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<h2 id="sec:foreigntypes"><a id="sec:10.3"><span class="sec-nr">10.3</span> <span class="sec-title">Interface
Data Types</span></a></h2>
<a id="sec:foreigntypes"></a>
<p><h3 id="sec:type-term-t"><a id="sec:10.3.1"><span class="sec-nr">10.3.1</span> <span class="sec-title">Type <code>term_t</code>:
a reference to a Prolog term</span></a></h3>
<a id="sec:type-term-t"></a>
<p>The principal data type is <code>term_t</code>. Type <code>term_t</code>
is what Quintus calls <code>QP_term_ref</code>. This name indicates
better what the type represents: it is a <em>handle</em> for a term
rather than the term itself. Terms can only be represented and
manipulated using this type, as this is the only safe way to ensure the
Prolog kernel is aware of all terms referenced by foreign code and thus
allows the kernel to perform garbage collection and/or stack-shifts
while foreign code is active, for example during a callback from C.
<p>A term reference is a C unsigned long, representing the offset of a
variable on the Prolog environment stack. A foreign function is passed
term references for the predicate arguments, one for each argument. If
references for intermediate results are needed, such references may be
created using <a class="func" href="foreigntypes.html#PL_new_term_ref()">PL_new_term_ref()</a>
or <a class="func" href="foreigntypes.html#PL_new_term_refs()">PL_new_term_refs()</a>.
These references normally live till the foreign function returns control
back to Prolog. Their scope can be explicitly limited using <a class="func" href="foreigninclude.html#PL_open_foreign_frame()">PL_open_foreign_frame()</a>
and <a class="func" href="foreigninclude.html#PL_close_foreign_frame()">PL_close_foreign_frame()</a>/<a class="func" href="foreigninclude.html#PL_discard_foreign_frame()">PL_discard_foreign_frame()</a>.
<p>A <code>term_t</code> always refers to a valid Prolog term (variable,
atom, integer, float or compound term). A term lives either until
backtracking takes us back to a point before the term was created, the
garbage collector has collected the term, or the term was created after
a
<a class="func" href="foreigninclude.html#PL_open_foreign_frame()">PL_open_foreign_frame()</a>
and <a class="func" href="foreigninclude.html#PL_discard_foreign_frame()">PL_discard_foreign_frame()</a>
has been called.
<p>The foreign interface functions can either <em>read</em>, <em>unify</em>
or
<em>write</em> to term references. In this document we use the following
notation for arguments of type <code>term_t</code>:
<blockquote>
<table class="latex frame-void">
<tr><td><code>term_t +t</code></td><td>Accessed in read-mode. The `+'
indicates the argument is `input'. </td></tr>
<tr><td><code>term_t -t</code></td><td>Accessed in write-mode. </td></tr>
<tr><td><code>term_t ?t</code></td><td>Accessed in unify-mode. </td></tr>
</table>
</blockquote>
<p>Term references are obtained in any of the following ways:
<p>
<ul class="latex">
<li><i>Passed as argument</i><br>
The C functions implementing foreign predicates are passed their
arguments as term references. These references may be read or unified.
Writing to these variables causes undefined behaviour.
<li><i>Created by <a class="func" href="foreigntypes.html#PL_new_term_ref()">PL_new_term_ref()</a></i><br>
A term created by <a class="func" href="foreigntypes.html#PL_new_term_ref()">PL_new_term_ref()</a>
is normally used to build temporary terms or to be written by one of the
interface functions. For example, <a class="func" href="foreigninclude.html#PL_get_arg()">PL_get_arg()</a>
writes a reference to the term argument in its last argument.
<li><i>Created by <a class="func" href="foreigntypes.html#PL_new_term_refs()">PL_new_term_refs(int
n)</a></i><br>
This function returns a set of term references with the same
characteristics as <a class="func" href="foreigntypes.html#PL_new_term_ref()">PL_new_term_ref()</a>.
See <a class="func" href="foreigninclude.html#PL_open_query()">PL_open_query()</a>.
<li><i>Created by <a class="func" href="foreigntypes.html#PL_copy_term_ref()">PL_copy_term_ref(term_t
t)</a></i><br>
Creates a new term reference to the same term as the argument. The term
may be written to. See <a class="fig" href="foreigninclude.html#fig:pl-display">figure
7</a>.
</ul>
<p>Term references can safely be copied to other C variables of type
<code>term_t</code>, but all copies will always refer to the same term.
<dl class="latex">
<dt class="pubdef"><a id="PL_new_term_ref()"><var>term_t</var> <strong>PL_new_term_ref</strong>(<var></var>)</a></dt>
<dd class="defbody">
Return a fresh reference to a term. The reference is allocated on the
<em>local</em> stack. Allocating a term reference may trigger a
stack-shift on machines that cannot use sparse memory management for
allocation of the Prolog stacks. The returned reference describes a
variable.
</dd>
<dt class="pubdef"><a id="PL_new_term_refs()"><var>term_t</var> <strong>PL_new_term_refs</strong>(<var>int
n</var>)</a></dt>
<dd class="defbody">
Return <var>n</var> new term references. The first term reference is
returned. The others are <var><var>t</var>+1</var>, <var><var>t</var>+2</var>,
etc. There are two reasons for using this function. <a class="func" href="foreigninclude.html#PL_open_query()">PL_open_query()</a>
expects the arguments as a set of consecutive term references, and <em>very</em>
time-critical code requiring a number of term references can be written
as:
<pre class="code">
pl_mypredicate(term_t a0, term_t a1)
{ term_t t0 = PL_new_term_refs(2);
term_t t1 = t0+1;
...
}
</pre>
</dd>
<dt class="pubdef"><a id="PL_copy_term_ref()"><var>term_t</var> <strong>PL_copy_term_ref</strong>(<var>term_t
from</var>)</a></dt>
<dd class="defbody">
Create a new term reference and make it point initially to the same term
as <var>from</var>. This function is commonly used to copy a predicate
argument to a term reference that may be written.
</dd>
<dt class="pubdef"><a id="PL_reset_term_refs()"><var>void</var> <strong>PL_reset_term_refs</strong>(<var>term_t
after</var>)</a></dt>
<dd class="defbody">
Destroy all term references that have been created after <var>after</var>,
including <var>after</var> itself. Any reference to the invalidated term
references after this call results in undefined behaviour.
<p>Note that returning from the foreign context to Prolog will reclaim
all references used in the foreign context. This call is only necessary
if references are created inside a loop that never exits back to Prolog.
See also <a class="func" href="foreigninclude.html#PL_open_foreign_frame()">PL_open_foreign_frame()</a>, <a class="func" href="foreigninclude.html#PL_close_foreign_frame()">PL_close_foreign_frame()</a>
and
<a class="func" href="foreigninclude.html#PL_discard_foreign_frame()">PL_discard_foreign_frame()</a>.
</dd>
</dl>
<p><h4 id="sec:foreign-gc"><a id="sec:10.3.1.1"><span class="sec-nr">10.3.1.1</span> <span class="sec-title">Interaction
with the garbage collector and stack-shifter</span></a></h4>
<a id="sec:foreign-gc"></a>
<p>Prolog implements two mechanisms for avoiding stack overflow: garbage
collection and stack expansion. On machines that allow for it, Prolog
will use virtual memory management to detect stack overflow and expand
the runtime stacks. On other machines Prolog will reallocate the stacks
and update all pointers to them. To do so, Prolog needs to know which
data is referenced by C code. As all Prolog data known by C is
referenced through term references (<code>term_t</code>), Prolog has all
the information necessary to perform its memory management without
special precautions from the C programmer.
<p><h3 id="sec:foreign-types"><a id="sec:10.3.2"><span class="sec-nr">10.3.2</span> <span class="sec-title">Other
foreign interface types</span></a></h3>
<a id="sec:foreign-types"></a>
<dl class="latex">
<dt><b>atom_t</b></dt>
<dd>
An atom in Prolog's internal representation. Atoms are pointers to an
opaque structure. They are a unique representation for represented text,
which implies that atom <var>A</var> represents the same text as atom <var>B</var>
if and only if <var>A</var> and <var>B</var> are the same pointer.
<p>Atoms are the central representation for textual constants in Prolog.
The transformation of a character string <var>C</var> to an atom implies
a hash-table lookup. If the same atom is needed often, it is advised to
store its reference in a global variable to avoid repeated lookup.
</dd>
<dt><b>functor_t</b></dt>
<dd>
A functor is the internal representation of a name/arity pair. They are
used to find the name and arity of a compound term as well as to
construct new compound terms. Like atoms they live for the whole Prolog
session and are unique.
</dd>
<dt><b>predicate_t</b></dt>
<dd>
Handle to a Prolog predicate. Predicate handles live forever (although
they can lose their definition).
</dd>
<dt><b>qid_t</b></dt>
<dd>
Query identifier. Used by
<a class="func" href="foreigninclude.html#PL_open_query()">PL_open_query()</a>, <a class="func" href="foreigninclude.html#PL_next_solution()">PL_next_solution()</a>
and <a class="func" href="foreigninclude.html#PL_close_query()">PL_close_query()</a>
to handle backtracking from C.
</dd>
<dt><b>fid_t</b></dt>
<dd>
Frame identifier. Used by
<a class="func" href="foreigninclude.html#PL_open_foreign_frame()">PL_open_foreign_frame()</a>
and <a class="func" href="foreigninclude.html#PL_close_foreign_frame()">PL_close_foreign_frame()</a>.
</dd>
<dt><b>module_t</b></dt>
<dd>
A module is a unique handle to a Prolog module. Modules are used only to
call predicates in a specific module.
</dd>
<dt><b>foreign_t</b></dt>
<dd>
Return type for a C function implementing a Prolog predicate.
</dd>
<dt><b>control_t</b></dt>
<dd>
Passed as additional argument to non-deterministic foreign functions.
See PL_retry*() and PL_foreign_context*().
</dd>
<dt><b>install_t</b></dt>
<dd>
Type for the install() and uninstall() functions of shared or dynamic
link libraries. See <a class="sec" href="foreignlink.html">section
10.2.3</a>.
</dd>
<dt><b>int64_t</b></dt>
<dd>
Actually part of the C99 standard rather than Prolog. As of version
5.5.6, Prolog integers are 64-bit on all hardware. The C99 type
<code>int64_t</code> is defined in the <code>stdint.h</code> standard
header and provides platform-independent 64-bit integers. Portable code
accessing Prolog should use this type to exchange integer values. Please
note that
<a class="func" href="foreigninclude.html#PL_get_long()">PL_get_long()</a>
can return <code>FALSE</code> on Prolog integers that cannot be
represented as a C long. Robust code should not assume any of the
integer fetching functions to succeed, <em>even</em> if the Prolog term
is known to be an integer.
</dd>
</dl>
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