content of a newly allocated blob that reuses the released handle.

int PL_free_blob(atom_t blob)
  New in 9.1.12. This function may be used on blobs with the `PL_BLOB_NOCOPY` flag set and a release() function. It causes the release() function to be called, after which the data and size are set to 0. The related atom remains existent and accessing it as a blob returns `NULL` for the data and 0 for the size.

#### 12.4.10.3 Considerations for non-C code

The blob API assumes that Prolog will take care of memory management, using the release(c)allback to handle any cleanup.

Other programming languages have their own memory management, which might not fit nicely with the Prolog memory management. For more details on blobs written with C++, see C++ interface to SWI-Prolog (Version 2).

### 12.4.11 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 number represented by mpq and return `TRUE` on success. Note that t is unified with an integer if the denominator is 1. The mpq argument is not changed.

### 12.4.12 Calling Prolog from C

The Prolog engine can be called from C. There are two interfaces for this. For the first, a term is created that could be used as an argument to call/1, and then PL_call() is used to call Prolog. This system is simple, but does not allow to inspect the different answers to a non-deterministic goal and is relatively slow as the runtime system needs to find the predicate. The other interface is based on PL_open_query(), PL_next_solution(), and PL_cut_query() or PL_close_query(). This mechanism is more powerful, but also more complicated to use.

#### 12.4.12.1 Predicate references

This section discusses the functions used to communicate about predicates. Though a Prolog predicate may be defined or not, redefined, etc., a Prolog predicate has a handle that is neither destroyed nor moved. This handle is known by the type `predicate_t`.

predicate_t PL_pred(functor_t f, module_t m)
  Return a handle to a predicate for the specified name/arity in the given module. If the module argument m is `NULL`, the current context module is used. If the target predicate does not exist a handle to a new undefined predicate is returned. The predicate may fail, returning `(predicate_t)0` after setting a resource exception, if the target module has a limit on the `program_space`, see set_module/1. Currently aborts the process with a fatal error when out of memory. Future versions may raise a resource exception and return `(predicate_t)0`.
predicate_t PL_predicate(const char *name, int arity, const char* module)
  Same as PL_pred(), but provides a more convenient interface to the C programmer. If the module argument module is `NULL`, the current context module is used. The `predicate_t` handle may be stored as global data and reused for future queries225PL_predicate() involves 5 hash lookups (two to get the atoms, one to get the module, one to get the functor and the final one to get the predicate associated with the functor in the module) as illustrated below.

      static predicate_t p = 0;

        ...
        if ( !p )
          p = PL_predicate("is_a", 2, "database");

  Note that PL_cleanup() invalidates the predicate handle. Foreign libraries that use the above mechanism must implement the module uninstall() function to clear the `predicate_t` global variable.

int PL_predicate_info(predicate_t p, atom_t *n, size_t *a, module_t *m)
  Return information on the predicate p. The name is stored over n, the arity over a, while m receives the definition module. Note that the latter need not be the same as specified with PL_predicate(). If the predicate is imported into the module given to PL_predicate(), this function will return the module where the predicate is defined. Any of the arguments n, a and m can be `NULL`. Currently always returns `TRUE`.

#### 12.4.12.2 Initiating a query from C

This section discusses the functions for creating and manipulating queries from C. Note that a foreign context can have at most one active query. This implies that it is allowed to make strictly nested calls between C and Prolog (Prolog calls C, calls Prolog, calls C, etc.), but it is not allowed to open multiple queries and start generating solutions for each of them by calling PL_next_solution(). Be sure to call PL_cut_query() or PL_close_query() on any query you opened before opening the next or returning control back to Prolog. Failure to do so results in "undefined behavior" (typically, a crash).

qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t +t0)

  Opens a query and returns an identifier for it. ctx is the context module of the goal. When `NULL`, the context module of the calling context will be used, or `user` if there is no calling context (as may happen in embedded systems). Note that the context module only matters for meta-predicates. See meta_predicate/1, context_module/1 and module_transparent/1. The term reference t0 is the first of a vector of term references as returned by PL_new_term_refs(n). Raise a resource exception and returns `(qid_t)0` on failure.

  Every use of PL_open_query() must have a corresponding call to PL_cut_query() or PL_close_query() before the foreign predicate returns either `TRUE` or `FALSE`.

  The flags arguments provides some additional options concerning debugging and exception handling. It is a bitwise or of the following values below. Note that exception propagation is defined by the flags `PL_Q_NORMAL`, `PL_Q_CATCH_EXCEPTION` and `PL_Q_PASS_EXCEPTION`. Exactly one of these flags must be specified (if none of them is specified, the behavior is as if `PL_Q_NODEBUG` is specified)..

  `PL_Q_NORMAL`
    Normal operation. It is named "normal" because it makes a call to Prolog behave as it did before exceptions were implemented, i.e., an error (now uncaught exception) triggers the debugger. See also the Prolog flag debug_on_error. This mode is still useful when calling Prolog from C if the C code is not willing to handle exceptions.
  `PL_Q_NODEBUG`
    Switch off the debugger while executing the goal. This option is used by many calls to hook-predicates to avoid tracing the hooks. An example is print/1 calling portray/1 from foreign code. This is the default if flags is `0`.
  `PL_Q_CATCH_EXCEPTION`
    If an exception is raised while executing the goal, make it available by calling `PL_exception(qid)`, where `qid` is the `qid_t` returned by PL_open_query(). The exception is implicitly cleared from the environment when the query is closed and the exception term returned from `PL_exception(qid)` becomes invalid. Use `PL_Q_PASS_EXCEPTION` if you wish to propagate the exception.
  `PL_Q_PASS_EXCEPTION`
    As `PL_Q_CATCH_EXCEPTION`, making the exception on the inner environment available using `PL_exception(0)` in the parent environment. If PL_next_solution() returns `FALSE`, you must call PL_cut_query() or PL_close_query(). After that you may verify whether failure was due to logical failure of the called predicate or an exception by calling `PL_exception(0)`. If the predicate failed due to an exception you should return with `FALSE` from the foreign predicate or call PL_clear_exception() to clear it. If you wish to process the exception in C, it is advised to use `PL_Q_CATCH_EXCEPTION` instead, but only if you have no need to raise an exception or re-raise the caught exception.

    Note that `PL_Q_PASS_EXCEPTION` is used by the debugger to decide whether the exception is caught. If there is no matching catch/3 call in the current query and the query was started using `PL_Q_PASS_EXCEPTION` the debugger searches the parent queries until it either finds a matching catch/3, a query with `PL_Q_CATCH_EXCEPTION` (in which case it considers the exception handled by C) or the top of the query stack (in which case it considers the exception uncaught). Uncaught exceptions use the `library(library(prolog_stack))` to add a backtrace to the exception and start the debugger as soon as possible if the Prolog flag debug_on_error is `true`.

  `PL_Q_ALLOW_YIELD`
    Support the `I_YIELD` instruction for engine-based coroutining. See $engine_yield/2 in `boot/init.pl` for details.
  `PL_Q_EXT_STATUS`
    Make PL_next_solution() return extended status. Instead of only `TRUE` or `FALSE` extended status as illustrated in the following table:

    Extended | Normal |
    PL_S_EXCEPTION | FALSE | Exception available through PL_exception()
    PL_S_FALSE | FALSE | Query failed
    PL_S_TRUE | TRUE | Query succeeded with choicepoint
    PL_S_LAST | TRUE | Query succeeded without choicepoint

  PL_open_query() can return the query identifier `0` if there is not enough space on the environment stack (and makes the exception available through `PL_exception(0)`). This function succeeds, even if the referenced predicate is not defined. In this case, running the query using PL_next_solution() may return an existence_error. See PL_exception().

  The example below opens a query to the predicate is_a/2 to find the ancestor of‘me'. The reference to the predicate is valid for the duration of the process or until PL_cleanup() is called (see PL_predicate() for details) and may be cached by the client.

      char *
      ancestor(const char *me)
      { term_t a0 = PL_new_term_refs(2);
        static predicate_t p;

        if ( !p )
          p = PL_predicate("is_a", 2, "database");

        PL_put_atom_chars(a0, me);
        PL_open_query(NULL, PL_Q_PASS_EXCEPTION, p, a0);
        ...
      }

int PL_next_solution(qid_t qid)
  Generate the first (next) solution for the given query. The return value is `TRUE` if a solution was found, or `FALSE` to indicate the query could not be proven. This function may be called repeatedly until it fails to generate all solutions to the query. The return value `PL_S_NOT_INNER` is returned if qid is not the innermost query.

  If the PL_open_query() had the flag `PL_Q_EXT_STATUS`, there are additional return values (see section 12.4.1.2).

int PL_cut_query(qid_t qid)
  Discards the query, but does not delete any of the data created by the query. It just invalidates qid, allowing for a new call to PL_open_query() in this context. PL_cut_query() may invoke cleanup handlers (see setup_call_cleanup/3) and therefore may experience exceptions. If an exception occurs the return value is `FALSE` and the exception is accessible through `PL_exception(0)`.

  An example of a handler that can trigger an exception in PL_cut_query() is:

      test_setup_call_cleanup(X) :-
          setup_call_cleanup(
              true,
              between(1, 5, X),
              throw(error)).

  where PL_next_solution() returns `TRUE` on the first result and the `throw(error)` will only run when PL_cut_query() or PL_close_query() is run. On the other hand, if the goal in setup_call_cleanup/3 has completed (failure, exception, determinitic success), the cleanup handler will have done its work before control gets back to Prolog and therefore PL_next_solution() will have generated the exception. The return value `PL_S_NOT_INNER` is returned if qid is not the innermost query.

int PL_close_query(qid_t qid)
  As PL_cut_query(), but all data and bindings created by the query are destroyed as if the query is called as `\+ \+ Goal`. This reduces the need for garbage collection, but also rewinds side effects such as setting global variables using b_setval/2. The return value `PL_S_NOT_INNER` is returned if qid is not the innermost query.
qid_t PL_current_query(void)
  Returns the query id of the current query or `0` if the current thread is not executing any queries.
PL_engine_t PL_query_engine(qid_t qid)
  Return the engine to which qid belongs. Note that interacting with a query or the Prolog terms associated with a query requires the engine to be current. See PL_set_engine().
int PL_call_predicate(module_t m, int flags, predicate_t pred, term_t +t0)
  Shorthand for PL_open_query(), PL_next_solution(), PL_cut_query(), generating a single solution. The arguments are the same as for PL_open_query(), the return value is the same as PL_next_solution().
int PL_call(term_t t, module_t m)
  Call term t just like the Prolog predicate once/1. t is called in the module m, or in the context module if m == NULL. Returns `TRUE` if the call succeeds, `FALSE` otherwise. If the goal raises an exception the return value is `FALSE` and the exception term is available using PL_exception(0).226Up to version 9.1.11 the debugger was started and the exception was not propagated. Figure 7 shows an example to obtain the number of defined atoms.

### 12.4.13 Discarding Data

The Prolog data created and term references needed to set up the call and/or analyse the result can in most cases be discarded right after the call. PL_close_query() allows for destroying the data, while leaving the term references. The calls below may be used to destroy term references and data. See figure 7 for an example.

fid_t PL_open_foreign_frame()

  Create a foreign frame, holding a mark that allows the system to undo bindings and destroy data created after it, as well as providing the environment for creating term references. This function is called by the kernel before calling a foreign predicate. Returns `(fid_t)0` on failure. Failure is either lack of space on the stacks, in which case a resource exception is scheduled or atom-gc being in progress in the current thread, in which case no exception is scheduled. The latter is an exceptional case that prevents doing a callback on Prolog from blob release handlers.227Such a callback would deadlock if the callback creates new atoms or requires stack shifts or garbage collection.

void PL_close_foreign_frame(fid_t id)
  Discard all term references created after the frame was opened. All other Prolog data is retained. This function is called by the kernel whenever a foreign function returns control back to Prolog.
void PL_discard_foreign_frame(fid_t id)
  Same as PL_close_foreign_frame(), but also undo all bindings made since the open and destroy all Prolog data.
void PL_rewind_foreign_frame(fid_t id)
  Undo all bindings and discard all term references created since the frame was created, but do not pop the frame. That is, the same frame can be rewound multiple times, and must eventually be closed or discarded.

It is obligatory to call either of the two closing functions to discard a foreign frame. Foreign frames may be nested.

    int
    count_atoms()
    { fid_t fid = PL_open_foreign_frame();
      term_t goal  = PL_new_term_ref();
      term_t a1    = PL_new_term_ref();
      term_t a2    = PL_new_term_ref();
      functor_t s2 = PL_new_functor(PL_new_atom("statistics"), 2);
      int atoms;

      PL_put_atom_chars(a1, "atoms");
      PL_cons_functor(goal, s2, a1, a2);
      PL_call(goal, NULL);         /* call it in current module */

      PL_get_integer(a2, &atoms);
      PL_discard_foreign_frame(fid);

      return atoms;
    }

Figure 7 : Calling Prolog

### 12.4.14 String buffering

Many of the functions of the foreign language interface involve strings. Some of these strings point into static memory like those associated with atoms. These strings are valid as long as the atom is protected against atom garbage collection, which generally implies the atom must be locked using PL_register_atom() or be part of an accessible term. Other strings are more volatile. Several functions provide a BUF_* flag that can be set to either `BUF_STACK` (default) or `BUF_MALLOC`. Strings returned by a function accepting `BUF_MALLOC` must be freed using PL_free(). Strings returned using `BUF_STACK` are pushed on a stack that is cleared when a foreign predicate returns control back to Prolog. More fine grained control may be needed if functions that return strings are called outside the context of a foreign predicate or a foreign predicate creates many strings during its execution. Temporary strings are scoped using these macros:

void PL_STRINGS_MARK()
void PL_STRINGS_RELEASE()
  These macros must be paired and create a C block ({...}). Any string created using `BUF_STACK` after PL_STRINGS_MARK() is released by the corresponding PL_STRINGS_RELEASE(). These macros should be used like below. Note that strings returned by any of the Prolog functions between this pair may be invalidated.

        ...
        PL_STRINGS_MARK();
          <operations involving strings>
        PL_STRINGS_RELEASE();
        ...

The Prolog flag string_stack_tripwire may be used to set a tripwire to help finding places where scoping strings may help reducing resources.

### 12.4.15 Foreign Code and Modules

Modules are identified via a unique handle. The following functions are available to query and manipulate modules.

module_t PL_context()
  Return the module identifier of the context module of the currently active foreign predicate. If there is no currently active predicate it returns a handle to the `user` module.
int PL_strip_module(term_t +raw, module_t *m, term_t -plain)
  Utility function. If raw is a term, possibly holding the module construct <module>`:`<rest>, this function will make plain a reference to <rest> and fill module * with <module>. For further nested module constructs the innermost module is returned via module *. If raw is not a module construct, raw will simply be put in plain. The value pointed to by m must be initialized before calling PL_strip_module(), either to the default module or to `NULL`. A `NULL` value is replaced by the current context module if raw carries no module. The following example shows how to obtain the plain term and module if the default module is the user module:

      { module m = PL_new_module(PL_new_atom("user"));
        term_t plain = PL_new_term_ref();

        PL_strip_module(term, &m, plain);
        ...
      }

  Returns `TRUE` on success and `FALSE` on error, leaving an exception. Currently the only exception condition is raw to be a cyclic term.

atom_t PL_module_name(module_t module)
  Return the name of module as an atom.
module_t PL_new_module(atom_t name)
  Find an existing module or create a new module with the name name. Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return `(module_t)0`.

### 12.4.16 Prolog exceptions in foreign code

This section discusses PL_exception() and PL_raise_exception(), the interface functions to detect and generate Prolog exceptions from C code. PL_raise_exception() from the C interface registers the exception term and returns `FALSE`. If a foreign predicate returns `FALSE`, while an exception term is registered, a Prolog exception will be raised by the virtual machine. This implies for a foreign function that implements a predicate and wishes to raise an exception, the function must call PL_raise_exception(), perform any necessary cleanup, and return the return code of PL_raise_exception() or explicitly `FALSE`. Calling PL_raise_exception() outside the context of a function implementing a foreign predicate results in undefined behaviour.

Note that many of the C API functions may call PL_raise_exception() and return `FALSE`. The user must test for this, cleanup, and make the foreign function return `FALSE`.

PL_exception() may be used to inspect the currently registered exception. It is normally called after a call to PL_next_solution() returns `FALSE`, and returns a term reference to an exception term if an exception is pending, and `(term_t)0` otherwise. It may also be called after, e.g., PL_unify() to distinguish a normal failing unification from a unification that raised an resource error exception. PL_exception() must only be called after a function such as PL_next_solution() or PL_unify() returns failure; if called elsewhere, the return value is undefined.

If a C function implementing a predicate that calls Prolog should use PL_open_query() with the flag `PL_Q_PASS_EXCEPTION` and make the function return FALSE if PL_next_solution() returns `FALSE` and PL_exception() indicates an exception is pending.

Both for C functions implementing a predicate and when Prolog is called while the main control of the process is in C, user code should always check for exceptions. As explained above, C functions implementing a predicate should normally cleanup and return with `FALSE`. If the C function whishes to continue it may call PL_clear_exception(). Note that this may cause any exception to be ignored, including time outs and abort. Typically the user should check the exception details before ignoring an exception (using `PL_exception(0)` or `PL_exception(qid)` as appropriate). If the C code does not implement a predicate it normally prints the exception and calls PL_clear_exception() to discard it. Exceptions may be printed by calling print_message/2 through the C interface.

int PL_raise_exception(term_t exception)
  Generate an exception (as throw/1) and return `FALSE`. If there is already a pending exception, the most urgent exception is kept; and if both are of the same urgency, the new exception is kept. Urgency of exceptions is defined as

  1. abort (`'$aborted'`).
  2. `time_limit_exceeded` (see call_with_time_limit/2).
  3. `resource_error` exceptions.
  4. Other `error(Formal, ImplDef)` exceptions.
  5. Other exceptions.

  This function is rarely used directly. Instead, errors are typically raised using the functions in section 12.4.7 or the C api functions that end in `_ex` such as PL_get_atom_ex(). Below we give an example returning an exception from a foreign predicate the verbose way. Note that the exception is raised in a sequence of actions connected using `&&`. This ensures that a proper exception is raised should any of the calls used to build or raise the exception themselves raise an exception. In this simple case PL_new_term_ref() is guaranteed to succeed because the system guarantees at least 10 available term references before entering the foreign predicate. PL_unify_term() however may raise a resource exception for the global stack.

      foreign_t
      pl_hello(term_t to)
      { char *s;

        if ( PL_get_atom_chars(to, &s) )
        { return Sfprintf(Scurrent_output, "Hello \"%s\"\n", s);
        } else
        { term_t except;

          return  ( (except=PL_new_term_ref()) &&
                    PL_unify_term(except,
                                  PL_FUNCTOR_CHARS, "type_error", 2,
                                    PL_CHARS, "atom",
                                    PL_TERM, to) &&
                    PL_raise_exception(except) );
        }
      }

  For reference, the preferred implementation of the above is below. The `CVT_EXCEPTION` tells the system to generate an exception if the conversion fails. The other `CVT_` flags define the admissible types and `REP_MB` requests the string to be provided in the current locale representation. This implies that Unicode text is printed correctly if the current environment can represent it. If not, a `representation_error` is raised.

      foreign_t
      pl_hello(term_t to)
      { char *s;

        if ( PL_get_chars(to, &s, CVT_ATOM|CVT_STRING|CVT_EXCEPTION|REP_MB) )
        { return Sfprintf(Scurrent_output, "Hello \"%s\"\n", s);
        }

        return FALSE;
      }

int PL_throw(term_t exception)
  Similar to PL_raise_exception(), but returns using the C longjmp() function to the innermost PL_next_solution(). This function is deprecated as it does not provide the opportunity to cleanup.
term_t PL_exception(qid_t qid)
  Return the pending exception. Exceptions may be raised by most of the API calls described in this chapter, a common possibility being `resource_error` exceptions. Some return `type_error` or `domain_error` exceptions. A call to Prolog using PL_next_solution() may return any exception, including those thrown by explicit calls to throw/1. If no exception is pending this function returns `(term_t)0`.

  Normally qid should be `0`. An explicit qid must be used after a call to PL_next_solution() that returns `FALSE` when the query was created using the `PL_Q_PASS_EXCEPTION` flag (see PL_open_query()).

  Note that an API may only raise an exception when it fails; if the API call succeeds, the result of `PL_exception(0)` will be 0.228Provided no exception was pending before calling the API function. As clients must deal with exceptions immediately after an API call raises one, this can not happen in a well behaved client. The implementation of a foreign predicate should normally cleanup and return `FALSE` after an exception is raised (and typically also after an API call failed for logical reasons; see PL_unify() for an elaboration on this topic). If the call to Prolog is not the implementation of a foreign predicate, e.g., when the overall process control is in some other language, exceptions may be printed by calling print_message/2 and should be discarded by calling PL_clear_exception().

void PL_clear_exception(void)
  Tells Prolog that the encountered exception must be ignored. This function must be called if control remains in C after a previous API call fails with an exception.229This feature is non-portable. Other Prolog systems (e.g., YAP) have no facilities to ignore raised exceptions, and the design of YAP's exception handling does not support such a facility. If there is no pending exception, PL_clear_exception() does nothing.

### 12.4.17 Catching Signals (Software Interrupts)

SWI-Prolog offers both a C and Prolog interface to deal with software interrupts (signals). The Prolog mapping is defined in section 4.12. This subsection deals with handling signals from C.

If a signal is not used by Prolog and the handler does not call Prolog in any way, the native signal interface routines may be used.

Any handler that wishes to call one of the Prolog interface functions should call PL_sigaction() to install the handler. PL_signal() provides a deprecated interface that is notably not capable of properly restoring the old signal status if the signal was previously handled by Prolog.

int PL_sigaction(int sig, pl_sigaction_t *act, pl_sigaction_t *oldact)
  Install or query the status for signal sig. The signal is an integer between 1 and 64, where the where the signals up to 32 are mapped to OS signals and signals above that are handled by Prolog's synchronous signal handling. The `pl_sigaction_t` is a struct with the following definition:

      typedef struct pl_sigaction
      { void        (*sa_cfunction)(int);     /* traditional C function */
        predicate_t sa_predicate;             /* call a predicate */
        int         sa_flags;                 /* additional flags */
      } pl_sigaction_t;

  The `sa_flags` is a bitwise or of `PLSIG_THROW`, `PLSIG_SYNC` and `PLSIG_NOFRAME`. Signal handling is enabled if `PLSIG_THROW` is provided, `sa_cfunction` or `sa_predicate` is provided. `sa_predicate` is a predicate handle for a predicate with arity 1. If no action is provided the signal handling for this signal is restored to the default before PL_initialise() was called.

  Finally, 0 (zero) may be passed for sig. In that case the system allocates a free signal in the Prolog range (32 ... 64). Such signal handler are activated using PL_thread_raise().

void (*)() PL_signal(sig, func)
  This function is equivalent to the BSD-Unix signal() function, regardless of the platform used. The signal handler is blocked while the signal routine is active, and automatically reactivated after the handler returns.

  After a signal handler is registered using this function, the native signal interface redirects the signal to a generic signal handler inside SWI-Prolog. This generic handler validates the environment, creates a suitable environment for calling the interface functions described in this chapter and finally calls the registered user-handler.

  By default, signals are handled asynchronously (i.e., at the time they arrive). It is inherently dangerous to call extensive code fragments, and especially exception related code from asynchronous handlers. The interface allows for synchronous handling of signals. In this case the native OS handler just schedules the signal using PL_raise(), which is checked by PL_handle_signals() at the call- and redo-port. This behaviour is realised by or-ing sig with the constant `PL_SIGSYNC`.230A better default would be to use synchronous handling, but this interface preserves backward compatibility.

  Signal handling routines may raise exceptions using PL_raise_exception(). The use of PL_throw() is not safe. If a synchronous handler raises an exception, the exception is delayed to the next call to PL_handle_signals();

int PL_raise(int sig)
  Register sig for synchronous handling by Prolog. Synchronous signals are handled at the call-port or if foreign code calls PL_handle_signals(). See also thread_signal/2.
int PL_handle_signals(void)
  Handle any signals pending from PL_raise(). PL_handle_signals() is called at each pass through the call- and redo-port at a safe point. Exceptions raised by the handler using PL_raise_exception() are properly passed to the environment.

  The user may call this function inside long-running foreign functions to handle scheduled interrupts. This routine returns the number of signals handled. If a handler raises an exception, the return value is -1 and the calling routine should return with `FALSE` as soon as possible.

int PL_get_signum_ex(term_t t, int *sig)
  Extract a signal specification from a Prolog term and store as an integer signal number in sig. The specification is an integer, a lowercase signal name without `SIG` or the full signal name. These refer to the same: `9`, `kill` and `SIGKILL`. Leaves a typed, domain or instantiation error if the conversion fails.

### 12.4.18 Miscellaneous

#### 12.4.18.1 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.

#### 12.4.18.2 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()).
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 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)
  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. Currently aborts the process with a fatal error on failure. Future versions may raise a resource exception and return `(char*)0`.

  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_record_external() will fail if the term contains blobs that cannot be serialized, such as streams.

  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.

#### 12.4.18.3 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);
      }

#### 12.4.18.4 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 filename as a wide-character string. This is intended for Windows to access the Unicode version of the Win32 API. Note that the flag `PL_FILE_OSPATH` must be provided to fetch a filename in OS native (e.g., `C:\x\y`) notation.

#### 12.4.18.5 Dealing with Prolog flags from C

Foreign code can set or create Prolog flags using PL_set_prolog_flag(). See set_prolog_flag/2 and create_prolog_flag/3. To retrieve the value of a flag you can use PL_current_prolog_flag().

int PL_set_prolog_flag(const char *name, int type, ...)
  Set/create a Prolog flag from C. name is the name of the affected flag. type is one of the values below, which also dictates the type of the final argument. The function returns `TRUE` on success and `FALSE` on failure. This function can be called before PL_initialise(), making the flag available to the Prolog startup code.
  `PL_BOOL`
    Create a boolean (`true` or `false`) flag. The argument must be an `int`.
  `PL_ATOM`
    Create a flag with an atom as value. The argument must be of type `const char *`.
  `PL_INTEGER`
    Create a flag with an integer as value. The argument must be of type `intptr_t *`.

int PL_current_prolog_flag(atom_t name, int type, void *value)
  Retrieve the value of a Prolog flag from C. name is the name of the flag as an `atom_t` (see current_prolog_flag/2). type specifies the kind of value to be retrieved, it is one of the values below. value is a pointer to a location where to store the value. The user is responsible for making sure this memory location is of the appropriate size/type (see the returned types below to determine the size/type). The function returns `TRUE` on success and `FALSE` on failure.
  `PL_ATOM`
    Retrieve a flag whose value is an `atom`. The returned value is an atom handle of type `atom_t`.
  `PL_INTEGER`
    Retrieve a flag whose value is an `integer`. The returned value is an integer of type `int64_t`.
  `PL_FLOAT`
    Retrieve a flag whose value is a `float`. The returned value is a floating point number of type `double`.
  `PL_TERM`
    Retrieve a flag whose value is a `term`. The returned value is a term handle of type `term_t`.

### 12.4.19 Errors and warnings

PL_warning() prints a standard Prolog warning message to the standard error (`user_error`) stream. Please note that new code should consider using PL_raise_exception() to raise a Prolog exception. See also section 4.10.

int PL_warning(format, a1, ...)
  Print an error message starting with‘`[WARNING:` ’, followed by the output from format, followed by a‘`]`’and a newline. Then start the tracer. format and the arguments are the same as for printf(2). Always returns `FALSE`.

### 12.4.20 Environment Control from Foreign Code

int PL_action(int, ...)
  Perform some action on the Prolog system. int describes the action. Remaining arguments depend on the requested action. The actions are listed below:
  PL_ACTION_TRACE
    Start Prolog tracer (trace/0). Requires no arguments.
  PL_ACTION_DEBUG
    Switch on Prolog debug mode (debug/0). Requires no arguments.
  PL_ACTION_BACKTRACE
    Print backtrace on current output stream. The argument (an `int`) is the number of frames printed.
  PL_ACTION_HALT
    Halt Prolog execution. This action should be called rather than Unix exit() to give Prolog the opportunity to clean up. This call does not return. The argument (an `int`) is the exit code. See halt/1.
  PL_ACTION_ABORT
    Generate a Prolog abort (abort/0). This call does not return. Requires no arguments.
  PL_ACTION_BREAK
    Create a standard Prolog break environment (break/0). Returns after the user types the end-of-file character. Requires no arguments.
  PL_ACTION_GUIAPP
    Windows: Used to indicate to the kernel that the application is a GUI application if the argument is not 0, and a console application if the argument is 0. If a fatal error occurs, the system uses a windows messagebox to report this on a GUI application, and otherwise simply prints the error and exits.
  PL_ACTION_TRADITIONAL
    Same effect as using --traditional. Must be called before PL_initialise().
  PL_ACTION_WRITE
    Write the argument, a `char *` to the current output stream.
  PL_ACTION_FLUSH
    Flush the current output stream. Requires no arguments.
  PL_ACTION_ATTACH_CONSOLE
    Attach a console to a thread if it does not have one. See attach_console/0.
  PL_GMP_SET_ALLOC_FUNCTIONS
    Takes an integer argument. If `TRUE`, the GMP allocations are immediately bound to the Prolog functions. If `FALSE`, SWI-Prolog will never rebind the GMP allocation functions. See mp_set_memory_functions() in the GMP documentation. The action returns `FALSE` if there is no GMP support or GMP is already initialised.
unsigned int PL_version_info(int key)
  Query version information. This function may be called before PL_initialise(). If the key is unknown the function returns 0. See section 2.22 for a more in-depth discussion on binary compatibility. Versions upto SWI-Prolog 8.5.2 defined this function as PL_version(). It was renamed to avoid a conflict with Perl affecting Yaswi. PL_version() is provided as a macro for compatibility. Defined keys are:
  PL_VERSION_SYSTEM
    SWI-Prolog version as 10,000 × major + 100 × minor + patch.
  PL_VERSION_FLI
    Incremented if the foreign interface defined in this chapter changes in a way that breaks backward compatibility.
  PL_VERSION_REC
    Incremented if the binary representation of terms as used by PL_record_external() and fast_write/2 changes.
  PL_VERSION_QLF
    Incremented if the QLF file format changes.
  PL_VERSION_QLF_LOAD
    Represents the oldest loadable QLF file format version.
  PL_VERSION_VM
    A hash that represents the VM instructions and their arguments.
  PL_VERSION_BUILT_IN
    A hash that represents the names, arities and properties of all built-in predicates defined in C. If this function is called before PL_initialise() it returns 0.

### 12.4.21 Querying Prolog

long PL_query(int)
  Obtain status information on the Prolog system. The actual argument type depends on the information required. int describes what information is wanted.231Returning pointers and integers as a long is bad style. The signature of this function should be changed. The options are given in table 9.

  > `PL_QUERY_ARGC` | Return an integer holding the number of arguments given to Prolog from Unix.
  > `PL_QUERY_ARGV` | Return a `char **` holding the argument vector given to Prolog from Unix.
  > `PL_QUERY_SYMBOLFILE` | Return a `char *` holding the current symbol file of the running process.
  > `PL_MAX_INTEGER` | Return a long, representing the maximal integer value represented by a Prolog integer.
  > `PL_MIN_INTEGER` | Return a long, representing the minimal integer value.
  > `PL_QUERY_VERSION` | Return a long, representing the version as 10,000 × M + 100 × m + p, where M is the major, m the minor version number and p the patch level. For example, `20717` means `2.7.17`.
  > `PL_QUERY_ENCODING` | Return the default stream encoding of Prolog (of type `IOENC`).
  > `PL_QUERY_USER_CPU` | Get amount of user CPU time of the process in milliseconds.

  Table 9 : PL_query() options

### 12.4.22 Registering Foreign Predicates

int PL_register_foreign_in_module(char *mod, char *name, int arity, foreign_t (*f)(), int flags, ...)
  Register the C function f to implement a Prolog predicate. After this call returns successfully a predicate with name name (a `char *`) and arity arity (a C `int`) is created in module mod. If mod is `NULL`, the predicate is created in the module of the calling context, or if no context is present in the module `user`.

  When called in Prolog, Prolog will call function. flags form a bitwise or’ed list of options for the installation. These are:

  `PL_FA_META` | Provide meta-predicate info (see below)
  `PL_FA_TRANSPARENT` | Predicate is module transparent (deprecated)
  `PL_FA_NONDETERMINISTIC` | Predicate is non-deterministic. See also PL_retry().
  `PL_FA_NOTRACE` | Predicate cannot be seen in the tracer
  `PL_FA_VARARGS` | Use alternative calling convention.

  If `PL_FA_META` is provided, PL_register_foreign_in_module() takes one extra argument. This argument is of type `const char*`. This string must be exactly as long as the number of arguments of the predicate and filled with characters from the set `0-9:^-+?`. See meta_predicate/1 for details. `PL_FA_TRANSPARENT` is implied if at least one meta-argument is provided (`0-9:^`). Note that meta-arguments are not always passed as <module>:<term>. Always use PL_strip_module() to extract the module and plain term from a meta-argument.232It is encouraged to pass an additional `NULL` pointer for non-meta-predicates.

  Predicates may be registered either before or after PL_initialise(). When registered before initialisation the registration is recorded and executed after installing the system predicates and before loading the saved state.

  Default calling (i.e. without `PL_FA_VARARGS`) function is passed the same number of `term_t` arguments as the arity of the predicate and, if the predicate is non-deterministic, an extra argument of type `control_t` (see section 12.4.1.1). If `PL_FA_VARARGS` is provided, function is called with three arguments. The first argument is a `term_t` handle to the first argument. Further arguments can be reached by adding the offset (see also PL_new_term_refs()). The second argument is the arity, which defines the number of valid term references in the argument vector. The last argument is used for non-deterministic calls. It is currently undocumented and should be defined of type `void*`. Here is an example:

      static foreign_t
      atom_checksum(term_t a0, int arity, void* context)
      { char *s;

        if ( PL_get_atom_chars(a0, &s) )
        { int sum;

          for(sum=0; *s; s++)
            sum += *s&0xff;

          return PL_unify_integer(a0+1, sum&0xff);
        }

        return FALSE;
      }

      install_t
      install()
      { PL_register_foreign("atom_checksum", 2,
                            atom_checksum, PL_FA_VARARGS);
      }

int PL_register_foreign(const char *name, int arity, foreign_t (*function)(), int flags, ...)
  Same as PL_register_foreign_in_module(), passing `NULL` for the module.
void PL_register_extensions_in_module(const char *module, PL_extension *e)
  Register a series of predicates from an array of definitions of the type `PL_extension` in the given module. If module is `NULL`, the predicate is created in the module of the calling context, or if no context is present in the module `user`. The `PL_extension` type is defined as

      typedef struct PL_extension
      { char          *predicate_name; /* Name of the predicate */
        short         arity;           /* Arity of the predicate */
        pl_function_t function;        /* Implementing functions */
        short         flags;           /* Or of PL_FA_... */
      } PL_extension;

  For details, see PL_register_foreign_in_module(). Here is an example of its usage:

      static PL_extension predicates[] = {
      { "foo",        1,      pl_foo, 0 },
      { "bar",        2,      pl_bar, PL_FA_NONDETERMINISTIC },
      { NULL,         0,      NULL,   0 }
      };

      main(int argc, char **argv)
      { PL_register_extensions_in_module("user", predicates);

        if ( !PL_initialise(argc, argv) )
          PL_halt(1);

        ...
      }

void PL_register_extensions( PL_extension *e)
  Same as PL_register_extensions_in_module() using `NULL` for the module argument.

### 12.4.23 Foreign Code Hooks

For various specific applications some hooks are provided.

PL_dispatch_hook_t PL_dispatch_hook(PL_dispatch_hook_t)
  If this hook is not NULL, this function is called when reading from the terminal. It is supposed to dispatch events when SWI-Prolog is connected to a window environment. It can return two values: `PL_DISPATCH_INPUT` indicates Prolog input is available on file descriptor 0 or `PL_DISPATCH_TIMEOUT` to indicate a timeout. The old hook is returned. The type `PL_dispatch_hook_t` is defined as:

      typedef int  (*PL_dispatch_hook_t)(void);

void PL_abort_hook(PL_abort_hook_t)
  Install a hook when abort/0 is executed. SWI-Prolog abort/0 is implemented using C setjmp()/longjmp() construct. The hooks are executed in the reverse order of their registration after the longjmp() took place and before the Prolog top level is reinvoked. The type `PL_abort_hook_t` is defined as:

      typedef void (*PL_abort_hook_t)(void);

int PL_abort_unhook(PL_abort_hook_t)
  Remove a hook installed with PL_abort_hook(). Returns `FALSE` if no such hook is found, `TRUE` otherwise.
void PL_on_halt(int (*f)(int, void *), void *closure)
  Register the function f to be called if SWI-Prolog is halted. The function is called with two arguments: the exit code of the process (0 if this cannot be determined) and the closure argument passed to the PL_on_halt() call. Handlers must return 0. Other return values are reserved for future use. See also at_halt/1.bugAlthough both PL_on_halt() and at_halt/1 are called in FIFO order, all at_halt/1 handlers are called before all PL_on_halt() handlers. These handlers are called before system cleanup and can therefore access all normal Prolog resources. See also PL_exit_hook().
void PL_exit_hook(int (*f)(int, void *), void *closure)
  Similar to PL_on_halt(), but the hooks are executed by PL_halt() instead of PL_cleanup() just before calling exit().
PL_agc_hook_t PL_agc_hook(PL_agc_hook_t new)
  Register a hook with the atom-garbage collector (see garbage_collect_atoms/0) that is called on any atom that is reclaimed. The old hook is returned. If no hook is currently defined, `NULL` is returned. The argument of the called hook is the atom that is to be garbage collected. The return value is an `int`. If the return value is zero, the atom is not reclaimed. The hook may invoke any Prolog predicate.

  The example below defines a foreign library for printing the garbage collected atoms for debugging purposes.

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

      static int
      atom_hook(atom_t a)
      { Sdprintf("AGC: deleting %s\n", PL_atom_chars(a));

        return TRUE;
      }

      static PL_agc_hook_t old;

      install_t
      install()
      { old = PL_agc_hook(atom_hook);
      }

      install_t
      uninstall()
      { PL_agc_hook(old);
      }

### 12.4.24 Storing foreign data

When combining foreign code with Prolog, it can be necessary to make data represented in the foreign language available to Prolog. For example, to pass it to another foreign function. At the end of this section, there is a partial implementation of using foreign functions to manage bit-vectors. Another example is the SGML/XML library that manages a‘parser' object, an object that represents the current state of the parser and that can be directed to perform actions such as parsing a document or make queries about the document content.

This section provides some hints for handling foreign data in Prolog. There are four options for storing such data:

- Natural Prolog data
  Uses the representation one would choose if no foreign interface was required. For example, a bitvector representing a list of small integers can be represented as a Prolog list of integers.

- Opaque packed data on the stacks
  It is possible to represent the raw binary representation of the foreign object as a Prolog string (see section 5.2). Strings may be created from foreign data using PL_put_string_nchars() and retrieved using PL_get_string_chars(). It is good practice to wrap the string in a compound term with arity 1, so Prolog can identify the type. The hook portray/1 rules may be used to streamline printing such terms during development.

- Opaque packed data in a blob
  Similar to the above solution, binary data can be stored in an atom. The blob interface (section 12.4.10) provides additional facilities to assign a type and hook-functions that act on creation and destruction of the underlying atom.

- Natural foreign data, passed as a pointer
  An alternative is to pass a pointer to the foreign data. Again, the pointer is often wrapped in a compound term.

The choice may be guided using the following distinctions

- Is the data opaque to Prolog
  With‘opaque' data, we refer to data handled in foreign functions, passed around in Prolog, but where Prolog never examines the contents of the data itself. If the data is opaque to Prolog, the selection will be driven solely by simplicity of the interface and performance.

- What is the lifetime of the data
  With‘lifetime' we refer to how it is decided that the object is (or can be) destroyed. We can distinguish three cases:

  1. The object must be destroyed on backtracking and normal Prolog garbage collection (i.e., it acts as a normal Prolog term). In this case, representing the object as a Prolog string (second option above) is the only feasible solution.

  2. The data must survive Prolog backtracking. This leaves two options. One is to represent the object using a pointer and use explicit creation and destruction, making the programmer responsible. The alternative is to use the blob-interface, leaving destruction to the (atom) garbage collector.

  3. The data lives as during the lifetime of a foreign function that implements a predicate. If the predicate is deterministic, foreign automatic variables are suitable. If the predicate is non-deterministic, the data may be allocated using malloc() and a pointer may be passed. See section 12.4.1.1.

#### 12.4.24.1 Examples for storing foreign data

In this section, we outline some examples, covering typical cases. In the first example, we will deal with extending Prolog's data representation with integer sets, represented as bit-vectors. Then, we discuss the outline of the DDE interface.

Integer sets with not-too-far-apart upper- and lower-bounds can be represented using bit-vectors. Common set operations, such as union, intersection, etc., are reduced to simple and’ing and or’ing the bit-vectors. This can be done using Prolog's unbounded integers.

For really demanding applications, foreign representation will perform better, especially time-wise. Bit-vectors are naturally expressed using string objects. If the string is wrapped in `bitvector/1`, the lower-bound of the vector is 0 and the upper-bound is not defined; an implementation for getting and putting the sets as well as the union predicate for it is below.

    #include <SWI-Prolog.h>

    #define max(a, b) ((a) > (b) ? (a) : (b))
    #define min(a, b) ((a) < (b) ? (a) : (b))

    static functor_t FUNCTOR_bitvector1;

    static int
    get_bitvector(term_t in, int *len, unsigned char **data)
    { if ( PL_is_functor(in, FUNCTOR_bitvector1) )
      { term_t a = PL_new_term_ref();

        PL_get_arg(1, in, a);
        return PL_get_string(a, (char **)data, len);
      }

      PL_fail;
    }

    static int
    unify_bitvector(term_t out, int len, const unsigned char *data)
    { if ( PL_unify_functor(out, FUNCTOR_bitvector1) )
      { term_t a = PL_new_term_ref();

        PL_get_arg(1, out, a);

        return PL_unify_string_nchars(a, len, (const char *)data);
      }

      PL_fail;
    }

    static foreign_t
    pl_bitvector_union(term_t t1, term_t t2, term_t u)
    { unsigned char *s1, *s2;
      int l1, l2;

      if ( get_bitvector(t1, &l1, &s1) &&
           get_bitvector(t2, &l2, &s2) )
      { int l = max(l1, l2);
        unsigned char *s3 = alloca(l);

        if ( s3 )
        { int n;
          int ml = min(l1, l2);

          for(n=0; n<ml; n++)
            s3[n] = s1[n] | s2[n];
          for( ; n < l1; n++)
            s3[n] = s1[n];
          for( ; n < l2; n++)
            s3[n] = s2[n];

          return unify_bitvector(u, l, s3);
        }

        return PL_warning("Not enough memory");
      }

      PL_fail;
    }

    install_t
    install()
    { PL_register_foreign("bitvector_union", 3, pl_bitvector_union, 0);

      FUNCTOR_bitvector1 = PL_new_functor(PL_new_atom("bitvector"), 1);
    }

The DDE interface (see section 4.44) represents another common usage of the foreign interface: providing communication to new operating system features. The DDE interface requires knowledge about active DDE server and client channels. These channels contains various foreign data types. Such an interface is normally achieved using an open/close protocol that creates and destroys a handle. The handle is a reference to a foreign data structure containing the relevant information.

There are a couple of possibilities for representing the handle. The choice depends on responsibilities and debugging facilities. The simplest approach is to use PL_unify_pointer() and PL_get_pointer(). This approach is fast and easy, but has the drawbacks of (untyped) pointers: there is no reliable way to detect the validity of the pointer, nor to verify that it is pointing to a structure of the desired type. The pointer may be wrapped into a compound term with arity 1 (i.e., `dde_channel(<Pointer>)`), making the type-problem less serious.

Alternatively (used in the DDE interface), the interface code can maintain a (preferably variable length) array of pointers and return the index in this array. This provides better protection. Especially for debugging purposes, wrapping the handle in a compound is a good suggestion.

### 12.4.25 Embedding SWI-Prolog in other applications

With embedded Prolog we refer to the situation where the‘main' program is not the Prolog application. Prolog is sometimes embedded in C, C++, Java or other languages to provide logic based services in a larger application. Embedding loads the Prolog engine as a library to the external language. Prolog itself only provides for embedding in the C language (compatible with C++). Embedding in Java is achieved using JPL using a C-glue between the Java and Prolog C interfaces.

The most simple embedded program is below. The interface function PL_initialise() must be called before any of the other SWI-Prolog foreign language functions described in this chapter, except for PL_initialise_hook(), PL_new_atom(), PL_new_functor() and PL_register_foreign(). PL_initialise() interprets all the command line arguments, except for the -t toplevel flag that is interpreted by PL_toplevel().

    int
    main(int argc, char **argv)
    { if ( !PL_initialise(argc, argv) )
        PL_halt(1);

      PL_halt(PL_toplevel() ? 0 : 1);
    }

int PL_initialise(int argc, char **argv)
  Initialises the SWI-Prolog heap and stacks, restores the Prolog state, loads the system and personal initialisation files, runs the initialization/1 hooks and finally runs the initialization goals registered using -g goal.

  Special consideration is required for `argv[0]`. On Unix, this argument passes the part of the command line that is used to locate the executable. Prolog uses this to find the file holding the running executable. The Windows version uses this to find a module of the running executable. If the specified module cannot be found, it tries the module `libswipl.dll`, containing the Prolog runtime kernel. In all these cases, the resulting file is used for two purposes:

  - See whether a Prolog saved state is appended to the file. If this is the case, this state will be loaded instead of the default `boot.prc` file from the SWI-Prolog home directory. See also qsave_program/[1,2] and section 12.5.
  - Find the Prolog home directory. This process is described in detail in section 12.6.

  PL_initialise() returns 1 if all initialisation succeeded and 0 otherwise.bugVarious fatal errors may cause PL_initialise() to call PL_halt(1), preventing it from returning at all.

  In most cases, argc and argv will be passed from the main program. It is allowed to create your own argument vector, provided `argv[0]` is constructed according to the rules above. For example:

      int
      main(int argc, char **argv)
      { char *av[10];
        int ac = 0;

        av[ac++] = argv[0];
        av[ac++] = "-x";
        av[ac++] = "mystate";
        av[ac]   = NULL;

        if ( !PL_initialise(ac, av) )
          PL_halt(1);
        ...
      }

  Please note that the passed argument vector may be referred from Prolog at any time and should therefore be valid as long as the Prolog engine is used.

  A good setup in Windows is to add SWI-Prolog's `bin` directory to your `PATH` and either pass a module holding a saved state, or `"libswipl.dll"` as `argv[0]`. If the Prolog state is attached to a DLL (see the -dll option of swipl-ld), pass the name of this DLL.

int PL_winitialise(int argc, wchar_t **argv)
  Wide character version of PL_initialise(). Can be used in Windows combined with the wmain() entry point.
int PL_is_initialised(int *argc, char ***argv)
  Test whether the Prolog engine is already initialised. Returns `FALSE` if Prolog is not initialised and `TRUE` otherwise. If the engine is initialised and argc is not `NULL`, the argument count used with PL_initialise() is stored in argc. Same for the argument vector argv.
int PL_set_resource_db_mem(const unsigned char *data, size_t size)
  This function must be called at most once and before calling PL_initialise(). The memory area designated by data and size must contain the resource data and be in the format as produced by qsave_program/2. The memory area is accessed by PL_initialise() as well as calls to open_resource/3.233This implies that the data must remain accessible during the lifetime of the process if open_resource/3 is used. Future versions may provide a function to detach the resource database and cause open_resource/3 to raise an exception.

  For example, we can include the bootstrap data into an embedded executable using the steps below. The advantage of this approach is that it is fully supported by any OS and you obtain a single file executable.

  1. Create a saved state using qsave_program/2 or

         % swipl -o state -c file.pl ...

  2. Create a C source file from the state using e.g., the Unix utility xxd(1):

         % xxd -i state > state.h

  3. Embed Prolog as in the example below. Instead of calling the toplevel you probably want to call your application code.

         #include <SWI-Prolog.h>
         #include "state.h"

         int
         main(int argc, char **argv)
         { if ( !PL_set_resource_db_mem(state, state_len) ||
                !PL_initialise(argc, argv) )
             PL_halt(1);

           return PL_toplevel();
         }

  Alternative to xxd, it is possible to use inline assembler, e.g. the gcc `incbin` instruction. Code for gcc was provided by Roberto Bagnara on the SWI-Prolog mailinglist. Given the state in a file `state`, create the following assembler program:

              .globl _state
              .globl _state_end
      _state:
              .incbin "state"
      _state_end:

  Now include this as follows:

      #include <SWI-Prolog.h>

      #if __linux
      #define STATE _state
      #define STATE_END _state_end
      #else
      #define STATE state
      #define STATE_END state_end
      #endif

      extern unsigned char STATE[];
      extern unsigned char STATE_END[];

      int
      main(int argc, char **argv)
      { if ( !PL_set_resource_db_mem(STATE, STATE_END - STATE) ||
             !PL_initialise(argc, argv) )
          PL_halt(1);
        return PL_toplevel();
      }

  As Jose Morales pointed at https://github.com/graphitemaster/incbin, which contains a portability layer on top of the above idea.

int PL_toplevel()
  Runs the goal of the -t toplevel switch (default prolog/0) and returns 1 if successful, 0 otherwise.
int PL_cleanup(int status_and_flags)
  This function may be called instead of PL_halt() to cleanup Prolog without exiting the process. It performs the reverse of PL_initialise(). It runs the PL_on_halt() and at_halt/1 handlers, closes all streams (except for the standard I/O streams, which are flushed only), restores all signal handlers and reclaims all memory unless asked not to. status_and_flags accepts the following flags:
  `PL_CLEANUP_NO_RECLAIM_MEMORY`
    Do not reclaim memory. This is the default when called from PL_halt() for the release versions because the OS will do so anyway.
  `PL_CLEANUP_NO_CANCEL`
    Do not allow Prolog and foreign halt hooks to cancel the cleanup.

  The return value of PL_cleanup() is one of the following:

  `PL_CLEANUP_CANCELED`
    A Prolog or foreign halt hook cancelled the cleanup. Note that some of the halt hooks may have been executed.
  `PL_CLEANUP_SUCCESS`
    Cleanup completed successfully. Unless `PL_CLEANUP_NO_RECLAIM_MEMORY` was specified this implies most of the memory was reclaimed and Prolog may be reinitialized in the same process using PL_initialise().
  `PL_CLEANUP_FAILED`
    Cleanup failed. This happens if the user requested to reclaim all memory but this failed because the system was not able to join all Prolog threads and could therefore not reclaim the memory.
  `PL_CLEANUP_RECURSIVE`
    PL_cleanup() was called from a hook called by the cleanup process.

  PL_cleanup() allows deleting and restarting the Prolog system in the same process. In versions older than 8.5.9 this did not work. As of version 8.5.9, it works for the basic Prolog engine. Many of the plugins that contain foreign code do not implement a suitable uninstall handler and will leak memory and possibly other resources. Note that shutting Prolog down and renitializing it is slow. For almost all scenarios there are faster alternatives such as reloading modified code, using temporary modules, using threads, etc.

void PL_cleanup_fork()
  Stop intervaltimer that may be running on behalf of profile/1. The call is intended to be used in combination with fork():

          if ( (pid=fork()) == 0 )
          { PL_cleanup_fork();
            <some exec variation>
          }

  The call behaves the same on Windows, though there is probably no meaningful application.

int PL_halt(int status)
  Clean up the Prolog environment using PL_cleanup() and if successful call exit() with the status argument. Returns `FALSE` if exit was cancelled by PL_cleanup().

#### 12.4.25.1 Threading, Signals and embedded Prolog

This section applies to Unix-based environments that have signals or multithreading. The Windows version is compiled for multithreading, and Windows lacks proper signals.

We can distinguish two classes of embedded executables. There are small C/C++ programs that act as an interfacing layer around Prolog. Most of these programs can be replaced using the normal Prolog executable extended with a dynamically loaded foreign extension and in most cases this is the preferred route. In other cases, Prolog is embedded in a complex application that---like Prolog---wants to control the process environment. A good example is Java. Embedding Prolog is generally the only way to get these environments together in one process image. Java VMs, however, are by nature multithreaded and appear to do signal handling (software interrupts).

On Unix systems, SWI-Prolog installs handlers for the following signals:

SIGUSR2
  has an empty signal handler. This signal is sent to a thread after sending a thread-signal (see thread_signal/2). It causes blocking system calls to return with `EINTR`, which gives them the opportunity to react to thread-signals.

  In some cases the embedded system and SWI-Prolog may both use `SIGUSR2` without conflict. If the embedded system redefines `SIGUSR2` with a handler that runs quickly and no harm is done in the embedded system due to spurious wakeup when initiated from Prolog, there is no problem. If SWI-Prolog is initialised after the embedded system it will call the handler set by the embedded system and the same conditions as above apply. SWI-Prolog's handler is a simple function only chaining a possibly previously registered handler. SWI-Prolog can handle spurious `SIGUSR2` signals.

SIGINT
  is used by the top level to activate the tracer (typically bound to control-C). The first control-C posts a request for starting the tracer in a safe, synchronous fashion. If control-C is hit again before the safe route is executed, it prompts the user whether or not a forced interrupt is desired.
SIGTERM, SIGABRT and SIGQUIT
  are caught to cleanup before killing the process again using the same signal.
SIGSEGV, SIGILL, SIGBUS, SIGFPE and SIGSYS
  are caught by to print a backtrace before killing the process again using the same signal.
SIGHUP
  is caught and causes the process to exit with status 2 after cleanup.

The --no-signals option can be used to inhibit all signal processing except for `SIGUSR2`. The handling of `SIGUSR2` is vital for dealing with blocking system call in threads. The used signal may be changed using the --sigalert=NUM option or disabled using `--sigalert=0`.