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 have two alternatives to return control back to Prolog:
return TRUE.
return FALSE.
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:
PL_FIRST_CALL)PL_REDO)PL_PRUNED)(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
_PL_retry(n).
See also PL_succeed().return _PL_retry_address(n).
See also
PL_succeed().PL_PRUNED case and should be aware that the other
arguments are not valid in this case.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).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);
}
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:
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.213the 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:
PL_release_stream(s);
return TRUE;
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
{ PL_release_stream(s);
return 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.214Possibly, 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.
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.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.
The following functions provide for communication using atoms and functors.
(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.
PL_new_functor(ATOM_dot,2).
It is advised to use
PL_get_list(), PL_put_list()
or PL_unify_list()
where applicable.
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().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
12.4.9), conversion to the requested encoding is not possible or a
resource error occurs.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).(atom_t)0.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:
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.
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.
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 12.4.9.2) |
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.
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.
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_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_INTEGER|CVT_RATIONAL|CVT_FLOATBUF_* is provided, BUF_STACK is implied.CVT_WRITE, but using write_canonical/2.CVT_WRITE, but using writeq/2.CVT_VARIABLE
and
CVT_WRITE*.BUF_RING is an alias
for BUF_STACK. See section
12.4.13.PL_get_chars(l, s,
CVT_LIST|flags), provided flags
contains none of the CVT_* flags.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().int64_t, assign its value over i. See also PL_get_int64_ex().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().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.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.
NULL.
In addition, the following functions are available for creating and inspecting atoms:
(size_t)-1,
it is computed from s using strlen(). See PL_new_atom()
for error handling.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.
(size_t)-1,
it is computed from s using wcslen(). See PL_new_atom()
for error handling.PL_ATOM, PL_STRING,
PL_CODE_LIST or PL_CHAR_LIST.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.
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:
term_t term reference for the
elements (head). This reference is reused for each element.
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.
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.
_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.
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)) )
Sprintf("%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.
It is allowed to pass 0 for tail and NULL for len.
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,
"ed, &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 intTRUE.
OPT_INT intOPT_INT64 int64_tOPT_UINT64 uint64_tOPT_SIZE size_tOPT_DOUBLE doubleOPT_STRING char*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_tOPT_TERM term_tThe 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.
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);
Sprintf("%s", s);
break;
case PL_STRING:
PL_get_string_chars(t, &s, &len);
Sprintf("\"%s\"", s);
break;
case PL_TERM:
{ term_t a = PL_new_term_ref();
PL_get_name_arity(t, &name, &arity);
Sprintf("%s(", PL_atom_chars(name));
for(n=1; n<=arity; n++)
{ PL_get_arg(n, t, a);
if ( n > 1 )
Sprintf(", ");
pl_display(a);
}
Sprintf(")");
break;
default:
PL_fail; /* should not happen */
}
}
PL_succeed;
}
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().
true or false in the term
reference See also PL_put_atom(), PL_unify_bool()
and PL_get_bool().
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().
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 12.4.23.
uint64_t values with the highest
bit set to 1. Without unbounded integer support, too large values raise
a
representation_error exception.
PL_unify_*() functions or use PL_cons_functor()../2[|]/2TRUE. 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.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.
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);
}
}
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.
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.
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.218Goal
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.
false or true,
according to whether a is zero or non-zero. If t
is instantiated, off and on are also accepted.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.
int64_t. If unbounded integers are not supported a
representation_error is raised.
./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.
[]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 nonePL_FUNCTOR.
PL_BOOL inttrue or false.
PL_ATOM atom_tPL_CHARS const char *char *,
as in PL_unify_atom_chars().
PL_NCHARS size_t, const char *char* as in PL_unify_atom_nchars().
PL_UTF8_CHARS const char *PL_UTF8_STRING const char *PL_MBCHARS const char *PL_MBCODES const char *PL_MBSTRING const char *PL_NWCHARS size_t, const wchar_t *PL_NWCODES size_t, const wchar_t *PL_NWSTRING size_t, const wchar_t *PL_SHORT shortshort is promoted to int, PL_SHORT
is a synonym for PL_INT.
PL_INTEGER longPL_INT intPL_LONG longPL_INT64 int64_tPL_INTPTR intptr_tPL_LONG. but on 64-bit
MS-Windows pointers are 64 bits while longs are only 32 bits.
PL_DOUBLE doublePL_FLOAT doublePL_POINTER void *PL_STRING const char *PL_TERM term_tPL_FUNCTOR functor_t, ...PL_FUNCTOR_CHARS const char *name, int arity,
...PL_FUNCTOR.
PL_LIST int length, ...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();
}
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);
}
'\'', 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.
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 12.4.7 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.long.int64_t.int64_t.intptr_t.size_t.true, false, on,
constoff, 1 or 0). Note that the pointer is to
an
int because C has no bool type.TRUE
and t is -1, p is filled with -1. Raises an
appropriate error if the conversion is not possible.while loop over PL_get_list_ex(),
it fails immediately if there is an exception pending (from PL_get_list_ex()).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().
instantiation_error. Culprit is ignored,
but should be bound to the term that is insufficiently instantiated. See
instantiation_error/1.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).representation_error(resource). See representation_error/1.type_error(expected, culprit). See type_error/2.domain_error(expected, culprit). See domain_error/2.existence_error(type, culprit). See type_error/2.permission_error(operation, type, culprit). See
permission_error/3.
resource_error(resource). See resource_error/1.
syntax_error(message). If arg is not NULL,
add information about the current position of the input stream.
In addition to the functions described in
section 12.4.3.2,
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.219_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 12.4.6.
int because C has no bool type. The return
value is either 0 or 1.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.
(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.220The CVT_EXCEPTION
was added in version 8.3.12.
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. 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.
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. 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_NOCOPY is also specified, in which case the
pointers are compared.PL_BLOB_UNIQUE
is also specified, uniqueness is determined by comparing the pointer
rather than the data pointed at. Using
PL_BLOB_UNIQUE| can
be used to make a blob reference an arbitrary pointer where the pointer
data may be reclaimed in the release() handler.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:
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.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().221Or
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().
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;
}
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 Sprintf(),
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.
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.222Details
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.
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().
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.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.
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.
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.
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.
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().TRUE. Otherwise return FALSE. Each result
pointer may be NULL, in which case the requested
information is ignored.Get the data and type associated to a blob. This function is mainly
used from the callback functions described in section
12.4.9.1. 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.
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.
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.
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.
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);
}
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.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.TRUE on success. The mpz argument is not
changed.TRUE on success. Note that t is unified with an
integer if the denominator is 1. The mpq argument is not
changed.
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.
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.
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.NULL, the current
context module is used. The predicate_t handle may be
stored as global data and reused for future queries224PL_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.
NULL. Currently always returns TRUE.
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).
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_NORMALPL_Q_NODEBUG0.PL_Q_CATCH_EXCEPTIONPL_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_EXCEPTIONPL_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_YIELDI_YIELD instruction for engine-based
coroutining. See $engine_yield/2 in boot/init.pl for
details.PL_Q_EXT_STATUSTRUE 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);
...
}
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).
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.
\+ \+ 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.0 if the
current thread is not executing any queries.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).225Up
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.
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.
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.226Such
a callback would deadlock if the callback creates new atoms or
requires stack shifts or garbage collection.
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;
}
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:
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.
Modules are identified via a unique handle. The following functions are available to query and manipulate modules.
user 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.
(module_t)0.
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.
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
'$aborted').
time_limit_exceeded (see call_with_time_limit/2).
resource_error exceptions.
error(Formal, ImplDef) exceptions.
This function is rarely used directly. Instead, errors are typically
raised using the functions in section
12.4.6 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) )
{ Sprintf("Hello \"%s\"\n", s);
return TRUE;
} 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) )
{ Sprintf("Hello \"%s\"\n", s);
return TRUE;
}
return FALSE;
}
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 0.227Provided 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().
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.
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().
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.229A
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();
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.
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.
TRUE if t1 and t2 refer to
physically the same compound term and FALSE otherwise.
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)0.(record_t)0 if the record is an
external record (see PL_record_external()).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().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:
(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.
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);
}
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.
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_ABSOLUTEPL_FILE_OSPATH\/PL_FILE_SEARCHPL_FILE_EXISTPL_FILE_READPL_FILE_WRITEPL_FILE_EXECUTEPL_FILE_NOERRORSPL_FILE_OSPATH must be provided to fetch a
filename in OS native (e.g., C:\x\y) notation.
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().
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_BOOLtrue or false) flag. The
argument must be an int.
PL_ATOMconst char *.
PL_INTEGERintptr_t *.
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_ATOMatom. The returned value
is an atom handle of type atom_t.
PL_INTEGERinteger. The returned
value is an integer of type int64_t.
PL_FLOATfloat. The returned value
is a floating point number of type double.
PL_TERMterm. The returned value
is a term handle of type term_t.
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.
[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.
int)
is the number of frames printed.int) is the exit code. See halt/1.char * to the current output stream.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.
PL_QUERY_ARGCReturn an integer holding the number of arguments given to Prolog from Unix. PL_QUERY_ARGVReturn a char **holding the argument vector given to Prolog from Unix.PL_QUERY_SYMBOLFILEReturn a char *holding the current symbol file of the running process.PL_MAX_INTEGERReturn a long, representing the maximal integer value represented by a Prolog integer. PL_MIN_INTEGERReturn a long, representing the minimal integer value. PL_QUERY_VERSIONReturn 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, 20717means2.7.17.PL_QUERY_ENCODINGReturn the default stream encoding of Prolog (of type IOENC).PL_QUERY_USER_CPUGet amount of user CPU time of the process in milliseconds.
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.231It
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);
}
NULL for the
module.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);
...
}
NULL for the module argument.
For various specific applications some hooks are provided.
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);
PL_abort_hook_t is defined as:
typedef void (*PL_abort_hook_t)(void);
FALSE if no such hook is found, TRUE
otherwise.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);
}
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:
The choice may be guided using the following distinctions
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.
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);
}
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:
boot.prc
file from the SWI-Prolog home directory. See also qsave_program/[1,2]
and section 12.5.
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.
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.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.
% swipl -o state -c file.pl ...
% xxd -i state > state.h
#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.
PL_CLEANUP_NO_RECLAIM_MEMORYPL_CLEANUP_NO_CANCELThe return value of PL_cleanup() is one of the following:
PL_CLEANUP_CANCELEDPL_CLEANUP_SUCCESSPL_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_FAILEDPL_CLEANUP_RECURSIVEPL_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.
if ( (pid=fork()) == 0 )
{ PL_cleanup_fork();
<some exec variation>
}
The call behaves the same on Windows, though there is probably no meaningful application.
FALSE
if exit was cancelled by PL_cleanup().
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:
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.
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.