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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
. Deterministic foreign functions have two
alternatives to return control back to Prolog:
- void PL_succeed()
-
Succeed deterministically. PL_succeed is defined as
return TRUE
.
- void PL_fail()
-
Fail and start Prolog backtracking. PL_fail is defined as
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:
- Initial call (
PL_FIRST_CALL
)
Prolog has just created a frame for the foreign function and asks it to
produce the first answer.
- Redo call (
PL_REDO
)
The previous invocation of the foreign function associated with the
current goal indicated it was possible to backtrack. The foreign
function should produce the next solution.
- Terminate call (
PL_CUTTED
)
The choice point left by the foreign function has been destroyed by a
cut. The foreign function is given the opportunity to clean the
environment.
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.
- void PL_retry(long)
-
The foreign function succeeds while leaving a choice point. On
backtracking over this goal the foreign function will be called again,
but the control argument now indicates it is a `Redo' call and the macro PL_foreign_context()
will return the handle passed via
PL_retry(). This handle is a 30
bits signed value (two bits are used for status indication).
- void PL_retry_address(void
*)
-
As PL_retry(), but ensures an
address as returned by malloc() is correctly recovered by PL_foreign_context_address().
- int PL_foreign_control(control_t)
-
Extracts the type of call from the control argument. The return values
are described above. Note that the function should be prepared to handle
the
PL_CUTTED
case and should be aware that the other
arguments are not valid in this case.
- long PL_foreign_context(control_t)
-
Extracts the context from the context argument. In the call type is
PL_FIRST_CALL
the context value is 0L. Otherwise it is the
value returned by the last PL_retry()
associated with this goal (both if the call type is PL_REDO
as PL_CUTTED
).
- void * PL_foreign_context_address(control_t)
-
Extracts an address as passed in by PL_retry_address().
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_CUTTED
will follow.
The code of figure 6 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, foreign_t handle)
{ struct context * ctxt;
switch( PL_foreign_control(handle) )
{ case PL_FIRST_CALL:
ctxt = malloc(sizeof(struct context));
...
PL_retry_address(ctxt);
case PL_REDO:
ctxt = PL_foreign_context_address(handle);
...
PL_retry_address(ctxt);
case PL_CUTTED:
free(ctxt);
PL_succeed;
}
}
Figure 6 : Skeleton for non-deterministic foreign
functions
The following functions provide for communication using atoms and
functors.
- atom_t PL_new_atom(const
char *)
-
Return an atom handle for the given C-string. This function always
succeeds. The returned handle is valid for the entire session.
- const char * PL_atom_chars(atom_t
atom)
-
Return a C-string for the text represented by the given atom. The
returned text will not be changed by Prolog. It is not allowed to modify
the contents, not even `temporary' as the string may reside in read-only
memory.
- functor_t PL_new_functor(atom_t
name, int arity)
-
Returns a functor identifier, a handle for the name/arity
pair. The returned handle is valid for the entire Prolog session.
- atom_t PL_functor_name(functor_t
f)
-
Return an atom representing the name of the given functor.
- int PL_functor_arity(functor_t
f)
-
Return the arity of the given functor.
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.
- int PL_term_type(term_t)
-
Obtain the type of a term, which should be a term returned by one of the
other interface predicates or passed as an argument. The function
returns the type of the Prolog term. The type identifiers are listed
below. Note that the extraction functions PL_ge_t*() 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) )
{ ...;
}
PL_VARIABLE | An unbound variable. The
value of term as such is a unique identifier for the variable. |
PL_ATOM | A Prolog atom. |
PL_STRING | A Prolog string. |
PL_INTEGER | A Prolog integer. |
PL_FLOAT | A Prolog floating point
number. |
PL_TERM | A compound term. Note that a
list is a compound term ./2 . |
The functions PL_is_<type> are an alternative to PL_term_type().
The test PL_is_variable(term)
is equivalent to
PL_term_type(term) ==
PL_VARIABLE
, but the first is considerably faster. On the other
hand, using a switch over PL_term_type()
is faster and more readable then using an if-then-else using the
functions below. All these functions return either TRUE
or FALSE
.
- int PL_is_variable(term_t)
-
Returns non-zero if term is a variable.
- int PL_is_atom(term_t)
-
Returns non-zero if term is an atom.
- int PL_is_string(term_t)
-
Returns non-zero if term is a string.
- int PL_is_integer(term_t)
-
Returns non-zero if term is an integer.
- int PL_is_float(term_t)
-
Returns non-zero if term is a float.
- int PL_is_compound(term_t)
-
Returns non-zero if term is a compound term.
- int PL_is_functor(term_t,
functor_t)
-
Returns non-zero if term is compound and its functor is functor.
This test is equivalent to PL_get_functor(),
followed by testing the functor, but easier to write and faster.
- int PL_is_list(term_t)
-
Returns non-zero if term is a compound term with functor ./2
or the atom
.
- int PL_is_atomic(term_t)
-
Returns non-zero if term is atomic (not variable or
compound).
- int PL_is_number(term_t)
-
Returns non-zero if term is an integer or float.
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.
- int PL_get_atom(term_t
+t, atom_t *a)
-
If t is an atom, store the unique atom identifier over a.
See also PL_atom_chars() and PL_new_atom().
If there is no need to access the data (characters) of an atom, it is
advised to manipulate atoms using their handle.
- int PL_get_atom_chars(term_t
+t, char **s)
-
If t is an atom, store a pointer to a 0-terminated C-string
in
s. It is explicitly not allowed to modify
the contents of this string. Some built-in atoms may have the string
allocated in read-only memory, so `temporary manipulation' can cause an
error.
- int PL_get_string(term_t
+t, char **s, int *len)
-
If t is a string object, store a pointer to a 0-terminated
C-string in s and the length of the string in len.
Note that this pointer is invalidated by backtracking,
garbage-collection and stack-shifts, so generally the only save
operations are to pass it immediately to a C-function that doesn't
involve Prolog.
- int PL_get_chars(term_t
+t, char **s, unsigned flags)
-
Convert the argument term t to a 0-terminated C-string. flags
is a bitwise disjunction from two groups of constants. The first
specifies which term-types should converted and the second how the
argument is stored. Below is a specification of these constants.
BUF_RING
implies, if the data is not static (as from an atom), the data is copied
to the next buffer from a ring of four (4) buffers. This is a convenient
way of converting multiple arguments passed to a foreign predicate to
C-strings. If BUF_MALLOC is used, the data must be freed using free()
when not needed any longer.
CVT_ATOM | Convert if term is an atom |
CVT_STRING | Convert if term is a
string |
CVT_LIST | Convert if term is a list
of integers between 1 and 255 |
CVT_INTEGER | Convert if term is an
integer (using %d ) |
CVT_FLOAT | Convert if term is a float
(using %f ) |
CVT_NUMBER | Convert if term is a
integer or float |
CVT_ATOMIC | Convert if term is atomic |
CVT_VARIABLE | Convert variable to
print-name |
CVT_ALL | Convert if term is any of
the above, except for variables |
BUF_DISCARDABLE | Data must copied
immediately |
BUF_RING | Data is stored in a ring of
buffers |
BUF_MALLOC | Data is copied to a new
buffer returned by
malloc(3) |
- int PL_get_list_chars(+term_t
l, char **s, unsigned flags)
-
Same as
PL_get_chars(l, s,
CVT_LIST|flags)
, provided flags
contains no of the CVT_* flags.
- int PL_get_integer(+term_t
t, int *i)
-
If t is a Prolog integer, assign its value over i.
On 32-bit machines, this is the same as PL_get_long(),
but avoids a warning from the compiler. See also PL_get_long().
- int PL_get_long(term_t
+t, long *i)
-
If t is a Prolog integer, assign its value over i.
Note that Prolog integers have limited value-range. If t is a
floating point number that can be represented as a long, this function
succeeds as well.
- int PL_get_pointer(term_t
+t, void **ptr)
-
In the current system, pointers are represented by Prolog integers, but
need some manipulation to make sure they do not get truncated due to the
limited Prolog integer range. PL_put_pointer()/PL_get_pointer()
guarantees pointers in the range of malloc() are handled without
truncating.
- int PL_get_float(term_t
+t, double *f)
-
If t is a float or integer, its value is assigned over f.
- int PL_get_functor(term_t
+t, functor_t *f)
-
If t is compound or an atom, the Prolog representation of the
name-arity pair will be assigned over f. See also
PL_get_name_arity() and PL_is_functor().
- int PL_get_name_arity(term_t
+t, atom_t *name, int *arity)
-
If t is compound or an atom, the functor-name will be
assigned over name and the arity over arity. See
also
PL_get_functor() and PL_is_functor().
- int PL_get_module(term_t
+t, module_t *module)
-
If t is an atom, the system will lookup or create the
corresponding module and assign an opaque pointer to it over module,.
- int PL_get_arg(int
index, term_t +t, term_t -a)
-
If t is compound and index is between 1 and arity
(including), assign a with a term-reference to the argument.
- int _PL_get_arg(int
index, term_t +t, term_t -a)
-
Same as PL_get_arg(), but no
checking is performed, nor whether t is actually a term, nor
whether index is a valid argument-index.
The functions from this section are intended to read a Prolog list
from C. Suppose we expect a list of atoms, the following code will print
the atoms, each on a line:
foreign_t
pl_write_atoms(term_t l)
{ term_t head = PL_new_term_ref(); /* variable for the elements */
term_t list = PL_copy_term_ref(l); /* copy as we need to write */
while( PL_get_list(list, head, list) )
{ char *s;
if ( PL_get_atom_chars(head, &s) )
Sprintf("%s\n", s);
else
PL_fail;
}
return PL_get_nil(list); /* test end for [] */
}
- int PL_get_list(term_t
+l, term_t -h, term_t -t)
-
If l is a list and not
assign a term-reference
to the head to h and to the tail to t.
- int PL_get_head(term_t
+l, term_t -h)
-
If l is a list and not
assign a term-reference
to the head to h.
- int PL_get_tail(term_t
+l, term_t -t)
-
If l is a list and not
assign a term-reference
to the tail to t.
- int PL_get_nil(term_t
+l)
-
Succeeds if represents the atom
.
Figure 7 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;
}
Figure 7 : A Foreign definition of display/1
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.
- void PL_put_variable(term_t
-t)
-
Put a fresh variable in the term. The new variable lives on the global
stack. Note that the initial variable lives on the local stack and is
lost after a write to the term-references. After using this function,
the variable will continue to live.
- void PL_put_atom(term_t
-t, atom_t a)
-
Put an atom in the term reference from a handle. See also
PL_new_atom() and PL_atom_chars().
- void PL_put_atom_chars(term_t
-t, const char *chars)
-
Put an atom in the term-reference constructed from the 0-terminated
string. The string itself will never be references by Prolog after this
function.
- void PL_put_string_chars(term_t
-t, const char *chars)
-
Put a zero-terminated string in the term-reference. The data will be
copied. See also PL_put_string_nchars().
- void PL_put_string_nchars(term_t
-t, unsigned int len, const char *chars)
-
Put a string, represented by a length/start pointer pair in the
term-reference. The data will be copied. This interface can deal with
0-bytes in the string. See also section 5.6.17.
- void PL_put_list_chars(term_t
-t, const char *chars)
-
Put a list of ASCII values in the term-reference.
- void PL_put_integer(term_t
-t, long i)
-
Put a Prolog integer in the term reference.
- void PL_put_pointer(term_t
-t, void *ptr)
-
Put a Prolog integer in the term-reference. Provided ptr is in the
`malloc()-area', PL_get_pointer()
will get the pointer back.
- void PL_put_float(term_t
-t, double f)
-
Put a floating-point value in the term-reference.
- void PL_put_functor(term_t
-t, functor_t functor)
-
Create a new compound term from functor and bind t
to this term. All arguments of the term will be variables. To create a
term with instantiated arguments, either instantiate the arguments using
the PL_unify_*() functions or use PL_cons_functor().
- void PL_put_list(term_t
-l)
-
Same as
PL_put_functor(l,
PL_new_functor(PL_new_atom("."), 2))
.
- void PL_put_nil(term_t
-l)
-
Same as
PL_put_atom_chars("[]")
.
- void PL_put_term(term_t
-t1, term_t +t2)
-
Make t1 point to the same term as t2.
- void PL_cons_functor(term_t
-h, functor_t f, ...)
-
Create a term, whose arguments are filled from variable argument list
holding the same number of term_t objects as the arity of the functor.
To create the term
animal(gnu, 50)
, use:
term_t a1 = PL_new_term_ref();
term_t a2 = PL_new_term_ref();
term_t t = PL_new_term_ref();
PL_put_atom_chars(a1, "gnu");
PL_put_integer(a2, 50);
PL_cons_functor(t, PL_new_functor(PL_new_atom("animal"), 2),
a1, a2);
After this sequence, the term-references a1 and a2
may be used for other purposes.
- void PL_cons_functor_v(term_t
-h, functor_t f, term_t a0)
-
Creates a compound term like PL_cons_functor(),
but a0 is an array of term references as returned by PL_new_term_refs().
The length of this array should match the number of arguments required
by the functor.
- void PL_cons_list(term_t
-l, term_t +h, term_t +t)
-
Create a list (cons-) cell in l from the head and tail. The
code below creates a list of atoms from a
char **
. The list
is built tail-to-head. The PL_unify_*() functions can be
used 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);
}
}
Note that l can be redefined within a PL_cons_list
call as shown here because operationally its old value is consumed
before its new value is set.
The functions of this sections unify terms with other terms
or translated C-data structures. Except for PL_unify(),
the functions of this section are specific to SWI-Prolog. They have been
introduced to make translation of old code easier, but also because they
provide for a faster mechanism for returning data to Prolog that
requires less term-references. 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, buf);
}
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;
}
- int PL_unify(term_t
?t1, term_t ?t2)
-
Unify two Prolog terms and return non-zero on success.
- int PL_unify_atom(term_t
?t, atom_t a)
-
Unify t with the atom a and return non-zero on
success.
- int PL_unify_atom_chars(term_t
?t, const char *chars)
-
Unify t with an atom created from chars and return
non-zero on success.
- int PL_unify_list_chars(term_t
?t, const char *chars)
-
Unify t with a list of ASCII characters constructed from
chars.
- void PL_unisy_string_chars(term_t
?t, const char *chars)
-
Unify t with a Prolog string object created from the
zero-terminated string chars. The data will be copied. See
also PL_put_string_nchars().
- void PL_put_string_nchars(term_t
?t, unsigned int len, const char *chars)
-
Unify t with a Prolog string object created from the string
created from the len/chars pair. The data will be
copied. This interface can deal with 0-bytes in the string. See also
section 5.6.17.
- int PL_unify_integer(term_t
?t, long n)
-
Unify t with a Prolog integer from n.
- int PL_unify_float(term_t
?t, double f)
-
Unify t with a Prolog float from f.
- int PL_unify_pointer(term_t
?t, void *ptr)
-
Unify t with a Prolog integer describing the pointer. See
also
PL_put_pointer() and PL_get_pointer().
- int PL_unify_functor(term_t
?t, functor_t f)
-
If t is a compound term with the given functor, just succeed.
If it is unbound, create a term and bind the variable, else fails. Not
that this function does not create a term if the argument is already
instantiated.
- int PL_unify_list(term_t
?l, term_t -h, term_t -t)
-
Unify l with a list-cell (./2). If successful,
write a reference to the head of the list to h and a
reference to the tail of the list in t. This reference 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_put_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_put_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.
foreign_t
pl_get_environ(term_t env)
{ term_t l = PL_copy_term_ref(env);
term_t a = PL_new_term_ref();
extern char **environ;
char **e;
for(e = environ; *e; e++)
{ if ( !PL_unify_list(l, a, l) ||
!PL_unify_atom_chars(a, *e) )
PL_fail;
}
return PL_unify_nil(l);
}
- int PL_unify_nil(term_t
?l)
-
Unify l with the atom
.
- int PL_unify_arg(int
index, term_t ?t, term_t ?a)
-
Unifies the index-th argument (1-based) of t
with
a.
- int PL_unify_term(term_t
?t, ...)
-
Unify t with a (normally) compound term. The remaining
arguments is a sequence of a type identifier, followed by the required
arguments. This predicate is an extension to the Quintus and SICStus
foreign interface from which the SWI-Prolog foreign interface has been
derived, but has proved to be a powerful and comfortable way to create
compound terms from C. Due to the vararg packing/unpacking and the
required type-switching this interface is slightly slower than using the
primitives. Please note that some bad C-compilers have fairly low limits
on the number of arguments that may be passed to a function.
The type identifiers are:
PL_VARIABLE
none-
No op. Used in arguments of
PL_FUNCTOR
.
PL_ATOM
atom_t-
Unify the argument with an atom, as in PL_unify_atom().
PL_INTEGER
long-
Unify the argument with an integer, as in PL_unify_integer().
PL_FLOAT
double-
Unify the argument with a float, as in PL_unify_float().
Note that, as the argument is passed using the C vararg conventions, a
float must be casted to a double explicitly.
PL_STRING
const char *-
Unify the argument with a string object, as in PL_unify_string_chars().
PL_TERM
term_t-
Unify a subterm. Note this may the return value of a PL_new_term_ref()
call to get access to a variable.
PL_CHARS
const char *-
Unify the argument with an atom, constructed from the C
char *
,
as in PL_unify_atom_chars().
PL_FUNCTOR
functor_t, ...-
Unify the argument with a compound term. This specification should be
followed by exactly as many specifications as the number of arguments of
the compound term.
PL_LIST
int length, ...-
Create a list of the indicated length. The following arguments contain
the elements of the list.
For example, to unify an argument with the term language(dutch)
,
the following skeleton may be used:
static functor_t FUNCTOR_language1;
static void
init_constants()
{ FUNCTOR_language1 = PL_new_functor(PL_new_atom("language"), 1);
}
foreign_t
pl_get_lang(term_t r)
{ return PL_unify_term(r,
PL_FUNCTOR, FUNCTOR_language1,
PL_CHARS, "dutch");
}
install_t
install()
{ PL_register_foreign("get_lang", 1, pl_get_lang, 0);
init_constants();
}
The Prolog engine can be called from C. There are to interfaces for
this. For the first, a term is created that could be used as an argument
to call/1
and next 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 defined or not, redefined,
etc., a Prolog predicate has a handle that is not destroyed, nor moved.
This handle is known by the type predicate_t
.
- predicate_t PL_pred(functor_t
f, module_t m)
-
Return a handle to a predicate for the specified name/arity in the given
module. This function always succeeds, creating a handle for an
undefined predicate if no handle was available.
- predicate_t PL_predicate(const
char *name, int arity, const char* module)
-
Same a PL_pred(), but provides a
more convenient interface to the C-programmer.
- void PL_predicate_info(predicate_t
p, atom_t *n, int *a, module_t *m)
-
Return information on the predicate p. The name is stored
over n, the arity over a, while m
receives the definition module. Note that the latter need not be the
same as specified with PL_predicate().
If the predicate was imported into the module given to PL_predicate(),
this function will return the module where the predicate was defined.
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 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.
- qid_t PL_open_query(module_t
ctx, int flags, predicate_t p, term_t +t0)
-
Opens a query and returns an identifier for it. This function always
succeeds, regardless whether the predicate is defined or not. 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 module_transparent
predicates. See context_module/1
and module_transparent/1.
The p argument specifies the predicate, and should be the
result of a call to PL_pred() or PL_predicate().
Note that it is allowed to store this handle as global data and reuse it
for future queries. The term-reference t0 is the first of a
vector of term-references as returned by
PL_new_term_refs(n).
The flags arguments provides some additional options
concerning debugging and exception handling. It is a bitwise or of the
following values:
PL_Q_NORMAL
-
Normal operation. The debugger inherits its settings from the
environment. If an exception occurs that is not handled in Prolog, a
message is printed and the tracer is started to debug the error. (26)
PL_Q_NODEBUG
-
Switch off the debugger while executing the goal. This option is used by
many calls to hook-predicates to avoid tracing the hooks. An example is print/1
calling portray/1
from foreign code.
PL_Q_CATCH_EXCEPTION
-
If an exception is raised while executing the goal, do not report it,
but make it available for PL_exception().
PL_Q_PASS_EXCEPTION
-
As
PL_Q_CATCH_EXCEPTION
, but do not invalidate the
exception-term while calling PL_close_query().
This option is experimental.
The example below opens a query to the predicate is_a/2 to find the
ancestor of for some name.
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_NORMAL, p, a0);
...
}
- int PL_next_solution(qid_t
qid)
-
Generate the first (next) solution for the given query. The return value
is
TRUE
if a solution was found, or FALSE
to
indicate the query could not be proven. This function may be called
repeatedly until it fails to generate all solutions to the query.
- void PL_cut_query(qid)
-
Discards the query, but does not delete any of the data created by the
query. It just invalidate qid, allowing for a new call to
PL_open_query() in this
context.
- void PL_close_query(qid)
-
As PL_cut_query(), but all
data and bindings created by the query are destroyed.
- int PL_call_predicate(module_t
m, int flags, predicate_t pred, term_t +t0)
-
Shorthand for PL_open_query(), PL_next_solution(), PL_cut_query(),
generating a single solution. The arguments are the same as for
PL_open_query(), the return
value is the same as PL_next_solution().
- int PL_call(term_t,
module_t)
-
Call term just like the Prolog predicate once/1. Term
is called in the specified module, or in the context module if module_t
= NULL. Returns
TRUE
if the call succeeds, FALSE
otherwise.
Figure 8 shows an example to
obtain the number of defined atoms. All checks are omitted to improve
readability.
The Prolog data created and term-references needed to setup the call
and/or analyse the result can in most cases be discarded right after the
call. PL_close_query()
allows for destructing the data, while leaving the term-references. The
calls below may be used to destroy term-references and data. See figure
8 for an example.
- fid_t PL_open_foreign_frame()
-
Created 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.
- void PL_close_foreign_frame(fid_t
id)
-
Discard all term-references created after the frame was opened. All
other Prolog data is retained. This function is called by the kernel
whenever a foreign function returns control back to Prolog.
- void PL_discard_foreign_frame(fid_t
id)
-
Same as PL_close_foreign_frame(),
but also undo all bindings made since the open and destroy all Prolog
data.
It is obligatory to call either of the two closing functions to
discard a foreign frame. Foreign frames may be nested.
int
count_atoms()
{ fid_t fid = PL_open_foreign_frame();
term_t goal = PL_new_term_ref();
term_t a1 = PL_new_term_ref();
term_t a2 = PL_new_term_ref();
functor_t s2 = PL_new_functor(PL_new_atom("statistics"), 2);
int atoms;
PL_put_atom_chars(a1, "atoms");
PL_cons_functor(goal, s2, a1, a2);
PL_call(goal, NULL); /* call it in current module */
PL_get_integer(a2, &atoms);
PL_discard_foreign_frame(fid);
return atoms;
}
Figure 8 : Calling Prolog
Modules are identified via a unique handle. The following functions
are available to query and manipulate modules.
- module_t PL_context()
-
Return the module identifier of the context module of the currently
active foreign predicate.
- int PL_strip_module(term_t
+raw, module_t *m, term_t -plain)
-
Utility function. If raw is a term, possibly holding the
module construct <module>:<rest>
this function will make
plain a reference to <rest> and fill module
* with <module>. For further nested module
constructs the inner most module is returned via module *. If raw
is not a module construct arg will simply be put in plain.
If module * is
NULL
it will be set to the
context module. Otherwise it will be left untouched. 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);
...
- atom_t PL_module_name(module_t)
-
Return the name of module as an atom.
- module_t PL_new_module(atom_t
name)
-
Find an existing or create a new module with name specified by the atom
name.
This section discusses PL_exception(), PL_throw()
and
PL_raise_exception(),
the interface functions to detect and generate Prolog exceptions from
C-code. PL_throw() and PL_raise_exception()
from the C-interface to raise an exception from foreign code. PL_throw()
exploits the C-function longjmp() to return immediately to the innermost
PL_next_solution(). PL_raise_exception()
registers the exception term and returns FALSE
. If a
foreign predicate returns FALSE, while and exception-term is registered
a Prolog exception will be raised by the virtual machine.
Calling these functions outside the context of a function
implementing a foreign predicate results in undefined behaviour.
PL_exception() may be used
after a call to PL_next_solution()
fails, and returns a term reference to an exception term if an exception
was raised, and 0 otherwise.
If a C-function, implementing a predicate calls Prolog and detects an
exception using PL_exception(),
it can handle this exception, or return with the exception. Some caution
is required though. It is
not allowed to call PL_close_query()
or
PL_discard_foreign_frame()
afterwards, as this will invalidate the exception term. Below is the
code that calls a Prolog defined arithmetic function (see arithmethic_function/1).
If PL_next_solution()
succeeds, the result is analysed and translated to a number, after which
the query is closed and all Prolog data created after PL_open_foreign_frame()
is destroyed. On the other hand, if
PL_next_solution() fails
and if an exception was raised, just pass it. Otherwise generate an
exception (PL_error() is an internal call for building the
standard error terms and calling PL_raise_exception()).
After this, the Prolog environment should be discarded using
PL_cut_query() and PL_close_foreign_frame()
to avoid invalidating the exception term.
static int
prologFunction(ArithFunction f, term_t av, Number r)
{ int arity = f->proc->definition->functor->arity;
fid_t fid = PL_open_foreign_frame();
qid_t qid;
int rval;
qid = PL_open_query(NULL, PL_Q_NORMAL, f->proc, av);
if ( PL_next_solution(qid) )
{ rval = valueExpression(av+arity-1, r);
PL_close_query(qid);
PL_discard_foreign_frame(fid);
} else
{ term_t except;
if ( (except = PL_exception(qid)) )
{ rval = PL_throw(except); /* pass exception */
} else
{ char *name = stringAtom(f->proc->definition->functor->name);
/* generate exception */
rval = PL_error(name, arity-1, NULL, ERR_FAILED, f->proc);
}
PL_cut_query(qid); /* donot destroy data */
PL_close_foreign_frame(fid); /* same */
}
return rval;
}
- int PL_raise_exception(term_t
exception)
-
Generate an exception (as throw/1)
and return
FALSE
. Below is an example returning an
exception from foreign predicate:
foreign_t
pl_hello(term_t to)
{ char *s;
if ( PL_get_atom_chars(to, &s) )
{ Sprintf("Hello \"%s\"\n", s);
PL_succeed;
} else
{ term_t except = PL_new_term_ref();
PL_unify_term(except,
PL_FUNCTOR, PL_new_functor(PL_new_atom("type_error"), 2),
PL_CHARS, "atom",
PL_TERM, to);
return PL_raise_exception(except);
}
}
- int PL_throw(term_t
exception)
-
Similar to PL_raise_exception(),
but returns using the C longjmp() function to the innermost PL_next_solution().
- term_t PL_exception(qid_t
qid)
-
If PL_next_solution()
fails, this can be due to normal failure of the Prolog call, or because
an exception was raised using throw/1.
This function returns a handle to the exception term if an exception was
raised, or 0 if the Prolog goal simply failed. (27).
- int PL_compare(term_t
t1, term_t t2)
-
Compares two terms using the standard order of terms and returns -1, 0
or 1. See also compare/3.
The interface functions below provide for efficient management of
Prolog terms in the Prolog database. They provide an alternative to
calling
asserta/1
or recorda/3
or friends.
- record_t PL_record(term_t
+t)
-
Record the term t into the Prolog database as recorda/3
and return an opaque handle to the term. The returned handle remains
valid until PL_erase() is called
on it. PL_recorded() is used to
copy recorded terms back to the Prolog stack.
- void PL_recorded(record_t
record, term_t -t)
-
Copy a recorded term back to the Prolog stack. The same record may be
used to copy multiple instances at any time to the Prolog stack. See
also PL_record() and PL_erase().
- void PL_erase(record_t
record)
-
Remove the recorded term from the Prolog database, reclaiming all
associated memory resources.
SWI-Prolog offers both a C and Prolog interface to deal with software
interrupts (signals). The Prolog mapping is defined in
section ??. 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.
Some versions of SWI-Prolog, notably running on popular Unix
platforms, handle SIG_SEGV
for guarding the Prolog stacks.
If the application whishes to handle this signal too, it should use PL_signal()
to install its handler after initialisating Prolog. SWI-Prolog will pass
SIG_SEGV
to the user code if it detected the signal is not
related to a Prolog stack overflow.
Any handler that wishes to call one of the Prolog interface functions
should call PL_signal() for its
installation.
- void (*)() PL_signal(sig,
func)
-
This function is equivalent to the BSD-Unix signal() function,
regardless of the platform used. The signal handler is blocked while the
signal routine is active, and automatically reactivated after the
handler returns.
After a signal handler is registered using this function, the native
signal interface redirects the signal to a generic signal handler inside
SWI-Prolog. This generic handler validates the environment, creates a
suitable environment for calling the interface functions described in
this chapter and finally calls the registered user-handler.
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 3.8.
- int PL_warning(format,
a1, ...)
-
Print an error message starting with `[WARNING: ',
followed by the output from format, followed by a `
]
'
and a newline. Then start the tracer. format and the
arguments are the same as for printf(2). Always returns FALSE
.
- int PL_action(int,
C_type)
-
Perform some action on the Prolog system. int describes the
action, C_type provides the argument if necessary. The
actions are listed in table 5.
PL_ACTION_TRACE | Start Prolog tracer |
PL_ACTION_DEBUG | Switch on Prolog
debug mode |
PL_ACTION_BACKTRACE | Print backtrace
on current output stream. The argument (an int) is the number of frames
printed. |
PL_ACTION_HALT | Halt Prolog
execution. This action should be called rather than Unix exit() to give
Prolog the opportunity to clean up. This call does not return. |
PL_ACTION_ABORT | Generate a Prolog
abort. This call does not return. |
PL_ACTION_BREAK | Create a standard
Prolog break environment. Returns after the user types control-D. |
PL_ACTION_SYMBOLFILE | The argument (a
char *) is considered to be hold the symbolfile for further incremental
loading. Should be called by user applications that perform incremental
loading as well and want to inform Prolog of the new symbol table. |
Table 5 : PL_action()
options
- C_type PL_query(int)
-
Obtain status information on the Prolog system. The actual argument type
depends on the information required. int describes what
information is wanted. The options are given in table
6.
PL_QUERY_ARGC | Return an integer
holding the number of arguments given to Prolog from Unix. |
PL_QUERY_ARGV | Return a char **
holding the argument vector given to Prolog from Unix. |
PL_QUERY_SYMBOLFILE | Return a char *
holding the current symbol file of the running process. |
PL_QUERY_ORGSYMBOLFILE | Return the
initial symbol file (before loading) of Prolog. By setting the symbol
file to this value no name clashes can occur with previously loaded
foreign files (but no symbols can be shared with earlier loaded modules
as well). |
PL_MAX_INTEGER | Return a long,
representing the maximal integer value represented by Prolog's tagged
integers. |
PL_MIN_INTEGER | Return a long,
represented the minimal integer value. |
PL_QUERY_VERSION | Return a long,
representing the version as
10,000 × M + 100 × m + p, where
M is the major, m the minor version number and p
the patch-level. For example,
20717 means 2.7.17 . |
Table 6 : PL_query()
options
- int PL_register_foreign(name,
arity, function, flags)
-
Register a C-function to implement a Prolog predicate. After this call
returns successfully a predicate with name name (a char *)
and arity arity (a C int) is created. When called in Prolog,
Prolog will call function. flags forms bitwise
or'ed list of options for the installation. These are:
PL_FA_NOTRACE | Predicate cannot be
seen in the tracer |
PL_FA_TRANSPARENT | Predicate is
module transparent |
PL_FA_NONDETERMINISTIC | Predicate is
non-deterministic. See also PL_retry(). |
- void PL_register_extensions(PL_extension
*e)
-
Register foreign predicates from a table of structures. The type
PL_extension
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;
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(predicates);
if ( !PL_initialise(argc, argv) )
PL_halt(1);
...
}
The function PL_register_extensions()
is the only PL_* function that may be called before PL_initialise().
The functions are registered after registration of the SWI-Prolog
builtin foreign predicates and before loading the initial saved state.
This implies that
initialization/1
directives can refer to them.
For various specific applications some hooks re provided.
- PL_dispatch_hook_t PL_dispatch_hook(PL_dispatch_hook_t)
-
If this hook is not NULL, this function is called when reading from the
terminal. It is supposed to dispatch events when SWI-Prolog is connected
to a window environment. It can return two values:
PL_DISPATCH_INPUT
indicates Prolog input is available on
file descriptor 0 or PL_DISPATCH_TIMEOUT
to indicate a
timeout. The old hook is returned. The type PL_dispatch_hook_t
is defined as:
typedef int (*PL_dispatch_hook_t)(void);
- void PL_abort_hook(PL_abort_hook_t)
-
Install a hook when abort/0
is executed. SWI-Prolog abort/0
is implemented using C setjmp()/longjmp() construct. The hooks are
executed in the reverse order of their registration after the longjmp()
took place and before the Prolog toplevel is reinvoked. The type
PL_abort_hook_t
is defined as:
typedef void (*PL_abort_hook_t)(void);
- int PL_abort_unhook(PL_abort_hook_t)
-
Remove a hook installed with PL_abort_hook().
Returns
FALSE
if no such hook is found, TRUE
otherwise.
This section provides some hints for handling foreign data in Prolog.
With foreign data, we refer to data that is used by foreign language
predicates and needs to be passed around in Prolog. Excluding
combinations, there are three principal options for storing such data
- Natural Prolog data
E.i. using the representation one would choose if there was no foreign
interface required.
- Opaque packed Prolog data
Data can also be represetented in a foreign structure and stored on the
Prolog stacks using PL_put_string_nchars()
and retrieved using
PL_get_string_chars(). It is generally good practice to wrap the
string in a compound term with arity 1, so Prolog can identify the type. portray/1
rules may be used to streamline printing such terms during development.
- Natural foreign data, passing a pointer
An alternative is to pass a pointer to the foreign data. Again, this
functor may be wrapped in a compound term.
The choice may be guided using the following distinctions
- Is the data opaque to Prolog
With `opaque' data, we refer to data handled in foreign functions,
passed around in Prolog, but of which Prolog never examines the contents
of the data itself. If the data is opaque to Prolog, the choosen
representation does not depend on simple analysis by Prolog, and the
selection will be driven solely by simplicity of the interface and
performance (both in time and space).
- How big is the data
Is effient encoding required? For examine, a boolean aray may be
expressed as a compound term, holding integers each of which contains a
number of bits, or as a list of true
and false
.
- What is the nature of the data
For examples in C, constants are often expressed using `enum' or
#define'd integer values. If prolog needs to handle this data, atoms are
a more logical choice. Whether or not this mapping is used depends on
whether Prolog needs to interpret the data, how important debugging is
and how important performance is.
- What is the lifetime of the data
We can distinguish three cases.
- The lifetime is dictated by the accesibility of the data on the
Prolog stacks. Their is no way by which the foreign code when the data
becomes `garbage', and the data thus needs to be represented on the
Prolog stacks using Prolog data-types. (2),
- The data lives on the `heap' and is explicitly allocated and
deallocated. In this case, representing the data using native foreign
representation and passing a pointer to it is a sensible choice.
- The data lives as during the lifetime of a foreign predicate. If the
predicate is deterministic, foreign automatic variables are suitable. if
the predicate is non-deterministic, the data may be allocated using
malloc() and a pointer may be passed. See section
5.6.1.1.
In this section, we wull 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. In the
second example, we look at handling a `netmask'. Finally, we discuss the
outline of the DDE interface.
Integer sets with not-to-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
bitvectors. This can be done in Prolog, using a compound term holding
integer arguments. Especially if the integers are kept below the maximum
tagged integer value (see feature/2),
this representation is fairly space-efficient (wasting 1 word for the
functor and and 7 bits per integer for the tags). Arithmetic can all be
performed in Prolog too.
For really demanding applications, foreign representation will
perform better, especially time-wise. Bit-vectors are natrually
expressed using string objects. If the string is wrapped in bitvector/1
, lower-bound of the vector is 0, and the upperbound is not defined, an
implementation for getting and putting the setes as well as the union
predicate for it is below.
#include
#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
Netmask's are used with TCP/IP configuration. Suppose we have
an application dealing with reasoning about a network configuration.
Such an application requires communicating netmask structures from the
operating system, reasoning about them and possibly communicate them to
the user. A netmask consists of 4 bitmasks between 0 and 255.
C-application normally see them as an 4-byte wide unsigned integer.
SWI-Prolog cannot do that, as integers are always signed.
We could use the string approach outlined above, but this makes it
hard to handle these terms in Prolog. A better choice is a compound term netmask/4,
holding the 4 submasks as integer arguments.
As the implementation is trivial, we will omit this here.
The DDE interface (see section 3.42)
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 aproach is to using 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, not to
verify 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.
As of version 2.1.0, SWI-Prolog may be embedded in a C-program. To
reach at a compiled C-program with SWI-Prolog as an embedded application
is very similar to creating a statically linked SWI-Prolog executable as
described in section 5.4.1.
The file ... /pl/include/stub.c
defines SWI-Prologs
default main program:
int
main(int argc, char **argv)
{ if ( !PL_initialise(argc, argv) )
PL_halt(1);
PL_install_readline(); /* delete if you don't want readline */
PL_halt(PL_toplevel() ? 0 : 1);
}
This may be replaced with your own main C-program. The interface
function PL_initialise() must
be called before any of the other SWI-Prolog foreign language functions
described in this chapter.
PL_initialise() interprets
all the command-line arguments, except for the -t toplevel
flag that is interpreted by PL_toplevel().
- int PL_initialise(int
argc, char **argv, char **environ)
-
Initialises the SWI-Prolog heap and stacks, restores the boot QLF file,
loads the system and personal initialisation files, runs the at_initialization/1
hooks and finally runs the -g goal hook.
PL_initialise() returns 1
if all initialisation succeeded and 0 otherwise. Various fatal errors
may cause PL_initialise to call
PL_halt(1), preventing it from
returning at all.
- void PL_install_readline()
-
Installs the GNU-readline line-editor. Embedded applications that do not
use the Prolog toplevel should normally delete this line, shrinking the
Prolog kernel significantly.
- int PL_toplevel()
-
Runs the goal of the -t toplevel switch
(default prolog/0)
and returns 1 if successful, 0 otherwise.
- void PL_halt(int
status)
-
Cleanup the Prolog environment and calls exit() with the status
argument.