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_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.
- int PL_is_variable(term_t)
- Returns non-zero if term is a variable.
- int PL_is_ground(term_t)
- Returns non-zero if term is a ground term. See also ground/1.
This function is cycle-safe.
- int PL_is_atom(term_t)
- Returns non-zero if term is an atom.
- int PL_is_string(term_t)
- Returns non-zero if term is a string.
- int PL_is_integer(term_t)
- Returns non-zero if term is an integer.
- int PL_is_rational(term_t)
- Returns non-zero if term is a rational number (P/Q).
Note that all integers are considered rational and this test thus
succeeds for any term for which PL_is_integer()
succeeds. See also
PL_get_mpq()
and PL_unify_mpq().
- int PL_is_float(term_t)
- Returns non-zero if term is a float. Note that the
corresponding
PL_get_float()
converts rationals (and thus integers).
- int PL_is_callable(term_t)
- Returns non-zero if term is a callable term. See callable/1
for details.
- int PL_is_compound(term_t)
- Returns non-zero if term is a compound term.
- int PL_is_functor(term_t,
functor_t)
- Returns non-zero if term is compound and its functor is functor.
This test is equivalent to PL_get_functor(),
followed by testing the functor, but easier to write and faster.
- int PL_is_list(term_t)
- Returns non-zero if term is a compound term using the list
constructor or the list terminator. See also PL_is_pair()
and
PL_skip_list().
- int PL_is_pair(term_t)
- Returns non-zero if term is a compound term using the list
constructor. See also PL_is_list()
and PL_skip_list().
- int PL_is_dict(term_t)
- Returns non-zero if term is a dict. See also PL_put_dict()
and PL_get_dict_key().
- int PL_is_atomic(term_t)
- Returns non-zero if term is atomic (not a variable or
compound).
- int PL_is_number(term_t)
- Returns non-zero if term is an rational (including integers)
or float.
- int PL_is_acyclic(term_t)
- Returns non-zero if term is acyclic (i.e. a finite tree).
The functions PL_get_*() read information from a Prolog
term. Most of them take two arguments. The first is the input term and
the second is a pointer to the output value or a term reference. The
return value is TRUE or FALSE, indicating the
success of the "get" operation. Most functions have a related "_ex"
function that raises an error if the argument is the operation cannot be
completed. If the Prolog term is not suitable, this is a type, domain or
instantiation error. If the receiving C type cannot represent the value
this is a representation error.
For integers an alternative interface exists, which helps deal with
the various integer types in C and C++. They are convenient for use with
_Generic selection or C++ overloading.
- int PL_get_atom(term_t
+t, atom_t *a)
- If t is an atom, store the unique atom identifier over a.
See also PL_atom_chars()
and PL_new_atom().
If there is no need to access the data (characters) of an atom, it is
advised to manipulate atoms using their handle. As the atom is
referenced by t, it will live at least as long as t
does. If longer live-time is required, the atom should be locked using PL_register_atom().
- int PL_get_atom_chars(term_t
+t, char **s)
- If t is an atom, store a pointer to a 0-terminated C-string
in
s. It is explicitly not allowed to modify
the contents of this string. Some built-in atoms may have the string
allocated in read-only memory, so‘temporary manipulation' can
cause an error.
- int PL_get_string_chars(term_t
+t, char **s, size_t *len)
- If t is a string object, store a pointer to a 0-terminated
C-string in s and the length of the string in len.
Note that this pointer is invalidated by backtracking, garbage
collection and stack-shifts, so generally the only safe operations are
to pass it immediately to a C function that doesn't involve Prolog.
- int PL_get_chars(term_t
+t, char **s, unsigned flags)
- Convert the argument term t to a 0-terminated C-string.
flags is a bitwise disjunction from two groups of constants.
The first specifies which term types should be converted and the second
how the argument is stored. Below is a specification of these constants.
BUF_STACK implies, if the data is not static (as from an
atom), that the data is pushed on a stack. If BUF_MALLOC is used, the
data must be freed using PL_free()
when no longer needed.
With the introduction of wide characters (see section
2.19.1), not all atoms can be converted into a char*.
This function fails if t is of the wrong type, but also if
the text cannot be represented. See the REP_* flags below
for details. See also PL_get_wchars().
- CVT_ATOM
- Convert if term is an atom.
- CVT_STRING
- Convert if term is a string.
- CVT_LIST
- Convert if term is a list of of character codes.
- CVT_INTEGER
- Convert if term is an integer.
- CVT_FLOAT
- Convert if term is a float. The characters returned are the same as
write/1
would write for the floating point number.
- CVT_NUMBER
- Convert if term is an integer or float.
- CVT_ATOMIC
- Convert if term is atomic.
- CVT_VARIABLE
- Convert variable to print-name
- CVT_WRITE
- Convert any term that is not converted by any of the other flags using
write/1.
If no
BUF_* is provided, BUF_STACK is implied.
- CVT_WRITE_CANONICAL
- As
CVT_WRITE, but using write_canonical/2.
- CVT_WRITEQ
- As
CVT_WRITE, but using writeq/2.
- CVT_ALL
- Convert if term is any of the above, except for
CVT_VARIABLE
and
CVT_WRITE*.
- CVT_EXCEPTION
- If conversion fails due to a type error, raise a Prolog type error
exception in addition to failure
- BUF_DISCARDABLE
- Data must copied immediately.
- BUF_STACK
- Data is stored on a stack. The older
BUF_RING is an alias
for BUF_STACK. See section
12.4.13.
- BUF_MALLOC
- Data is copied to a new buffer returned by PL_malloc(3).
When no longer needed the user must call PL_free()
on the data.
- REP_ISO_LATIN_1
- Text is in ISO Latin-1 encoding and the call fails if text cannot be
represented. This flag has the value 0 and is thus the default.
- REP_UTF8
- Convert the text to a UTF-8 string. This works for all text.
- REP_MB
- Convert to default locale-defined 8-bit string. Success depends on the
locale. Conversion is done using the wcrtomb() C library function.
- int PL_get_list_chars(+term_t
l, char **s, unsigned flags)
- Same as
PL_get_chars(l, s,
CVT_LIST|flags), provided flags
contains none of the CVT_* flags.
- int PL_get_integer(+term_t
t, int *i)
- If t is a Prolog integer, assign its value over i.
On 32-bit machines, this is the same as PL_get_long(),
but avoids a warning from the compiler. See also PL_get_long()
and
PL_get_integer_ex().
- int PL_get_long(term_t
+t, long *i)
- If t is a Prolog integer that can be represented as a long,
assign its value over i. If t is an integer that
cannot be represented by a C long, this function returns
FALSE.
If t is a floating point number that can be represented as a
long, this function succeeds as well. See also PL_get_int64()
and PL_get_long_ex().
- int PL_get_int64(term_t
+t, int64_t *i)
- If t is a Prolog integer or float that can be represented as
a
int64_t, assign its value over i. See also PL_get_int64_ex().
- int PL_get_uint64(term_t
+t, uint64_t *i)
- If t is a Prolog integer that can be represented as a
uint64_t, assign its value over i. Note that
this requires GMP support for representing uint64_t values
with the high bit set. See also PL_get_uint64_ex().
- int PL_get_intptr(term_t
+t, intptr_t *i)
- Get an integer that is at least as wide as a pointer. On most platforms
this is the same as PL_get_long(),
but on Win64 pointers are 8 bytes and longs only 4. Unlike PL_get_pointer(),
the value is not modified.
- int PL_get_bool(term_t
+t, int *val)
- If t has the value
true, false, set val
to the C constant TRUE or FALSE and return
success, otherwise return failure. The values on, 1, off,
const0 and are also accepted.
- int PL_get_pointer(term_t
+t, void **ptr)
- Together with PL_put_pointer()
and PL_unify_pointer(),
these functions allow representing a C pointer as a Prolog integer. The
integer value is derived from the pointer, but not equivalent. The
translation aims at producing smaller integers that fit more often in
the tagged integer range. Representing C pointers as integers
is unsafe. The blob API described in section
12.4.9 provides a safe way for handling foreign resources that
cooperates with Prolog garbage collection.
- int PL_get_float(term_t
+t, double *f)
- If t is a float, integer or rational number, its value is
assigned over f. Note that if t is an integer or
rational conversion may fail because the number cannot be represented as
a float.
- int PL_get_functor(term_t
+t, functor_t *f)
- If t is compound or an atom, the Prolog representation of the
name-arity pair will be assigned over f. See also
PL_get_name_arity()
and PL_is_functor().
- int PL_get_name_arity(term_t
+t, atom_t *name, size_t *arity)
- If t is compound or an atom, the functor name will be
assigned over name and the arity over arity
(either or both may be NULL). See also PL_get_functor()
and PL_is_functor().
- int PL_get_compound_name_arity(term_t
+t, atom_t *name, size_t *arity)
- If t is compound term, the functor name will be assigned over
name and the arity over arity. This is the same as
PL_get_name_arity(),
but this function fails if t is an atom.
- int PL_get_module(term_t
+t, module_t *module)
- If t is an atom, the system will look up or create the
corresponding module and assign an opaque pointer to it over module.
- int PL_get_arg(size_t
index, term_t +t, term_t -a)
- If t is compound and index is between 1 and arity
(inclusive), assign a with a term reference to the argument.
- int _PL_get_arg(size_t
index, term_t +t, term_t -a)
- Same as PL_get_arg(),
but no checking is performed, neither whether t is actually a
term nor whether index is a valid argument index.
- int PL_get_dict_key(atom_t
key, term_t +dict, term_t -value)
- If dict is a dict, get the associated value in value.
Fails silently if key does not appear in dict or
if if dict is not a dict.
All internal text representation in SWI-Prolog is represented using
char * plus length and allow for 0-bytes in them.
The foreign library supports this by implementing a *_nchars() function
for each applicable *_chars() function. Below we briefly present the
signatures of these functions. For full documentation consult the
*_chars() function.
- int PL_get_atom_nchars(term_t
t, size_t *len, char **s)
- See PL_get_atom_chars().
- int PL_get_list_nchars(term_t
t, size_t *len, char **s)
- See PL_get_list_chars().
- int PL_get_nchars(term_t
t, size_t *len, char **s, unsigned int flags)
- See PL_get_chars().
- int PL_put_atom_nchars(term_t
t, size_t len, const char *s)
- See PL_put_atom_chars().
- int PL_put_string_nchars(term_t
t, size_t len, const char *s)
- See PL_put_string_chars().
- int PL_put_list_ncodes(term_t
t, size_t len, const char *s)
- See PL_put_list_codes().
- int PL_put_list_nchars(term_t
t, size_t len, const char *s)
- See PL_put_list_chars().
- int PL_unify_atom_nchars(term_t
t, size_t len, const char *s)
- See PL_unify_atom_chars().
- int PL_unify_string_nchars(term_t
t, size_t len, const char *s)
- See PL_unify_string_chars().
- int PL_unify_list_ncodes(term_t
t, size_t len, const char *s)
- See PL_unify_codes().
- int PL_unify_list_nchars(term_t
t, size_t len, const char *s)
- See PL_unify_list_chars().
In addition, the following functions are available for creating and
inspecting atoms:
- atom_t PL_new_atom_nchars(size_t
len, const char *s)
- Create a new atom as PL_new_atom(),
but using the given length and characters. If len is
(size_t)-1,
it is computed from s using strlen(). See PL_new_atom()
for error handling.
- const char * PL_atom_nchars(atom_t
a, size_t *len)
- Extract the text and length of an atom. If you do not need the length,
pass NULL as the value of len. If PL_atom_nchars()
is called for a blob, NULL is returned.
Support for exchange of wide-character strings is still under
consideration. The functions dealing with 8-bit character strings return
failure when operating on a wide-character atom or Prolog string object.
The functions below can extract and unify both 8-bit and wide atoms and
string objects. Wide character strings are represented as C arrays of
objects of the type pl_wchar_t, which is guaranteed to be
the same as wchar_t on platforms supporting this type. For
example, on MS-Windows, this represents a 16-bit UTF-16 string, while
using the GNU C library (glibc) this represents 32-bit UCS4 characters.
- atom_t PL_new_atom_wchars(size_t
len, const pl_wchar_t *s)
- Create atom from wide-character string as PL_new_atom_nchars()
does for ISO-Latin-1 strings. If s only contains ISO-Latin-1
characters a normal byte-array atom is created. If len is
(size_t)-1,
it is computed from s using wcslen(). See PL_new_atom()
for error handling.
- const pl_wchar_t* PL_atom_wchars(atom_t
atom, size_t *len)
- Extract characters from a wide-character atom. Succeeds on any atom
marked as‘text'. If the underlying atom is a wide-character atom,
the returned pointer is a pointer into the atom structure. If the atom
is represented as an ISO-Latin-1 string, the returned pointer comes from
Prolog's‘buffer stack' (see section
12.4.13).
- int PL_get_wchars(term_t
t, size_t *len, pl_wchar_t **s, unsigned flags)
- Wide-character version of PL_get_chars().
The flags argument is the same as for PL_get_chars().
Note that this operation may return a pointer into Prolog's‘buffer
stack' (see section
12.4.13).
- int PL_unify_wchars(term_t
t, int type, size_t len, const pl_wchar_t *s)
- Unify t with a textual representation of the C wide-character
array s. The type argument defines the Prolog
representation and is one of
PL_ATOM, PL_STRING,
PL_CODE_LIST or PL_CHAR_LIST.
- int PL_unify_wchars_diff(term_t
+t, term_t -tail, int type, size_t len, const pl_wchar_t *s)
- Difference list version of PL_unify_wchars(),
only supporting the types
PL_CODE_LIST and PL_CHAR_LIST.
It serves two purposes. It allows for returning very long lists from
data read from a stream without the need for a resizing buffer in C.
Also, the use of difference lists is often practical for further
processing in Prolog. Examples can be found in packages/clib/readutil.c
from the source distribution.
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(); /* the elements */
term_t list = PL_copy_term_ref(l); /* copy (we modify list) */
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 [] */
}
Note that as of version 7, lists have a new representation
unless the option --traditional is used. see section
5.1.
- int PL_get_list(term_t
+l, term_t -h, term_t -t)
- If l is a list and not the empty list, assign a term
reference to the head to h and to the tail to t.
- int PL_get_head(term_t
+l, term_t -h)
- If l is a list and not the empty list, assign a term
reference to the head to h.
- int PL_get_tail(term_t
+l, term_t -t)
- If l is a list and not the empty list, assign a term
reference to the tail to t.
- int PL_get_nil(term_t
+l)
- Succeeds if l represents the list termination constant.
- int PL_skip_list(term_t
+list, term_t -tail, size_t *len)
- This is a multi-purpose function to deal with lists. It allows for
finding the length of a list, checking whether something is a list, etc.
The reference tail is set to point to the end of the list,
len is filled with the number of list-cells skipped, and the
return value indicates the status of the list:
- PL_LIST
- The list is a‘proper' list: one that ends in the list terminator
constant and tail is filled with the terminator constant.
- PL_PARTIAL_LIST
- The list is a‘partial' list: one that ends in a variable and
tail is a reference to this variable.
- PL_CYCLIC_TERM
- The list is cyclic (e.g. X = [a|X]). tail points to an
arbitrary cell of the list and len is at most twice the cycle
length of the list.
- PL_NOT_A_LIST
- The term list is not a list at all. tail is bound
to the non-list term and len is set to the number of
list-cells skipped.
It is allowed to pass 0 for tail and NULL for len.
- int PL_scan_options(term_t
options, int flags, const char* opttype, PL_option_t specs[], ...)
- Process an option list as we find with, e.g., write_term/2
and many other builtin predicates. This function takes an option list
(or dict) and in the variadic argument list pointers that receive the
option values. PL_scan_options()
takes care of validating the list, ensuring the list is not cyclic,
validating the option type and storing the converted values using the
supplied pointers.
Below is an example. While PL_option_t is a struct, its
members are initialised using the PL_OPTION() macro. The data
structure is not constant because PL_scan_options()
adds the option names as
atoms to speed up option processing. The macro PL_OPTIONS_END
terminates the option list.
static PL_option_t mypred_options[] =
{ PL_OPTION("quoted", OPT_BOOL),
PL_OPTION("length", OPT_SIZE),
PL_OPTION("callback", OPT_TERM),
PL_OPTIONS_END
};
static foreign_t
mypred(term_t a1, term_t options)
{ int quoted = FALSE;
size_t length = 10;
term_t callback = 0;
if ( !PL_scan_options(options, 0, "mypred_options", mypred_options,
"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 int- Convert the option value to a bool. This converts the values described
by PL_get_bool().
In addition, an option without a value (i.e., a plain atom that denotes
the option name) can act as a boolean
TRUE.
OPT_INT intOPT_INT64 int64_tOPT_UINT64 uint64_tOPT_SIZE size_tOPT_DOUBLE double- Numeric values of various types. Raises an error if the Prolog value
cannot be represented by the C type.
OPT_STRING char*- Uses PL_get_chars()
using the flags
CVT_ALL|REP_UTF8|BUF_STACK|CVT_EXCEPTION. The buffered
string must be guarded using PL_STRINGS_MARK()
and PL_STRINGS_RELEASE().
OPT_ATOM atom_t- Accepts an atom. Note that if the C function that implements the
predicate wishes to keep hold of the atom after it returns it must use PL_register_atom().
OPT_TERM term_t- Accepts an arbitrary Prolog term. The term handle is scoped by the
foreign predicate invocation. Terms can be preserved using
PL_record().
The ISO standard demands that if an option is repeated the last
occurance holds. This implies that PL_scan_options()
must scan the option list to the end.
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;
}