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<h2 id="sec:foreignnotes"><a id="sec:10.8"><span class="sec-nr">10.8</span> <span class="sec-title">Notes
on Using Foreign Code</span></a></h2>
<a id="sec:foreignnotes"></a>
<p><h3 id="sec:foreign-malloc"><a id="sec:10.8.1"><span class="sec-nr">10.8.1</span> <span class="sec-title">Memory
Allocation</span></a></h3>
<a id="sec:foreign-malloc"></a>
<p>SWI-Prolog's heap memory allocation is based on the <strong>malloc</strong>(3)
library routines. SWI-Prolog provides the functions below as a wrapper
around malloc(). Allocation errors in these functions trap SWI-Prolog's
fatal-error handler, in which case <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>
or <a class="func" href="foreignnotes.html#PL_realloc()">PL_realloc()</a>
do not return.
<p>Portable applications must use <a class="func" href="foreignnotes.html#PL_free()">PL_free()</a>
to release strings returned by <a class="func" href="foreigninclude.html#PL_get_chars()">PL_get_chars()</a>
using the <code>BUF_MALLOC</code> argument. Portable applications may
use both <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>
and friends or malloc() and friends but should not mix these two sets of
functions on the same memory.
<dl class="latex">
<dt class="pubdef"><a id="PL_malloc()"><var>void *</var> <strong>PL_malloc</strong>(<var>size_t
bytes</var>)</a></dt>
<dd class="defbody">
Allocate <var>bytes</var> of memory. On failure SWI-Prolog's fatal-error
handler is called and <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>
does not return. Memory allocated using these functions must use <a class="func" href="foreignnotes.html#PL_realloc()">PL_realloc()</a>
and <a class="func" href="foreignnotes.html#PL_free()">PL_free()</a>
rather than realloc() and free().</dd>
<dt class="pubdef"><a id="PL_realloc()"><var>void *</var> <strong>PL_realloc</strong>(<var>void
*mem, size_t size</var>)</a></dt>
<dd class="defbody">
Change the size of the allocated chunk, possibly moving it. The
<var>mem</var> argument must be obtained from a previous <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>
or
<a class="func" href="foreignnotes.html#PL_realloc()">PL_realloc()</a>
call.</dd>
<dt class="pubdef"><a id="PL_free()"><var>void</var> <strong>PL_free</strong>(<var>void
*mem</var>)</a></dt>
<dd class="defbody">
Release memory. The <var>mem</var> argument must be obtained from a
previous <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>
or <a class="func" href="foreignnotes.html#PL_realloc()">PL_realloc()</a>
call.
</dd>
</dl>
<p><h4 id="sec:boehm-gc"><a id="sec:10.8.1.1"><span class="sec-nr">10.8.1.1</span> <span class="sec-title">Boehm-GC
support</span></a></h4>
<a id="sec:boehm-gc"></a>
<p><a id="idx:BoehmGC:1902"></a>To accommodate future use of the Boehm
garbage collector<sup class="fn">159<span class="fn-text"><a class="url" href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">http://www.hpl.hp.com/personal/Hans_Boehm/gc/</a></span></sup>
for heap memory allocation, the interface provides the functions
described below. Foreign extensions that wish to use the Boehm-GC
facilities can use these wrappers. Please note that if SWI-Prolog is not
compiled to use Boehm-GC (default), the user is responsible for calling <a class="func" href="foreignnotes.html#PL_free()">PL_free()</a>
to reclaim memory.
<dl class="latex">
<dt class="pubdef"><a id="PL_malloc_atomic()"><var>void*</var> <strong>PL_malloc_atomic</strong>(<var>size_t
bytes</var>)</a></dt>
<dt class="pubdef"><a id="PL_malloc_uncollectable()"><var>void*</var> <strong>PL_malloc_uncollectable</strong>(<var>size_t
bytes</var>)</a></dt>
<dt class="pubdef"><a id="PL_malloc_atomic_uncollectable()"><var>void*</var> <strong>PL_malloc_atomic_uncollectable</strong>(<var>size_t
bytes</var>)</a></dt>
<dd class="defbody">
If Boehm-GC is not used, these are all the same as <a class="func" href="foreignnotes.html#PL_malloc()">PL_malloc()</a>.
With Boehm-GC, these map to the corresponding Boehm-GC functions.
<em>Atomic</em> means that the content should not be scanned for
pointers, and <em>uncollectable</em> means that the object should never
be garbage collected.
</dd>
<dt class="pubdef"><a id="PL_malloc_stubborn()"><var>void*</var> <strong>PL_malloc_stubborn</strong>(<var>size_t
bytes</var>)</a></dt>
<dt class="pubdef"><a id="PL_end_stubborn_change()"><var>void</var> <strong>PL_end_stubborn_change</strong>(<var>void
*memory</var>)</a></dt>
<dd class="defbody">
These functions allow creating objects, promising GC that the content
will not change after <a class="func" href="foreignnotes.html#PL_end_stubborn_change()">PL_end_stubborn_change()</a>.
</dd>
</dl>
<p><h3 id="sec:foreign-compat"><a id="sec:10.8.2"><span class="sec-nr">10.8.2</span> <span class="sec-title">Compatibility
between Prolog versions</span></a></h3>
<a id="sec:foreign-compat"></a>
<p>Great care is taken to ensure binary compatibility of foreign
extensions between different Prolog versions. Only the much less
frequently used stream interface has been responsible for binary
incompatibilities.
<p><a id="idx:PLVERSION:1903"></a>Source code that relies on new
features of the foreign interface can use the macro <code>PLVERSION</code>
to find the version of
<code>SWI-Prolog.h</code> and <a class="func" href="foreigninclude.html#PL_query()">PL_query()</a>
using the option
<code>PL_QUERY_VERSION</code> to find the version of the attached Prolog
system. Both follow the same numbering schema explained with <a class="func" href="foreigninclude.html#PL_query()">PL_query()</a>.
<p><h3 id="sec:foreign-debug-and-profile"><a id="sec:10.8.3"><span class="sec-nr">10.8.3</span> <span class="sec-title">Debugging
and profiling foreign code (valgrind)</span></a></h3>
<a id="sec:foreign-debug-and-profile"></a>
<p><a id="idx:valgrind:1904"></a><a id="idx:profilingforeigncode:1905"></a>This
section is only relevant for Unix users on platforms supported by
<a class="url" href="http://valgrind.org/">valgrind</a>. Valgrind is an
excellent binary instrumentation platform. Unlike many other
instrumentation platforms, valgrind can deal with code loaded through
dlopen().
<p>The <b>callgrind</b> tool can be used to profile foreign code loaded
under SWI-Prolog. Compile the foreign library adding <strong>-g</strong>
option to <b>gcc</b> or <b>swipl-ld</b>. By setting the environment
variable <code>VALGRIND</code> to <code>yes</code>, SWI-Prolog will <em>not</em>
release loaded shared objects using dlclose(). This trick is required to
get source information on the loaded library. Without, valgrind claims
that the shared object has no debugging information.<sup class="fn">160<span class="fn-text">Tested
using valgrind version 3.2.3 on x64.</span></sup> Here is the complete
sequence using <b>bash</b> as login shell:
<pre class="code">
% VALGRIND=yes valgrind --tool=callgrind pl <args>
<prolog interaction>
% kcachegrind callgrind.out.<pid>
</pre>
<p><h3 id="sec:foreign-name-conflicts"><a id="sec:10.8.4"><span class="sec-nr">10.8.4</span> <span class="sec-title">Name
Conflicts in C modules</span></a></h3>
<a id="sec:foreign-name-conflicts"></a>
<p>In the current version of the system all public C functions of
SWI-Prolog are in the symbol table. This can lead to name clashes with
foreign code. Someday I should write a program to strip all these
symbols from the symbol table (why does Unix not have that?). For now I
can only suggest you give your function another name. You can do this
using the C preprocessor. If---for example---your foreign package uses a
function warning(), which happens to exist in SWI-Prolog as well, the
following macro should fix the problem:
<pre class="code">
#define warning warning_
</pre>
<p>Note that shared libraries do not have this problem as the shared
library loader will only look for symbols in the main executable for
symbols that are not defined in the library itself.
<p><h3 id="sec:foreign-quintus-sicstus"><a id="sec:10.8.5"><span class="sec-nr">10.8.5</span> <span class="sec-title">Compatibility
of the Foreign Interface</span></a></h3>
<a id="sec:foreign-quintus-sicstus"></a>
<p>The term reference mechanism was first used by Quintus Prolog version 3.
SICStus Prolog version 3 is strongly based on the Quintus interface. The
described SWI-Prolog interface is similar to using the Quintus or
SICStus interfaces, defining all foreign-predicate arguments of type
<code>+term</code>. SWI-Prolog explicitly uses type <code>functor_t</code>,
while Quintus and SICStus use <<var>name</var>> and <<var>arity</var>>.
As the names of the functions differ from Prolog to Prolog, a simple
macro layer dealing with the names can also deal with this detail. For
example:
<pre class="code">
#define QP_put_functor(t, n, a) \
PL_put_functor(t, PL_new_functor(n, a))
</pre>
<p>The <code>PL_unify_*()</code> functions are lacking from the Quintus
and SICStus interface. They can easily be emulated, or the put/unify
approach should be used to write compatible code.
<p>The <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>
combination is lacking from both other Prologs. SICStus has <a class="func" href="foreigntypes.html#PL_new_term_refs()">PL_new_term_refs(0)</a>,
followed by <a class="func" href="foreigntypes.html#PL_reset_term_refs()">PL_reset_term_refs()</a>,
that allows for discarding term references.
<p>The Prolog interface for the graphical user interface package XPCE
shares about 90% of the code using a simple macro layer to deal with
different naming and calling conventions of the interfaces.
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